U.S. patent application number 09/933998 was filed with the patent office on 2001-12-20 for semiconductor memory with built-in cache.
This patent application is currently assigned to Oki Electric Industry Co., Ltd.. Invention is credited to Tanabe, Tetsuya, Tanaka, Yasuhiro, Tanoi, Satoru.
Application Number | 20010053103 09/933998 |
Document ID | / |
Family ID | 27517928 |
Filed Date | 2001-12-20 |
United States Patent
Application |
20010053103 |
Kind Code |
A1 |
Tanaka, Yasuhiro ; et
al. |
December 20, 2001 |
Semiconductor memory with built-in cache
Abstract
A semiconductor memory device has memory cells for storing data,
sense amplifiers for amplifying the stored data, and cache cells in
which the amplified data can be placed for quick recall. The cache
cells can continue to hold data during memory-cell refresh cycles,
permitting the cached data to be accessed quickly afterward. The
cache cells may be coupled to column data lines that can be
disconnected from the sense amplifiers, enabling memory cells to be
refreshed while cache access is in progress. Write buffers may be
provided so that when cache data are replaced, the old cache data
can be copied back to the memory cells while the new cache data are
being accessed.
Inventors: |
Tanaka, Yasuhiro; (Tokyo,
JP) ; Tanabe, Tetsuya; (Tokyo, JP) ; Tanoi,
Satoru; (Tokyo, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN &
LANGER & CHICK, PC
767 THIRD AVENUE
25TH FLOOR
NEW YORK
NY
10017-2023
US
|
Assignee: |
Oki Electric Industry Co.,
Ltd.
Tokyo
JP
|
Family ID: |
27517928 |
Appl. No.: |
09/933998 |
Filed: |
August 21, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09933998 |
Aug 21, 2001 |
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09788262 |
Feb 16, 2001 |
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09788262 |
Feb 16, 2001 |
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09458894 |
Dec 10, 1999 |
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6249450 |
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09458894 |
Dec 10, 1999 |
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09003736 |
Jan 17, 1998 |
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6011709 |
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09003736 |
Jan 17, 1998 |
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08365970 |
Dec 29, 1994 |
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5596521 |
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Current U.S.
Class: |
365/207 ;
711/E12.041 |
Current CPC
Class: |
G11C 15/043 20130101;
G11C 11/4096 20130101; G11C 15/04 20130101; G11C 7/1051 20130101;
G06F 12/0893 20130101; G11C 7/106 20130101; G11C 7/1006 20130101;
G11C 2207/2245 20130101; G11C 7/1078 20130101 |
Class at
Publication: |
365/207 |
International
Class: |
G11C 007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 6, 1994 |
JP |
000210/94 |
Jan 11, 1994 |
JP |
001298/94 |
Claims
What is claimed is:
1. A semiconductor memory device, comprising: a memory cell array
for storing rows of data having row addresses; a cache for
temporarily storing at least one row of data having an arbitrary
row address from among said row addresses; a plurality of sense
amplifiers coupled to said memory cell array and said cache, for
amplifying data stored in said memory cell array at a first rate,
and amplifying data stored in said cache at a second rate faster
than said first rate; and a data bus coupled to said sense
amplifiers and said cache, for input and output of data.
2. The semiconductor memory device of claim 1, comprising a tag
circuit coupled to said cache, for storing row addresses of the
data stored in said cache.
3. The semiconductor memory device of claim 1, wherein said
semiconductor memory device is a dynamic random-access memory,
requiring periodic refreshing of data stored in said memory cell
array, and said cache stores first data having one row address
while said sense amplifiers refresh second data having another
address by amplifying said second data in said memory cell
array.
4. The semiconductor memory device of claim 1, further comprising
switching elements for disconnecting said cache and said data bus
from said sense amplifiers, wherein said semiconductor memory
device is a dynamic random-access memory, requiring periodic
refreshing of data stored in said memory cell array, and said-sense
amplifiers amplify and thereby refresh first data stored in said
memory cell array while second data are transferred between said
cache and said data bus.
5. The semiconductor memory device of claim 1, comprising write
buffers coupled to said sense amplifiers, for receiving first data
stored in said cache, storing said first data while said first data
are replaced in said cache by second data from said memory cell
array, and transferring said first data to said memory cell array
after said first data have been thus replaced.
6. A semiconductor memory device, comprising: a rectangular array
of memory cells disposed in intersecting rows and columns, for
storing data; a plurality of bit lines extending parallel to said
columns and coupled to the memory cells in respective columns, for
transporting data to and from said memory cells; a plurality of
word lines extending parallel to said rows and coupled to the
memory cells in respective rows, for controlling transfer of data
between said memory cells and said bit lines; a row of first
switching elements coupled to respective bit lines; a plurality of
sense lines coupled to respective first switching elements, for
transferring data via said first switching elements to and from
said bit lines; a row of sense amplifiers coupled to said of sense
lines, for amplifying data on said sense lines; a row of second
switching elements coupled to respective sense lines; a data bus
for input and output of data; a plurality of column data lines
coupled to respective second switching elements, for interfacing
said sense lines to said data bus; a cache having at least one row
of cache cells coupled to said column data lines, for storing data
transferred from an arbitrary row of said memory cells; and a tag
circuit coupled to said cache, for storing, for each said row of
cache cells, a row address indicating which row of said memory
cells has data stored in said row of cache cells, and controlling
transfer of data between said row of cache cells and said column
data lines responsive to said row address.
7. The memory device of claim 6, wherein: said bit lines are
grouped into complementary pairs, with one complementary pair of
bit lines per column; said sense lines are grouped into
corresponding complementary pairs; and said column data lines are
grouped into corresponding complementary pairs.
8. The memory device of claim 7, wherein each of said cache cells
comprises: a third switching element coupled to one of said column
data lines and switched on and off by said tag circuit; and a
storage element coupled to said switching element.
9. The memory device of claim 8, wherein: said third switching
element is a transistor; and said storage element is a capacitor
having one electrode coupled to said transistor and another
electrode coupled to a fixed potential.
10. The memory device of claim 7, wherein each of said cache cells
comprises: a third switching element coupled to one of said column
data lines and switched on and off by said tag circuit; a fourth
switching element coupled to another one of said column data lines
and switched on and off by said tag circuit; and a storage element
coupled in series between said third switching element and said
fourth switching element.
11. The memory device of claim 7, wherein each of said sense
amplifiers is supplied with two different fixed potentials for use
in amplifying data on said complementary pairs of sense lines,
further comprising: sense-line equalizing circuits coupled to
respective complementary pairs of sense lines, for equalizing said
sense lines to a third potential intermediate between said two
different fixed potentials; a sense-amplifier equalizing circuit
coupled to said row of sense amplifiers, for supplying said sense
amplifiers with said third potential in place of said two different
fixed potentials; and a delay circuit coupled to said
sense-amplifier equalizing circuit, for delaying supply of said
third potential to said sense amplifiers until a certain time after
equalization of said sense lines has begun.
12. The memory device of claim 11, also comprising a hit/miss
detector coupled to said delay circuit, for preventing the supply
of said third potential to said sense amplifiers prior to transfer
of data from said cache cells to said sense lines.
13. The memory device of claim 12, wherein said hit/miss detector
comprises: a hit detector for detecting, from an output of said tag
circuit, access to said cache cells, and generating a first signal;
a miss detector for detecting, from an output of said tag circuit,
access to said memory cells, and generating a second signal that
activates said sense-amplifier equalizing circuit via said delay
circuit; and a logic gate for combining said first signal and said
second signal to produce a third signal, which activates said
sense-line equalizing circuits both when access to said cache cells
is detected and when access to said memory cells is detected.
14. The memory device of claim 7, further comprising a row of write
buffers coupled between respective complementary pairs of sense
lines, for storing data to be copied back from said cache cells to
said memory cell array, while other data are transferred from said
memory cell array to said cache cells.
15. The memory device of claim 14, wherein each of said write
buffers comprises a switching element, a storage element, and
another switching element coupled in series between a complementary
pair of said sense lines.
16. The memory device of claim 14, comprising a transfer control
circuit for turning off said first switching elements after data
have been transferred from said bit lines to said sense lines, but
before said sense amplifiers have finished amplifying said
data.
17. The memory device of claim 16, wherein said transfer control
circuit comprises: a control circuit for generating a first
transfer signal for controlling said first switching elements when
data are transferred unidirectionally from said bit lines to said
sense lines; a logic circuit for receiving said first transfer
signal, also receiving a sense amplifier enable signal that enables
and disables said sense amplifiers, and generating a second
transfer signal that is active if and only if said first transfer
signal is active and said sense amplifier enable signal is
inactive, said second transfer signal being used to control said
first switching elements.
18. The memory device of claim 6, wherein said memory cells are
dynamic memory cells that are refreshed periodically by said sense
amplifiers, comprising a refresh control circuit coupled to said
tag circuit, for determining whether data for a row of memory cells
to be refreshed are stored in said cache, and if so, refreshing
said row of memory cells by transferring data from said cache to
said row of memory cells.
19. The memory device of claim 18, wherein said refresh control
circuit generates: a first set of refresh timing signals that turn
said first switching elements on and said second switching elements
off, thereby permitting data in said memory cells to be refreshed
by said sense amplifiers while data are transferred between said
column data lines and said data bus; and a second set of refresh
timing signals that turn on both said first switching elements and
said second switching elements, and cause data to be transferred
from said cache to said sense lines, amplified by said sense
amplifiers, then transferred via said bit lines to said memory
cells.
20. The memory device of claim 19, wherein said refresh control
circuit comprises: a refresh timing generator for generating said
first set of refresh timing signals and said second set of refresh
timing signals; a multiplexer coupled to said tag circuit, for
providing a refresh address to said tag circuit; a logic circuit
coupled to said tag circuit, for deciding whether said refresh
address matches a row address of data stored in said cache; and a
switch coupled to and controlled by said logic circuit, for
selecting one set of refresh timing signals generated by said
refresh timing generator.
21. The memory device of claim 6, comprising three-level driver
circuits for driving said word lines and said first switching
elements at a first active level to transfer data from said sense
lines to said memory cells, and at a second active level lower than
said first active level to transfer data from said memory cells via
said sense lines to said cache cells.
22. The memory device of claim 7, wherein: said cache cells are
disposed in a cache element area; and the second switching elements
coupled to each complementary pair of sense lines are disposed on
opposite sides of said cache element area.
23. The memory device of claim 22, wherein said data bus comprises
a complementary pair read data lines disposed on respective
opposite sides of said cache element area, and a complementary pair
of write data lines also disposed on respective opposite sides of
said cache element area.
24. The memory device of claim 7, wherein: said cache cells are
disposed in a cache element area; and both of the second switching
elements coupled to each complementary pair of sense lines are
disposed on one side of said cache element area.
25. The memory device of claim 24, wherein said data bus comprises
a complementary pair of read data lines both disposed on one side
of said cache element area, and a complementary pair of write data
lines both disposed on another side of said cache element area.
26. The memory device of claim 7, wherein: said cache cells are
disposed in a cache element area; said cache element area also has
dummy cells which are not coupled to said column data lines and are
not coupled to said sense lines; and in their combined arrangement,
said cache cells and said dummy cells match row and column pitches
of said memory cells in said memory cell array.
27. The memory device of claim 26, wherein: each complementary pair
of said column data lines is disposed between a complementary pair
of said sense lines; said cache cells are disposed between
complementary column data lines; and said dummy cells are disposed
between said column data lines and said sense lines.
28. A semiconductor memory device, comprising: a rectangular array
of memory cells disposed in intersecting rows and columns, for
storing data; a plurality of bit lines extending parallel to said
columns and coupled to the memory cells in respective columns, for
transporting data t o and from said memory cells; a plurality of
word lines extending parallel to said rows and coupled to the
memory cells in respective rows, for controlling transfer of data
between said memory cells and said bit lines; a row of first
switching elements coupled to respective bit lines; a plurality of
sense lines coupled to respective first switching elements, for
transferring data via said first switching elements to and
from-said bit lines; a row of sense amplifiers coupled to said
sense lines, for amplifying the data on said sense lines; a row of
cache cells coupled to said sense lines, for storing data of an
arbitrary row of said memory cells; and a tag circuit coupled to
said row of cache cells, for storing a row address indicating the
row of said memory cells having data stored in said cache cells,
and controlling transfer of data between said cache cells and said
sense lines responsive to said row address.
29. The memory device of claim 28, comprising: a plurality of rows
of cache cells as described in claim 31, for storing data of an
arbitrary plurality of rows of said memory cells.
30. The memory device of claim 29, wherein said tag circuit
comprises a plurality of tag memories for storing row addresses of
data stored in corresponding rows of cache cells.
31. The memory device of claim 29, wherein: said memory cell array
is divided into left and right banks; said sense lines are
switchably coupled to both said left and right banks; and any row
of cache cells in said plurality of rows of cache cells can store
data of either of said left and right banks.
32. The memory device of claim 28, wherein: said bit lines are
grouped into complementary pairs, with one complementary pair of
bit lines per column; and said sense lines are grouped into
corresponding complementary pairs.
33. The memory device of claim 32, wherein each of said cache cells
comprises: a second switching element coupled to one of said sense
lines and switched on and off by said tag circuit; and a storage
element coupled to said second switching element.
34. The memory device of claim 33, wherein: said second switching
element is a transistor; and said storage element is a capacitor
having one electrode coupled to said transistor and another
electrode coupled to a fixed potential.
35. The memory device of claim 32, wherein each of said cache cells
comprises: a second switching element coupled to one of said sense
lines and switched on and off by said tag circuit; a third
switching element coupled to another one of said sense lines and
switched on and off by said tag circuit; and a storage element
coupled in series between said second switching element and said
third switching element.
36. The memory device of claim 28, wherein: said memory cells are
dynamic memory cells that are refreshed periodically by said sense
amplifiers; and said tag circuit prevents transfer of data between
said cache cells and said sense lines while said sense amplifiers
are refreshing said memory cells.
37. The memory device of claim 32, wherein each of said sense
amplifiers is supplied with two different fixed potentials for use
in amplifying data on said complementary pairs of sense lines,
further comprising: sense-line equalizing circuits coupled to
respective complementary pairs of sense lines, for equalizing said
sense lines to a third potential intermediate between said two
different fixed potentials; a sense-amplifier equalizing circuit
coupled to said row of sense amplifiers, for supplying said sense
amplifiers with said third potential in place of said two different
fixed potentials; and a delay circuit coupled to said
sense-amplifier equalizing circuit, for delaying supply of said
third potential to said sense amplifiers until a certain time after
equalization of said sense lines has begun.
38. The memory device of claim 37, also comprising a hit/miss
detector coupled to said sense-amplifier equalizing circuit, for
preventing the supply of said third potential to said sense
amplifiers prior to transfer of data from said cache cells to said
sense lines.
39. The memory device of claim 38, wherein said hit/miss detector
comprises: a hit detector for detecting, from an output of said tag
circuit, access to said cache cells, and generating a first signal;
a miss detector for detecting, from an output of said tag circuit,
access to said memory cells, and generating a second signal that
activates said sense-amplifier equalizing circuit via said delay
circuit; and a logic gate for combining said first signal and said
second signal to produce a third signal, which activates said
sense-line equalizing circuits both when access to said cache cells
is detected and when access to said memory cells is detected.
40. In a semiconductor memory device having word lines, rows of
memory cells coupled to respective word lines, sense lines, sense
amplifiers coupled to said sense lines, cache cells coupled to said
sense lines, and a data bus, a method of executing a memory access
cycle, comprising the steps of: receiving a row address designating
a row of memory cells; determining if data of said row of memory
cells are currently present in said sense amplifiers and said cache
cells; transferring said data from said cache cells to said sense
lines, if said data are currently present in said cache cells but
not in said sense amplifiers, and amplifying the data thus
transferred; transferring said data from said memory cells to said
sense lines, if said data are currently present in neither said
cache cells nor said sense amplifiers, and amplifying the data thus
transferred; receiving at least one column address and transferring
data between corresponding sense lines and said data bus, thereby
completing said memory cycle; and leaving said sense amplifiers
enabled when said memory cycle ends, so that said sense amplifiers
continue to hold amplified data of the row of memory cells
designated by said row address.
41. The method of claim 40, comprising the further steps of:
activating a word line corresponding to said row address;
transferring amplified data from said sense lines to the memory
cells coupled to said word line; and leaving said word line active
when said memory cycle ends.
42. The method of claim 40, wherein the step of transferring data
from said memory cells to said sense lines comprises the further
steps of: disabling said sense amplifiers; interconnecting pairs of
said sense lines; waiting a certain time, thus allowing charge on
said sense lines to discharge into said sense amplifiers; then
supplying a fixed potential to said sense lines and said sense
amplifiers.
43. The method of claim 40, wherein the step of transferring data
from said cache cells to said sense lines comprises the further
steps of: disabling said sense amplifiers; interconnecting pairs of
said sense lines; waiting a certain time, thus allowing charge on
said sense lines to discharge into said sense amplifiers; then
supplying a fixed potential to said sense lines but not to said
sense amplifiers.
44. In a semiconductor memory device having word lines, rows of
memory cells coupled to respective word lines, sense lines, sense
amplifiers coupled to said sense lines, column data lines coupled
to said sense lines, cache cells coupled to said column data lines,
and a data bus, a method of refreshing said memory cells,
comprising the steps of: receiving a row address corresponding to
data stored in said cache cells; transferring said data from said
cache cells via said column data lines to said sense lines, and
amplifying the data thus transferred; disconnecting said column
data lines from said sense lines, thereby leaving amplified data on
said column data lines; activating a first word line; using said
sense amplifiers to refresh the memory cells coupled to said first
word line; and receiving at least one column address and
transferring data between corresponding column data lines and said
data bus, while said sense amplifiers are refreshing said memory
cells.
45. The method of claim 44, comprising the further steps of:
disabling said sense amplifiers after said memory cells have been
refreshed; coupling said column data lines to said sense lines
again; and enabling said sense amplifiers, thereby again amplifying
the data on said column data lines.
46. The method of claim 45, comprising the further steps of:
deactivating said first word line after said memory cells have been
refreshed; activating a second word line corresponding to said row
address; and transferring data from said column data lines via said
sense lines to the memory cells coupled to said second word
line.
47. In a semiconductor memory device having word lines, bit lines,
memory cells coupled to said word lines and bit lines, sense lines,
sense amplifiers coupled to said sense lines, write buffers coupled
to said sense lines, column data lines coupled to said sense lines,
cache cells coupled to said column data lines, and a data bus, a
method of executing a cache load cycle, comprising the steps of:
receiving a row address not corresponding to data stored in any of
said cache cells; coupling said bit lines to said sense lines;
activating a word line corresponding to said row address;
transferring data from memory cells coupled to said word line via
said bit lines to said sense lines; amplifying the data on said
sense lines; coupling said column data lines to said sense lines;
transferring the data on said sense lines via said column data
lines to said cache cells; and receiving at least one column
address and transferring data between said column data lines and
said data bus, thereby completing said memory cycle.
48. The method of claim 47, wherein said sense amplifiers are left
enabled and when said cache load cycle ends.
49. The method of claim 47, comprising the further steps of:
disconnecting said bit lines from said sense lines before
amplification of the data on said sense lines is completed;
deactivating said word line; and precharging said bit lines to a
fixed potential.
50. The method of claim 47, wherein the step of transferring data
from said memory cells to said sense lines comprises the further
steps of: disabling said sense amplifiers; interconnecting pairs of
said sense lines; waiting a certain time, thus allowing charge on
said sense lines to discharge into said sense amplifiers; then
supplying a fixed potential to said sense lines and said sense
amplifiers.
51. In a semiconductor memory device having word lines, bit lines,
memory cells coupled to said word lines and bit lines, sense lines,
sense amplifiers coupled to said sense lines, write buffers coupled
to said sense lines, column data lines coupled to said sense lines,
cache cells coupled to said column data lines, and a data bus, a
method of executing a cache hit cycle, comprising the steps of:
receiving a row address corresponding to data stored in said cache
cells: transferring data from said cache cells via said column data
lines to said sense lines; amplifying the data on said sense lines
and said column data lines; and receiving at least one column
address and transferring data between said column data lines and a
data bus, thereby completing said cache hit cycle.
52. The method of claim 51, wherein said sense amplifiers are left
enabled and when said cache hit cycle ends.
53. The method of claim 51, wherein said bit lines are left
disconnected from said sense lines throughout said cache hit cycle,
and data are not transferred to said memory cells.
54. The method of claim 51, wherein the step of transferring data
from said cache cells to said sense lines comprises the further
steps of: disabling said sense amplifiers; interconnecting pairs of
said sense lines; waiting a certain time, thus allowing charge on
said sense lines to discharge into said sense amplifiers; then
supplying a fixed potential to said sense lines but not to said
sense amplifiers.
55. In a semiconductor memory device having word lines, bit lines,
memory cells coupled to said word lines and bit lines, sense lines,
sense amplifiers coupled to said sense lines, write buffers coupled
to said sense lines, column data lines coupled to said sense lines,
at least one row of cache cells coupled to respective column data
lines, and a data bus, a method of executing a cache replace cycle,
comprising the steps of: receiving a row address not corresponding
to data not stored in any of said cache cells; selecting a row of
cache cells; transferring data from said row of cache cells to said
write buffers, with amplification by said sense amplifiers;
transferring data from said memory cells via said sense lines to
said column data lines and said row of cache cells, with
amplification by said sense amplifiers; disconnecting said column
data lines from said sense lines; receiving at least one column
address and transferring data between corresponding column data
lines and said data bus; and transferring data from said write
buffers to said memory cells, with amplification by said sense
amplifiers.
56. The method of claim 55, wherein transferring data between said
column data lines and said data bus and transferring data from said
write buffers to said memory cells are carried out
concurrently.
57. The method of claim 55, comprising the further steps of:
coupling said column data lines to said sense lines, after
transferring said data from said write buffers to said memory
cells; and amplifying the data on said column data lines again.
58. The method of claim 55, wherein said sense amplifiers are left
enabled when said cache replace cycle ends.
59. The method of claim 55, wherein the step of transferring data
from said memory cells comprises: coupling said bit lines to said
sense lines; activating a first word line corresponding to said row
address, thereby transferring said data from the memory cells
coupled to said first word line to said sense lines; and
disconnecting said bit lines from said sense lines when said data
have been transferred to said sense lines, before amplification of
said data is completed.
60. The method of claim 59, comprising the further steps of:
deactivating said first word line; precharging said bit lines to a
fixed potential; activating a second word line corresponding to the
data transferred from said row of cache cells to said write buffer;
deactivating said second word line after data have been transferred
from said write buffer to the memory cells coupled to said second
word line; and precharging said bit lines to said fixed potential
again.
61. In a semiconductor memory device having word lines, bit lines,
memory cells coupled to said word lines and bit lines, sense lines,
sense amplifiers coupled to said sense lines, column data lines
coupled to said sense lines, cache cells coupled to said column
data lines, and a data bus, a method of refreshing said memory
cells, comprising the steps of: obtaining a refresh address;
determining whether said refresh address corresponds to data stored
in said cache cells; transferring data from said cache cells via
said column data lines, said sense lines, and said bit lines to
said memory cells, with amplification by said sense amplifiers, if
said refresh address corresponds to data stored in said cache
cells; and disconnecting said column data lines from said sense
lines and using said sense amplifiers to refresh said memory cells,
if said refresh address does not correspond to data stored in said
cache cells.
62. The method of claim 61, wherein said cache cells are organized
into at least two rows of cache cells, and the step of transferring
data from said cache cells comprises: selecting a first row of
cache cells corresponding to said refresh address; transferring
data from said first row of cache cells to said column data lines;
transferring data from said column data lines to said sense lines;
amplifying the data on said sense lines; disconnecting said column
data lines from said sense lines; transferring data from said sense
lines to said bit lines; receiving a row address corresponding to a
second row of cache cells different from said first row of cache
cells; transferring data from said second row of cache cells to
said column data lines; and receiving at least one column address
and transferring data between corresponding column data lines and
said data bus.
63. The method of claim 62, wherein transferring data from said
second row of cache cells to said column data lines and
transferring data between said column data lines and said data bus
are carried out concurrently.
64. The method of claim 61, comprising the further steps of:
coupling said column data lines to said sense lines after said
memory cells have been refreshed; transferring data from said
column data lines to said sense lines; and amplifying the data on
said column data lines and said sense lines.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a semiconductor memory device such
as a dynamic random-access memory (DRAM), more particularly to a
semiconductor memory device with a built-in cache in which data can
be stored for quick recall.
[0002] A DRAM has, among other circuits, an array of memory cells
identified by row and column addresses, and a row of sense
amplifiers. In read or write access, a row address is input, the
memory cells in the addressed row are coupled to the sense
amplifiers, and their data are amplified. Next a column address is
input, and in read access for example, the amplified data for the
addressed column are output to a data bus. Further column addresses
may then be input to access the data of other columns in the same
row. When such access ends, in a conventional DRAM, the sense
amplifiers are disabled, and the data remain held only in the
memory cell array.
[0003] A problem in the conventional DRAM is that before each
access cycle, the sense amplifiers, and the bit lines that couple
them to the memory cells, must be precharged by equalizing them to
a certain potential. Furthermore, at the beginning of every cycle,
the sense amplifiers must amplify the data in an entire row of
memory cells. Substantial time is required for these operations,
particularly in a large-scale memory with long bit lines, which
have large intrinsic capacitances that must be charged and
discharged. This severely limits the access speed of the
device.
[0004] One possible solution to this problem is to retain data in
the sense amplifiers by leaving them enabled at the end of an
access cycle, thereby using the sense amplifiers as a cache. Then
if the same row address is input again, the addressed data are
immediately available from the sense amplifiers and do not have to
be read from the memory cells. This scheme is particularly suited
to a device in which the memory cell array is divided into multiple
banks, and multiple rows of sense amplifiers are provided,
permitting data from different banks to be cached in different
sense amplifier rows.
[0005] The utility of this method is restricted, however, by the
need to refresh data in DRAM memory cells periodically. Each time a
refresh is carried out, the data being held in the relevant sense
amplifiers are lost. The data are also lost if the device is placed
in standby and the sense amplifiers are powered down.
[0006] Another problem is that each row of sense amplifiers can
hold data for only a single row of memory cells. If access
alternates between two rows in the same bank, for example, the
above method confers no benefit.
SUMMARY OF THE INVENTION
[0007] It is accordingly an object of the present invention to
enable rapid access to data in a semiconductor memory device.
[0008] Another object of the invention is to avoid loss of rapidly
accessible data when a refresh occurs.
[0009] Yet another object is to enable refresh cycles and access
cycles to take place simultaneously.
[0010] Still another object is to reduce current dissipation when
data are transferred from memory cells to sense amplifiers.
[0011] The invented semiconductor memory device has a row-column
array of memory cells for storing data. Bit lines extending in the
column direction transport data to and from memory cells selected
by word lines extending in the row direction. The bit lines are
coupled via a row of first switching elements to corresponding
sense lines, to which sense amplifiers are coupled.
[0012] According to a first aspect of the invention, one or more
rows of cache cells are coupled to the sense lines. According to a
second aspect of the invention, column data lines are coupled via
second switching elements to the sense lines, and one or more rows
of cache cells are coupled to the column data lines.
[0013] In either aspect of the invention, each row of cache cells
can store data from an arbitrary row of memory cells. Data
transferred from the memory cells to the sense lines and amplified
by the sense amplifiers can be placed in the cache cells for quick
recall later. The cache cells can continue to store such data while
the sense amplifiers are refreshing the memory cells.
[0014] If the cache cells are coupled to column data lines, then
during refresh operations, the column data lines can be
disconnected from the sense lines, and access to data on the column
data lines can continue while the refresh is in progress.
[0015] A tag circuit is provided to keep track of the row addresses
of the memory cells having data stored in the cache cells, and
control transfers of data to and from the cache cells.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a block diagram of a novel DRAM with a cache.
[0017] FIG. 2 is a circuit diagram of a cache cell.
[0018] FIG. 3 is a circuit diagram of another cache cell.
[0019] FIG. 4 is a timing diagram illustrating the operation of the
DRAM in FIG. 1.
[0020] FIG. 5 is a block diagram of a DRAM with two cache rows per
row of sense amplifiers.
[0021] FIG. 6 is a timing diagram illustrating the operation of the
DRAM in FIG. 5.
[0022] FIG. 7 is a block diagram of a DRAM with four cache rows per
row of sense amplifiers.
[0023] FIG. 8 is a block diagram of a DRAM with a novel circuit for
equalizing the sense lines and sense amplifiers.
[0024] FIG. 9 is a timing diagram illustrating the operation of the
circuit in FIG. 8.
[0025] FIG. 10 is a block diagram of a DRAM with another novel
circuit for equalizing the sense lines and sense amplifiers.
[0026] FIG. 11 is a circuit diagram of the hit/miss detector in
FIG. 10.
[0027] FIG. 12 is a timing diagram illustrating the operation of
the circuit in FIG. 10.
[0028] FIG. 13 is another timing diagram illustrating the operation
of the circuit in FIG. 10.
[0029] FIG. 14 illustrates a DRAM having column switching circuits
with internal cache cells.
[0030] FIG. 15 is a timing diagram illustrating the operation of
the DRAM in FIG. 14.
[0031] FIG. 16 is another timing diagram illustrating the operation
of the DRAM in FIG. 14.
[0032] FIG. 17 illustrates a DRAM having write buffers, as well as
column switching circuits with internal cache cells.
[0033] FIG. 18 is a timing diagram illustrating the operation of
the DRAM in FIG. 17.
[0034] FIG. 19 is a timing diagram illustrating two types of
refresh operations in the DRAM of FIG. 17.
[0035] FIG. 20 illustrates a circuit for controlling the coupling
of bit lines to sense lines.
[0036] FIG. 21 illustrates a refresh control circuit.
[0037] FIG. 22 illustrates a three-level driver circuit.
[0038] FIG. 23 illustrates a preferred layout of the column
switching circuits in the DRAM of FIG. 14 or 17.
[0039] FIG. 24 illustrates another preferred layout of the column
switching circuits in the DRAM of FIG. 14 or 17.
[0040] FIG. 25 illustrates a DRAM with rows of sense amplifiers
disposed on both sides of a memory cell array.
[0041] FIG. 26 illustrates a preferred layout of the column
switching circuits in the DRAM of FIG. 25.
DETAILED DESCRIPTION OF THE INVENTION
[0042] Several embodiments of the invention will now be described
with reference to the attached illustrative drawings. The drawings
have been somewhat simplified, to avoid obscuring the invention
with irrelevant detail. Control signal lines will be designated by
the names of the control signals they carry. All control signals
are shown as active high, and the high (active) level is denoted by
Vcc. The low (inactive) level is denoted by Vss, or by the
conventional ground symbol. Some signals may be boosted to an
active level higher than Vcc, as will be explained later.
[0043] The terms row and column will be used frequently. In the
drawings, rows are vertical and columns are horizontal. In the
specification and claims, the term row will often be used to denote
a plurality of identical objects aligned in the row direction. For
example, a "row of cache cells" describes a row of possibly
thousands of cache cells extending in the vertical direction in the
drawings, even though only one cache cell in the row is actually
shown.
[0044] FIG. 1 is a block diagram of a DRAM provided with a novel
cache that shadows the data held in the sense amplifiers, so that
these data need not be lost in a refresh or standby.
[0045] The DRAM comprises a memory cell array 2 having memory cells
4 coupled to word lines WL1, WL2, . . . which run in the row
direction. They are intersected by pairs of complementary bit lines
BLi and {overscore (BLi)} running in the column direction. Each
memory cell 4 is coupled to one word line and one bit line. Each
pair of complementary bit lines forms one column in the array, BLi
and {overscore (BLi)} being the bit lines of the i-th column. The
bit lines transport data (electrical charges) to and from the
memory cells. The word lines control the transfer of data between
their coupled memory cells and the bit lines.
[0046] Each column has an equalizing circuit 6 coupled between its
pair of complementary bit lines BLi and {overscore (BL)}i. These
equalizing circuits are all controlled by a common signal line
EQB.
[0047] The memory cell array 2 has word lines, bit lines, and
memory cells other than the ones shown in FIG. 1, but they have
been omitted to simplify the drawing.
[0048] The bit lines BLi and {overscore (BL)}i are coupled through
a transfer circuit 8 to corresponding pairs of complementary sense
lines Si and {overscore (S)}i. The transfer circuit 8 comprises
switching elements such as N-channel metal-oxide-semiconductor
transistors (NMOS transistors) 9 controlled by a transfer gate
signal TG. Each NMOS transistor 9 is coupled in series between one
bit line and the corresponding sense line, and the gates of all
these NMOS transistors 9 are coupled in common to the TG control
signal line.
[0049] Coupled to the sense lines Si and {overscore (S)}i are a
cache 10 comprising a row of cache cells 12 for temporarily storing
data having an arbitrary row address, a sense amplifier row 14
comprising a row of sense amplifiers 16 and a row of sense-line
equalizing circuits 18, and a plurality of column switching
circuits 20. As illustrated, there is one cache cell 12, sense
amplifier 16, sense-line equalizing circuit 18, and column
switching circuit 20 per column in the memory cell array 2. The
column switching circuit 20 selectively 8 couples the sense lines
Si and {overscore (S)}i to a data bus 21 comprising complementary
read data lines RDB and {overscore (RDB)} for output of read data,
and complementary write data lines WDB and {overscore (WDB)} for
input of write data.
[0050] Each cache cell 12 is coupled to at least one of the two
sense lines Si and {overscore (S)}i in its column, and all the
cache cells 12 are coupled to a common control line that carries a
cache control signal SWc. Each sense amplifier 16 is coupled to
both sense lines Si and {overscore (S)}i in its column, and all the
sense amplifiers 16 are coupled to a common control line that
carries a sense amplifier enable signal SAE. Each equalizing
circuit 18 is coupled to both sense lines Si and {overscore (S)}i
in its column, and all the equalizing circuits are coupled to a
common control line EQS.
[0051] The cache 10, sense amplifier row 14, and column switching
circuits 20 need not be coupled to the sense lines Si and
{overscore (S)}i in the order shown in FIG. 1. Another possible
order will be illustrated in later embodiments. Furthermore,
although the sense lines Si and {overscore (S)}i are shown in the
drawings as single lines, it is possible for the sense amplifiers
16 to have nodes which are coupled by separate signal lines to the
transfer circuit 8, cache cells 12, and column switching circuit
20; the term "sense lines" then encompasses both these nodes and
the separate signal lines connected to them.
[0052] An address buffer 22 is provided for input of row and column
addresses. An address multiplexer 24 sends the row addresses to a
control circuit 26. The control circuit 26 has a tag circuit 28 for
storing the row address of the data currently held in the cache 10,
and a flag indicating whether the cache contents are valid. When
the cache contents are flagged as invalid, the cache is said to be
empty.
[0053] The tag circuit 28 activates a control signal SW when a
cache hit occurs, i.e. when the flag indicates that the cache
contents are valid and the address stored in the tag circuit 28
matches the current input row address. Control signal SW is used in
generating the SWc signal that controls the cache 10. The control
circuit 26 also has registers (not shown) for holding the current
and preceding row address inputs, and various logic and timing
circuits that generate control signals EQB, TG, and SAE.
[0054] The control circuit 26 furnishes row addresses as
appropriate to a row decoder 30, which activates drivers (not
shown) to drive the corresponding word lines in the memory cell
array 2. The address multiplexer 24 sends column addresses to a
column decoder 32, which controls the column switching circuits 20.
For the i-th column, the column decoder generates a read column
signal RCLi and a write column signal WCLi.
[0055] The SW signal output by the control circuit 26 becomes one
input of a two-input NAND gate 34. The other input to this NAND
gate 34 is a REF signal that is activated during refresh cycles.
The REF signal is generated by another control circuit, not shown
in the drawing, and is inverted by an inverter 35 before input to
the NAND gate 34. The output of the NAND gate 34 is inverted by
another inverter 36 to generate the cache control signal SWc.
[0056] The SAE signal output by the control circuit 26 is inverted
by an inverter 38 to generate the EQS signal that controls the
equalizing circuits 18 in the sense amplifier row 14. Additional
circuit elements (not shown) are used to provide timing delays
between EQS and SAE.
[0057] FIG. 2 shows one preferred configuration of the cache cells
12. The cache cell 12 in this drawing comprises a single switching
element such as an NMOS transistor 40, and a storage element such
as a capacitor 42. The transistor 40 is coupled in series between a
sense line Si and one electrode of the capacitor 42. The other
electrode of the capacitor 42 is coupled to an HVcc line (not shown
in FIG. 1) which is held at a fixed potential intermediate between
the two fixed potential levels Vcc and Vss. (Typically, HVcc is
halfway between Vcc and Vss.) The cache cell configuration in FIG.
2 is similar to the configuration of the memory cells 4 in the
memory cell array 2. In the present embodiment, however, the
transistor 40 and capacitor 42 preferably have larger dimensions
than the corresponding transistors and capacitors of the memory
cells 4, so that the cache can be accessed quickly and does not
require frequent refreshing.
[0058] FIG. 3 shows another preferred cache-cell configuration,
having two NMOS transistors 40 and 44 coupled in series between
complementary sense lines Si and {overscore (S)}i. The gates of
both transistors 40 and 44 are driven by the cache control line
SWc. The capacitor 42 is coupled in series between the two
transistors 40 and 44. This cache cell takes up more space, but has
the advantage of faster readout than the cache cell in FIG. 2,
because it creates a greater potential swing between the two sense
lines Si and {overscore (S)}i.
[0059] Detailed descriptions of the other circuits in FIG. 1 will
be omitted at this point, as these circuits will be familiar to
those skilled in the semiconductor memory art, but the functions of
their control signals will be briefly explained.
[0060] When EQB is active, the equalizing circuit 6 equalizes the
bit lines BLi and {overscore (BL)}i to the intermediate potential
HVcc.
[0061] When TG is active, the transfer circuit 8 couples the bit
lines BLi and {overscore (BL)}i to the sense lines Si and
{overscore (S)}i.
[0062] When SWc is active, the storage elements in the cache cells
12 are coupled to the sense lines Si and {overscore (S)}i,
permitting data transfer. This occurs when SW is active and REF is
inactive.
[0063] When SAE is inactive and EQS is active, the sense amplifiers
are disabled, meaning that their Vcc and Vss power supplies (not
shown) are switched off, and the sense lines Si and {overscore
(S)}i are equalized to HVcc. When SAE is active and EQS is
inactive, the sense amplifiers 16 are enabled, meaning that they
are supplied with Vcc and Vss, and if a potential difference exists
on the sense lines Si and {overscore (S)}i, they amplify it by
pulling the higher sense line up to Vcc and the lower sense line
down to Vss.
[0064] When RCLi is active, the column switching circuit couples
sense lines Si and {overscore (S)}i to the read data lines RDB and
{overscore (RDB)}. The coupling may be effected through a simple
and well-known transistor amplifier circuit, which will be
illustrated later. When WCLi is active, sense lines Si and
{overscore (S)}i are coupled to the write data lines WDB and
{overscore (WDB)}.
[0065] FIG. 4 is a timing diagram of the DRAM in FIG. 1, showing
several types of memory access cycles. For simplicity, it will be
assumed that all accesses are read accesses.
[0066] At the extreme left of FIG. 4, the DRAM is in the standby
state: a state in which the sense amplifiers are disabled because
no access is occurring. All word lines are inactive, control
signals TG, SW, SWc, and SAE are inactive, and EQB and EQS are
active. The bit lines BLi and {overscore (BL)}i and sense lines Si
and {overscore (S)}i are equalized to the intermediate potential
HVcc. The cache 10 is disconnected from the sense lines, but holds
the data of, for example, row RA.sub.1. The tag circuit 28 holds
the matching row address RA.sub.0.
[0067] The first cycle illustrated in FIG. 4 is a miss cycle in
which new data are loaded into the sense amplifiers, while the
cache 10 continues to hold the data of row RA.sub.1.
[0068] When row address RA.sub.0 is input, the control circuit 26
compares it with the address RA.sub.1 stored in the tag circuit 28
and finds that the two do not match. Since no data are currently
held in the sense amplifiers, the control circuit 26 outputs row
address RA.sub.0 to the address decoder 30, which drives the
corresponding word line WL0 to the active state. The data
(electrical charges) in the memory cells 4 in row RA.sub.1 are
thereby transferred onto the bit lines, causing a slight potential
divergence on each pair of complementary bit lines BLi and
{overscore (BL)}i, as shown in the drawing.
[0069] Next, the control circuit 26 activates control line TG,
turning on the transistors 9 in the transfer circuit 8, and
activates control line SAE, enabling the sense amplifiers 16. The
pairs of bit lines BLi and {overscore (BL)}i are accordingly
coupled to the sense lines Si and {overscore (S)}i and the
potential differences on them are amplified. The data in one row of
memory cells have now been read onto the sense lines Si and
{overscore (S)}i and amplified.
[0070] A series of column addresses CA.sub.00, CA.sub.01, and
CA.sub.02 is now input and decoded by the column decoder 32,
causing it to generate three read column signals RCL. Although all
three RCL signals are shown for convenience on a single line in
FIG. 4, this represents three read column signals RCLi with
possibly different values of "i," designating different columns.
Each read column signal couples one pair of sense lines Si and
{overscore (S)}i to the read data lines RDB and {overscore (RDB)}
and transfers the corresponding data D.sub.00, D.sub.01, and
D.sub.02 onto these data lines.
[0071] Control line SW is left inactive throughout this load cycle,
retaining the data of row RA.sub.1 in the cache 10. At the end of
the cycle, word line WL0 and control lines TG and SAE are left
active, and the sense amplifiers 16 remain enabled.
[0072] Next, the same row address RA.sub.0 is input again, and a
sense-amplifier hit cycle commences. The control circuit 26
recognizes that the input row address matches the row address of
the data present in the sense amplifiers 16, so it leaves word line
WL0 and control lines TG and SAE active. Column addresses
CA.sub.03, CA.sub.04, and CA.sub.05 are input, and data D.sub.03,
D.sub.04, and Do.sub.5 from three more columns are output onto the
read data lines RDB and {overscore (RDB)}. These data can be
accessed very rapidly because they are already amplified and
present on the sense lines Si and {overscore (Si)}.
[0073] The cache control signal SWc remains inactive throughout
this cycle as well, and the cache 10 continues to hold the data of
row RA.sub.1.
[0074] In the next cycle, row address RA.sub.1 is input and a cache
hit cycle takes place.
[0075] Recognizing that row address RA.sub.1 differs from the row
address RA.sub.0 of the data in the sense amplifiers 16, the
control circuit 26 deactivates word line WL0 and control lines TG
and SAE. When SAE goes low, EQS goes high and the sense lines Si
and {overscore (S)}i are equalized. This does not affect the bit
lines, because the transfer circuit 8 is switched off and the bit
lines are disconnected from the sense lines.
[0076] In the memory cell array 2, when WL0 has gone low, the
storage elements in the memory cells in row RA.sub.0 are
disconnected from the bit lines, but they continue to store the
data that were on the bit lines. Control line EQB is now driven
active and the bit lines are equalized to HVcc, as shown in the
drawing.
[0077] The term "precharge" will be used hereinafter to denote the
two operations of deactivating the currently-active word line and
then equalizing the bit lines to HVcc. The length of interval 46 is
the precharge time of the memory cell array 2.
[0078] After the sense lines Si and {overscore (S)}i have been
equalized, the sense amplifiers 16 are again enabled and control
line SW is activated. Since REF is inactive, this activates control
line SWc, permitting transfer of charge from the cache cells 12 to
the sense lines Si and {overscore (S)}i. The data in the cache 10
are thus recalled onto the sense lines Si and {overscore (S)}i and
amplified. This recall of the cached data can be completed more
quickly than the reading of data from the memory cell array 2,
because the sense lines Si and {overscore (S)}i are shorter than
the bit lines BLi and {overscore (BL)}i and have less capacitance,
and because the potential swing created on the sense lines by the
cache cells 12 is greater than the potential swing created on the
bit lines and sense lines by the memory cells 4. The sense
amplifiers 16 can accordingly amplify data stored in the cache 10
faster than they can amplify data stored in the memory cell array
2.
[0079] Moreover, recall of the cached data can begin while the
memory cell array 2 is still being precharged, as shown. This also
speeds up the response to a cache hit.
[0080] As soon as the cached data have been recalled onto the sense
lines, they are available for output in response to column
addresses CA.sub.10, CA.sub.11, and CA.sub.12. At a certain point
during the output, control line TG is reactivated and word line WL1
is driven, coupling the sense lines via the bit lines to the memory
cells 4 in row RA.sub.1. One reason for this is to refresh the data
in row RA.sub.1 of the memory cell array 2. Another reason is to
update the memory cell contents in any columns in which a write
access occurs. As a result, when the cache hit cycle ends, valid
data for the accessed row are not only present in the sense
amplifiers 16 and cache 10, but have also been copied back to the
memory cell array 2, maintaining data consistency between the
memory cell array 2 and cache 10.
[0081] At the end of this cycle the sense amplifiers 16 are left
enabled, word line WL1 is left active, and control lines TG and SWc
are left active, in case the same row address RA.sub.1 is input
again in the next cycle.
[0082] In the next cycle illustrated, however, the row address
RA.sub.2 matches neither the address (RA.sub.1) of the data in the
sense amplifiers 16 nor the address (also RA.sub.1) of the data in
the cache 10, so a replace cycle begins.
[0083] At the beginning of this cycle, control lines TG and SAE are
deactivated, disconnecting the memory cell array 2 from the sense
lines Si and {overscore (S)}i and disabling the sense amplifiers
16. The sense lines Si and {overscore (S)}i and connected cache
cells 12 are equalized to HVcc, and the memory cell array 2 is
precharged, storing the data of the previous cycle in the memory
cells 4 of row RA.sub.1.
[0084] At the end of the precharge interval 48, the control circuit
26 outputs row address RA.sub.2 to the row decoder 30, which drives
word line WL2, transferring data from the memory cells in row
RA.sub.2 onto the bit lines BLi and {overscore (BL)}i. Then control
signals TG and SAE are activated to read these data onto the sense
lines Si and {overscore (S)}i and amplify them. The amplified data
become available for output in response to column addresses
CA.sub.20, CA.sub.21, and CA.sub.22. Control line SW is also
activated, so the amplified data are stored in the cache 10. That
is, the cache contents are replaced, so that instead of holding the
data for row RA.sub.1, the cache 10 now holds the data for row
RA.sub.2. The reason for making this replacement is so that in a
refresh cycle or standby, the cache 10 will retain the most
recently accessed data.
[0085] The next cycle is a refresh cycle, to refresh the data in
row RAr0. Control line TG is deactivated and the memory cell array
2 is precharged. Control line SAE is deactivated and EQS is
activated, disabling the sense amplifiers 16 and equalizing the
sense lines Si and {overscore (S)}i. Then word line WLr0 and
control lines TG and SAE are activated, and the sense amplifiers
amplify and refresh the data in row RA.sub.r0.
[0086] When the REF signal goes high at the beginning of the
refresh cycle, it deactivates control signal SWc. This signal
remains inactive for the entire duration of the refresh cycle. At
the end of the refresh cycle, accordingly, although the data for
row RA.sub.2 have been lost from the sense amplifiers, they are
retained in the cache 10. If the next access is again to row
RA.sub.2, the data can be recalled quickly from the cache 10
instead of having to be read from the memory cell array 2.
[0087] Thus when the same row is accessed repeatedly, even if the
accesses are interrupted by standby intervals in which the sense
amplifiers are disabled and the sense lines Si and {overscore (S)}i
are equalized, or refresh cycles in which the sense amplifiers
acquire the data of a different row, after each interruption the
required data can be quickly recalled from the cache 10.
[0088] To retain data in the standby state, the control circuit 26
refreshes the cache 10 by periodically activating the SAE and SW
control lines, and refreshes the memory cell array 2 by
periodically activating the word lines, SAE, and TG, in response to
refresh addresses generated by a counter (not shown).
Alternatively, instead of refreshing the cache 10, the control
circuit 26 may abandon the cache contents after a certain time, by
resetting a flag in the tag circuit 28 to indicate that the cache
contents are no longer valid.
[0089] FIG. 5 is a block diagram showing a DRAM with a larger
cache, capable of storing two rows of data. Elements identical to
elements in FIG. 1 have the same reference numerals, and
descriptions will be omitted.
[0090] The cache 10 in this DRAM has two rows of cache cells, so
there are two cache cells 12.sub.1 and 12.sub.2 per column. The tag
circuit 28 in the control circuit 26 accordingly has two tag
memories 29.sub.1 and 29.sub.2, which store the row addresses of
data held in cache cells 12.sub.1 and 12.sub.2, respectively, and
output corresponding control signals SW1 and SW2. These are fed
through respective NAND gates 34.sub.1 and 34.sub.2 and inverters
36.sub.1 and 36.sub.2 to generate two cache control signals SWc1
and SWc2. Besides storing row addresses, the tag memories 29.sub.1
and 29.sub.2 store flag information indicating whether the
corresponding cache rows are occupied or empty.
[0091] The cache cells such as cell 12.sub.1 coupled to control
line SWc1 will be referred to as the first row of cache cells, or
the first cache row; the cache cells such as cell 12.sub.2 coupled
to control line SWc2 as the second row of cache cells, or the
second cache row.
[0092] The cache cells in FIG. 5 are of the type shown in FIG. 2,
cache cell 12.sub.1 being coupled to sense line Si and cache cell
12.sub.2 to sense line {overscore (S)}i. This arrangement saves
space, but other arrangements are possible. For example, both cache
cells 12.sub.1 and 12.sub.2 could be of the type shown in FIG.
3.
[0093] The operation of this DRAM will be described with reference
to the timing diagram in FIG. 6.
[0094] FIG. 6, like FIG. 4, starts from a standby state in which
the data of row RA.sub.1 are cached. The cached data are held in
the first cache row, controlled by signal SWc1. The second cache
row is empty.
[0095] The sense amplifier load cycle and sense amplifier hit cycle
in FIG. 6 are the same as in FIG. 4, except that control line SWc2
is activated, so the data loaded from the memory cell array 2 into
the sense amplifiers 16 are also loaded into the second row of
cache cells 122.
[0096] The cache hit cycle that follows is also identical to the
one in FIG. 4, except th at control line SWc2 is deactivated at the
beginning of the cycle, so that while the data with row address
RA.sub.1 are being recalled from the first cache row, the data with
row address RA.sub.0 remain held in the second cache row.
[0097] Accordingly, when access reverts from row RA.sub.1 to row
RA.sub.0 in the next cycle, another cache hit occurs. The data of
row RA.sub.0 are now recalled quickly from the second cache row,
instead of having to be read from the memory cell array 2. This is
done by deactivating control line SWc1, and activating control line
SWc2 again. In both cache hit cycles, the relevant word line (WL1
in the first cache hit cycle, WL0 in the second) is activated to
refresh the data in the memory cell array 2 and store any updates
that might occur due to write access, thereby maintaining data
consistency between the memory cell array 2 and cache 10.
[0098] In a replace cycle, when the row address RA.sub.2 does not
match the row of data held in the sense amplifiers 16 or in either
cache row, the control circuit 26 selects the cache row holding the
least recently accessed data, and replaces the data in that row. In
the present case, the data in the first cache row are replaced by
activating control line SWc1.
[0099] In a refresh cycle, the REF signal deactivates both cache
control lines SWc1 and SWc2, and the existing contents of both
cache rows are retained. Access to both rows of data held in the
cache 10 can accordingly resume quickly after the refresh.
[0100] This DRAM is particularly effective when access alternates
between two rows, because it allows the data of both rows to be
cached for quick recall.
[0101] FIG. 7 shows a DRAM in which the memory cell array is
divided into two banks, a left bank 2L and a right bank 2R. A
single cache 10 and sense amplifier row 14 are disposed between and
shared by the two banks 2L and 2R. The cache 10 now has four cache
cells 12.sub.1, 12.sub.2, 12.sub.3, and 12.sub.4 per column; that
is, there are four rows of cache cells. The two banks 2L and 2R
share the same column switching circuits 20, and have the same
column addresses. The sense lines Si and {overscore (S)}i are
coupled to banks 2L and 2R by respective transfer circuits 8L and
8R, which are controlled by respective control lines TGL and
TGR.
[0102] The two banks 2L and 2R have separate row addresses,
separate row decoders 30L and 30R, and separate bank control
circuits 27L and 27R. These bank control circuits are similar to
the control circuit 26 in FIGS. 1 and 5, except that the tag
circuit 28 is now external to them. This enables the same tag
circuit 28 to be shared by both bank control circuits 27L and
27R.
[0103] The tag circuit 28 has four tag memories 29.sub.1, 29.sub.2,
29.sub.3, and 29.sub.4, storing the row addresses of data held in
the four cache rows. The four tag memories 29.sub.1, 29.sub.2,
29.sub.3, and 29.sub.4 generate respective control signals SWc1,
SWc2, SWc3, and SWc4 that control the cache rows. Although not
explicitly shown, the tag circuit 28 also contains logic gates
similar to the inverters and NAND gates in FIGS. 1 and 5 for
deactivating control signals SWc1, SWc2, SWc3, and SWc4 during
refresh cycles, and other circuits for adjusting the timing of
control signals SWc1, SWc2, SWc3, and SWc4. The tag circuit 28
receives address input directly from the address multiplexer
24.
[0104] Bidirectional signal lines XL and XR enable the bank control
circuits 27L and 27R to learn when a cache hit occurs, and direct
operations such as cache replacement. The bank control circuits 27L
and 27R generate the control signals TGL and TGR, as well as
control signals EQBL and EQBR to equalize the bit lines in their
respective memory banks 2L and 2R, and sense amplifier enable
signals SAEL and SAER. A switch 49 controlled by the address
multiplexer 24 selects either SAEL or SAER to generate the signals
SAE and EQS that control the sense amplifier row 14.
[0105] Elements in FIG. 7 not mentioned above are similar to the
elements in FIGS. 1 and 5 with the same reference numerals. Omitted
to simplify the drawing is a sense-amplifier tag circuit that
stores the row address of the data currently held in the sense
amplifiers 16.
[0106] When a row address is input, the address multiplexer
determines which bank 2L or 2R the address lies in, sends the
address to the appropriate bank control circuit 27L or 27R, and
sets switch 49 to the corresponding position. The row address is
also sent to the tag circuit 28, which responds by reporting hit or
miss to the bank control circuits. Subsequent operational details
are similar to those of the DRAM in FIG. 5, so a timing diagram
will be omitted.
[0107] The cache 10 in FIG. 7 can be employed in various ways. In
one preferred scheme, any of the cache cells 12.sub.1, 12.sub.2,
12.sub.3, and 12.sub.4 can hold data from either memory bank 2L or
2R. For example, all four cache rows can hold data from the same
bank, or three cache rows can hold data from one bank while the
fourth row holds data from the other bank, or two cache rows can be
used for each bank. This full associativity of the cache 10 results
in a high cache hit rate and improves the average data access
speed.
[0108] In another possible scheme, two of the cache rows, e.g. the
two left rows containing cache cells 12.sub.1 and 12.sub.2, are
reserved for storing data from the left bank 2L. The other two
cache rows are reserved for storing data from the right bank.
[0109] The concept shown in FIG. 7 can be extended by providing
additional memory banks and sense amplifier rows, disposed in an
alternating arrangement. Each memory bank can then be served by two
adjacent rows of sense amplifiers, so that each sense amplifier row
needs sense amplifiers for only one half of the columns. Each sense
amplifier row has its own cache.
[0110] As mentioned above, one of the advantages of the invented
caching scheme is that when a cache hit occurs, cache access can
start before the precharging of the memory cell array is completed.
Nor is it necessary to wait for existing data to be copied back
from the sense amplifiers to the memory cells, because this
copy-back is complete at the end of each access cycle, as
illustrated in FIGS. 4 and 6. However, cache access still has to
wait for the equalization of the sense lines.
[0111] FIG. 8 shows a scheme for speeding up the equalization of
the sense lines. Elements similar to elements in FIGS. 1 to 7 have
the same reference numerals. The sense lines Si and {overscore
(S)}i are shown coupled to a single memory cell array 2, but this
array 2 may be divided into right and left banks as in FIG. 7. The
cache 10 has n cache cells 12.sub.1, . . . , 12.sub.n per column,
where n may be any positive integer, from one on up. The cache
cells shown are of the type in FIG. 2, but this is not a
restriction; the type in FIG. 3 or other types may be used instead.
The tag circuit 28 has n tag memories 29.sub.1, . . . , 29.sub.n,
one for each row of cache cells.
[0112] FIG. 8 shows a sense-amplifier equalizing circuit 50 that
was omitted to simplify FIGS. 1, 5, and 7. The sense-amplifier
equalizing circuit 50 comprises three NMOS transistors 51, 52, and
53, the gates of which are all driven by the same control signal
line EQSA. NMOS transistor 53 is connected across two
sense-amplifier drive lines P1 and N1, its source coupled to N1 and
its drain to P1. NMOS transistors 51 and 52 are coupled between
these two sense-amplifier drive lines P1 and N1, respectively, and
a source of the intermediate potential HVcc.
[0113] FIG. 8 also shows the detailed structure of the equalizing
circuit 18 that equalizes the sense lines Si and {overscore (S)}i.
This sense-line equalizing circuit 18 comprises NMOS transistors 55
and 56 coupled in series between the sense lines Si and {overscore
(S)}i, the gates of both transistors 55 and 56 being controlled by
control signal EQS. The intermediate potential HVcc is supplied
through a switch 57 to a node between the two NMOS transistors 55
and 56. Switch 57 is controlled by control signal EQSA.
[0114] FIG. 8 furthermore shows the detailed structure of the sense
amplifier 16. A sense amplifier is configured as a differential
amplifier comprising P-channel metal-oxide-semiconductor (PMOS)
transistors 61 and 62 and NMOS transistors 63 and 64 cross-coupled
between the sense lines Si and {overscore (S)}i. The sources of
PMOS transistors 61 and 62 are coupled to drive line P1, which
receives the power-supply potential Vcc through a switch 65. The
sources of NMOS transistors 63 and 64 are coupled to drive line N1,
which receives the ground potential through a switch 66. Switches
65 and 66 are controlled by the sense-amplifier enable signal SAE.
The drive lines PI and N1 are also coupled through capacitors 67
and 68 to fixed potentials such as the ground potential, to
stabilize the sense-amplifier driving potentials.
[0115] The sense lines Si and {overscore (S)}i have intrinsic
capacitances, which are indicated in FIG. 8 by capacitor symbols
Casi and Cbsi. These symbols do not denote separate capacitors
connected to the sense lines. Rather, they denote the so-called
stray capacitance of the sense lines themselves.
[0116] Control signal EQSA is generated from control signal EQS by
a delay circuit 70 that inputs EQS and outputs EQSA. This delay
circuit is preferably configured so that the rise of EQSA is
delayed from the rise of EQS, but EQSA and EQS fall together. Delay
circuits of this type are well known: one possible configuration
comprises an even number of inverters coupled in series with an AND
gate.
[0117] The sense-amplifier drive lines P1 and N1 extend in the row
direction and are coupled to all sense amplifiers 16 in the sense
amplifier row 14. Similarly, switch 57 controls the supply of the
HVcc potential to all equalizing circuits 18 in the sense amplifier
row 14.
[0118] FIG. 9 is a timing diagram illustrating the equalization of
the sense lines in two cache hit cycles. On the address bus ADD at
the top of FIG. 9, X.sub.1 and X.sub.2 are row addresses matching
the addresses in tag circuits 29.sub.1 and 29.sub.2, indicating
that the data addressed by X.sub.1 and X.sub.2 are held in the
first two rows in the cache 10. Row address X.sub.0 is a row
address for which data were not held in the cache 10, resulting in
access to the memory cell array 2. Column addresses are omitted,
since they are not relevant to the equalization process.
[0119] Access to row X.sub.0 results in word line WLm being
activated, and the data from its coupled memory cells being read
from the memory cell array 2 onto the sense lines Si and {overscore
(S)}i and amplified. Signal line SAE is active to enable the sense
amplifiers 16, while EQS and EQSA are inactive. The Vcc and Vss
(ground) potentials are accordingly supplied to the sense-amplifier
drive lines P1 and N1, and the sense lines Si and {overscore (S)}i
are not equalized.
[0120] When row address X.sub.1 is input and a cache hit occurs,
control signals TG and SAE are deactivated, placing the transfer
circuit 8 in the off state, and turning off the switches 65 and 66
in the sense amplifiers. The sense-amplifier drive lines P1 and N1
are thereby disconnected from the power supply and ground, but they
remain for the time being at their existing Vcc and Vss potentials.
Word line WLm is also deactivated and the memory cell array 2 is
precharged.
[0121] Shortly after control line SAE becomes inactive, control
line EQS becomes active, turning on transistors 55 and 56 in the
equalizing circuit 18. The opposite charges stored in the stray
capacitances Casi and Cbsi of the sense lines Si and {overscore
(S)}i now begin to discharge toward each other through these
transistors 55 and 56. This event is indicated by arrow 71 in FIG.
9. For example, if sense line Si is high while {overscore (S)}i is
low, then charge is transported through transistors 55 and 56 from
sense line Si to sense line {overscore (S)}i, causing the potential
of sense line {overscore (S)}i to rise and the potential of sense
line Si to fall.
[0122] In the sense amplifier 16, because Si was high and
{overscore (S)}i low, transistors 61 and 64 were turned on, while
transistors 62 and 63 were turned off. When the potential of sense
line Si falls below Vcc-Vtp (where Vtp is the PMOS threshold
value), PMOS transistor 62 turns on, allowing charge to flow from
sense-amplifier drive line P1, which is still at Vcc, onto sense
line {overscore (S)}i, thereby accelerating the equalization of
sense line {overscore (S)}i. This is indicated by arrow 72 in FIG.
9. Likewise, when the potential of sense line {overscore (S)}i
rises above the NMOS transistor threshold value V.sub.tn,
transistor 63 turns on, and charge can flow from sense line Si to
the ground-level sense-amplifier drive line N1, as indicated by
arrow 73. Thus the charges remaining on sense-amplifier drive lines
P1 and N1 are used to speed up the equalization of sense lines Si
and {overscore (S)}i.
[0123] After a delay due to the delay circuit 70, control line EQSA
becomes active, turning on transistors 51, 52, and 53 in the
sense-amplifier equalizing circuit 50, and supplying HVcc to the
sense-amplifier drive lines P1 and N1. EQSA also turns on switch 57
and supplies the intermediate potential HVcc to the sense-line
equalizing circuits 18. The sense-amplifier drive lines P1 and N1
and sense lines Si and {overscore (S)}i are thereby all equalized
to HVcc.
[0124] As explained above, delaying the activation of control
signal EQSA accelerates the equalization of the sense lines Si and
{overscore (S)}i by providing additional charge and discharge paths
72 and 73 through the sense amplifier 16. These paths would not
exist if the sense-amplifier drive lines P1 and N1 were equalized
immediately to HVcc. In the case described above, transistors 62
and 63 would not turn on; the gate potential of NMOS transistor 63,
for example, would be lower than its HVcc source potential.
[0125] After equalization is completed, signals EQS and EQSA are
deactivated simultaneously, control signal SWc1 is activated, and
the data in the first cache row are read onto the sense lines Si
and {overscore (S)}i. Control line SAE is activated, the
sense-amplifier drive lines P1 and N1 are powered, and the cached
data are amplified. Column addresses (not indicated in FIG. 9) can
then be input to access the data.
[0126] The next access cycle is a cache hit by row address X.sub.2.
Control signal SWc1 is deactivated, and the pair of sense lines Si
and {overscore (S)}i are equalized in the same rapid manner as
before. This is followed by activation of control signal SWc2 and
access to the data in the second cache row.
[0127] The quick equalization of sense lines Si and {overscore
(S)}i due to the delay of EQSA from EQS is beneficial in cache miss
cycles as well as cache hit cycles, but it is particularly valuable
in permitting rapid access to the cache 10.
[0128] FIG. 10 shows a scheme for further speeding up access to the
cache 10, and reducing current dissipation. Elements which are the
same as elements in FIG. 8 have the same reference numerals.
[0129] In FIG. 10 the sense-amplifier equalizing circuit 50 and
switch 57 are activated by separate control signals EQSA and EQSL,
which have respective delay circuits 70 and 74. The inputs EQS2 and
EQS3 to these delay circuits 70 and 74 are generated by a hit/miss
detector 75, the function of which is to prevent activation of the
sense-amplifier equalizing circuit 50 in a cache-hit cycle.
[0130] FIG. 11 shows an example of the internal structure of the
hit/miss detector 75. This hit/miss detector 75 comprises a hit
detector 76 that outputs an internal signal EQS1, a miss detector
77 that outputs control signal EQS2, and a logic circuit, more
specifically an OR gate 78, that takes the logical OR of EQS1 and
EQS2 to generate control signal EQS3.
[0131] The hit detector 76 receives, for example, a hit/miss flag
signal from the tag circuit 28, and activates signal EQS1 when a
cache hit occurs. The miss detector 77 also receives the hit/miss
flag signal, together with a REF signal indicating a refresh cycle,
and a STBY signal indicating standby. Control signal EQS2 is
activated when a cache miss occurs, or a refresh cycle occurs, or
the standby state is entered.
[0132] The hit detector 76 and miss detector 77 also receive
signals indicating occurrence of a sense-amplifier hit, in which
case neither EQS1 nor EQS2 is activated, because the data are
already present in the sense amplifiers 16.
[0133] FIG. 12 is a timing diagram illustrating equalization of the
sense lines in two cache hit cycles by the circuits shown in FIGS.
10 and 11. Row addresses X.sub.0, X.sub.1, and X.sub.2 have the
same meaning as in FIG. 9.
[0134] Equalization of the sense lines Si and {overscore (S)}i
proceeds as in FIG. 9, speeded by the transfer of charge between
the sense lines Si and {overscore (S)}i and the sense-amplifier
drive lines P1 and N1. After a delay due to delay circuit 74,
control line EQSL is activated and the sense lines Si and
{overscore (S)}i are brought to HVcc. In a hit cycle, however,
control line EQSA remains inactive, so HVcc is not supplied to the
sense-amplifier drive lines P1 and N1.
[0135] Assuming again that sense line Si was high and {overscore
(S)}i low, when the potential on sense-amplifier drive line P1
falls to HVcc+V.sub.tp, the potential on sense line Si, which is at
least HVcc, can no longer turn on PMOS transistor 62, so no more
charge flows from sense-amplifier drive line P1 to sense line
{overscore (S)}i. Similarly, when the potential on sense-amplifier
drive line N1 reaches HVcc-Vtn, NMOS transistor 63 turns off, and
the flow of charge from sense line Si to sense-amplifier drive line
N1 ceases. Sense-amplifier drive line P1 therefore remains at
HVcc+V.sub.tp, while sense-amplifier drive line N1 remains at
HVcc-Vtn.
[0136] When equalization of the sense lines Si and {overscore (S)}i
has ended, sense-amplifier drive lines P1 and N1 remain unequalized
at HVcc+V.sub.tp and HVcc-V.sub.tn, and control line SWc1 is
activated by the tag circuit 28. Since the sense lines are short
and their intrinsic capacitances Casi and Cbsi are comparatively
small, the charge in cache cell 12.sub.1 produces a fairly large
potential swing on sense line Si. Accordingly, when control line
SAE is activated to supply power to the sense amplifier 16, even
though the sense amplifier drive lines P1 and N1 were not fully
equalized, the potential difference between sense lines Si and
{overscore (S)}i is large enough to be amplified correctly.
[0137] In the following cache hit cycle the same process is
repeated to access the data in cache cell 12.sub.2. The
sense-amplifier drive lines P1 and N1 are again left
unequalized.
[0138] When control line SAE is activated to amplify the cached
data (arrow 79), control line TG must be inactive so that the sense
amplifiers 16 do not have to contend with the large stray
capacitance of the bit lines BLi and {overscore (BL)}i. After
amplification is completed, although not shown in FIG. 12, control
line TG can be activated to copy data back from the sense lines Si
and {overscore (S)}i to the memory cells 4.
[0139] Omitting the equalization of the sense-amplifier drive lines
P1 and N1 makes it possible to advance very quickly to the access
of cached data. In addition, not equalizing the sense-amplifier
drive lines P1 and N1 reduces current dissipation in a cache
hit.
[0140] FIG. 13 illustrates the equalization operations performed by
the circuits in FIGS. 10 and 11 in a cache miss cycle.
[0141] After a cache hit on, for example, the third cache row
caused by row address X.sub.3, the next row address X.sub.4 does
not match the address held in any tag memory, so the memory cells 4
must be accessed. Control line SWc3 is deactivated by the tag
circuit 28, control line SAE is deactivated, switches 65 and 66 in
sense amplifier 16 are placed in the off state, and sense-amplifier
drive lines P1 and N1 are disconnected from the power supply and
ground. The hit/miss detector 75 activates control lines EQS2 and
EQS3, and after a delay inserted by the delay circuit 74, control
line EQSL is activated, equalizing the sense lines Si and
{overscore (S)}i. Delay circuit 70 keeps control line EQSA inactive
during this time so as to speed up the equalization by using the
charge remaining on the sense-amplifier drive lines P1 and N1, as
already explained.
[0142] After the sense lines Si and {overscore (S)}i have been
equalized, the delay circuit 70 activates control line EQSA,
equalizing the sense-amplifier drive lines P1 and N1 to HVcc. Next,
control lines EQS2, EQS3, EQSA, and EQSL are deactivated, control
line TG is activated to couple the sense lines to the bit lines,
the word line WL4 corresponding to row address X.sub.4 is
activated, control line SAE is activated to enable the sense
amplifiers 16, and data are transferred from the memory cells 4
coupled to word line WL4 onto the bit lines BLi and {overscore
(BL)}i and sense lines Si and {overscore (S)}i. Due to the large
capacitance of the bit lines BLi and {overscore (BL)}i, the
resulting potential swing is small, but the potential difference is
sensed and amplified correctly by the sense amplifiers 16, because
their drive lines P1 and N1 start from the equalized potential
HVcc.
[0143] The next cycle is also a cache miss, the input row address
X.sub.5 not matching the address in any tag circuit. Once again,
this is detected by the hit/miss detector 75, and the
sense-amplifier drive lines P1 and N1 as well as the sense lines Si
and {overscore (S)}i are equalized to the intermediate potential
HVcc.
[0144] The hit/miss detector 75 activates the sense-amplifier
equalizing circuit 50 to equalize the sense-amplifier drive lines
P1 and N1 not only in cache miss cycles, but also in refresh
cycles, because in refresh cycles as well, the sense amplifiers 16
must amplify small potential differences on the bit lines.
Furthermore, the sense-amplifier drive lines P1 and N1 are
equalized in the standby state, to prevent the flow of unwanted
leakage current.
[0145] In the embodiments shown so far, the cache has been coupled
directly to the sense lines. Although the cache provides major
benefits by enabling quick recall of cached data, and retaining
cached data during refresh cycles, this arrangement does not permit
the cached data to be accessed during refresh cycles. Thus if the
memory device has an auto-refresh function and generates its own
refresh cycles, each time one of these refresh cycles occurs,
external access must be delayed until the refresh cycle ends.
[0146] FIG. 14 is a circuit drawing of part of a DRAM in which
cache cells are disposed in the column switching circuits. Elements
identical to elements in FIGS. 1 to 10 have the same reference
numerals. Although only a single memory cell array 2 is shown, it
may be divided into two banks as in FIG. 7. The sense amplifier row
14 preferably has equalizing circuits structured as in FIG. 8 or
10, but for simplicity only control lines SAE and EQS are
shown.
[0147] The novel column switching circuit 200.sub.i, like the
conventional column switching circuit 20 in FIGS. 1 to 7,
interfaces the sense lines Si and {overscore (S)}i to the read data
lines RDB and {overscore (RDB)} and write data lines WDB and
{overscore (WDB)}. Unlike the conventional circuit, the column
switching circuit 200.sub.i has n cache cells 12.sub.i1, . . . ,
12.sub.in, where n may be any positive integer. The subscript i is
being used to identify elements belonging to the i-th column, which
will be useful in discussing write access.
[0148] The cache cells 12.sub.ij are coupled to a pair of
complementary column data lines CDi and {overscore (CD)}i. Data
transfer between the cache cells 12.sub.ij and column data lines
CDi and {overscore (CD)}i is controlled by cache control signals
SWcj (j=1, 2, . . . , n). As in the preceding embodiments, these
cache control signals are output from a tag circuit 28 with tag
memories 29 (i=1, 2, . . . , n). The cache cells 12.sub.1j,
12.sub.2j, . . . controlled by the same control line SWcj will be
referred to as the j-th cache row 210.sub.j. FIG. 14 explicitly
indicates the first cache row 210.sub.1. All of the cache cells
12.sub.ij will continue to be referred to collectively as the
cache, even though the cache cells 12.sub.ij are disposed in
different column switching circuits 200.sub.i.
[0149] The cache cells 12.sub.ij have, for example, the structure
shown in FIG. 2 or 3, except that they are coupled to the column
data lines instead of the sense lines.
[0150] The read column line RCLi drives the gates of two NMOS
transistors 219 and 220 which are coupled to read data lines
{overscore (RDB)} and RDB, respectively. Two more NMOS transistors
221 and 222, with gates controlled by the column data lines CDi and
{overscore (CD)}i, are coupled in series between transistors 219
and 220. The sources of NMOS transistors 221 and 22 are supplied
with a fixed potential such as the ground potential (Vss).
Transistors 221 and 222 form a simple amplifier that enables a
small potential swing on the column data lines CDi and CDi to
produce a larger potential swing on the read data lines RDB and
{overscore (RDB)}. (Similar amplifiers may be employed in the
column switching circuits 20 of FIGS. 1 to 7.) Sense lines Si and
{overscore (S)}i are coupled to the corresponding column data lines
CDi and {overscore (CD)}i through respective NMOS transistors 223
and 224, the gates of which are coupled to a control line SWa. The
column data lines CDi and {overscore (CD)}i are coupled to the
write data lines WDB and {overscore (WDB)} through respective NMOS
transistors 225 and 226, the gates of which are driven by the write
column line WCLi. An equalizing circuit 227, similar to the sense
line equalizing circuit 18 for example, is provided to equalize the
column data lines CDi and {overscore (CD)}i. The control line of
the column-data-line equalizing circuit 227 is not explicitly
shown, but it is different from the EQS control line of the
sense-line equalizing circuit 18, permitting the sense lines Si and
{overscore (S)}i to be equalized while the column data lines CDi
and {overscore (CD)}i remain unequalized, or vice versa.
[0151] First, the operation of control lines SWa, WCLi, and RCLi
will be briefly described.
[0152] When control line SWa is activated, NMOS transistors 223 and
224 turn on, coupling the sense lines Si and {overscore (S)}i to
the column data lines CDi and {overscore (CD)}i, so that the same
data appear on both Si/{overscore (S)}i and CDi/{overscore (CD)}i.
This occurs simultaneously in all columns. When SWa is inactive,
the sense lines Si and {overscore (S)}i are disconnected from the
column data lines CDi and {overscore (CD)}i.
[0153] When control line WCLi is activated, NMOS transistors 225
and 226 turn on, and data present on the write data lines WDB and
{overscore (WDB)} are transferred to the column data lines CDi and
{overscore (CD)}i.
[0154] When control line RCLi is activated, transistors 219 and 220
turn on. Both read data lines RDB and {overscore (RDB)} are
initially precharged to a certain potential such as the
power-supply potential Vcc. If data are present on the column data
lines CDi and {overscore (CD)}i, so that one of these lines is high
while the other is low, then one of the NMOS transistor transistors
221 and 222 will be turned on while the other is turned off. If
column data line CDi is high, for example, then transistor 221 will
turn on, pulling read data line {overscore (RDB)} down to the
ground level. Transistor 222 will remain off, so read data line RDB
is not coupled to ground and remains high. In this way the data on
the column data lines CDi and {overscore (CD)}i are transferred to
the read data lines RDB and {overscore (RDB)}.
[0155] Although the input-output circuits coupled to the data bus
lines are not shown, these circuits should be configured so that
data can be transferred from the write data lines WDB and
{overscore (WDB)} to the column data lines CDi and {overscore
(CD)}i and then to the read data lines RDB and {overscore (RDB)},
or from the cache cells 12.sub.ij to the column data lines CDi and
{overscore (CD)}i and then to the read data lines RDB and
{overscore (RDB)}, without amplification by the sense amplifiers
16.
[0156] FIGS. 15 and 16 illustrate the operation of the DRAM in FIG.
14 in a cache load cycle, cache hit cycle, cache replace cycle, and
combined cache hit and refresh cycle. Both read and write access
are shown. On the address bus ADD, addresses X.sub.0, X.sub.1,
X.sub.2, . . . are row addresses that select corresponding word
lines WL0, WL1, WL2, . . . , while Y.sub.i, Y'.sub.i, Y".sub.i, . .
. are column addresses.
[0157] A feature of the operation of this DRAM is that, as in the
preceding embodiments, the cache contents are kept consistent with
the memory cell contents; in every cycle, any newly input write
data are copied back to the memory cell array 2. More precisely,
all data present on the sense lines at the end of a cycle have also
been copied back to the memory cell array 2 by the end of that
cycle.
[0158] (1) Cache Load Cycle
[0159] A cache load cycle occurs when an input row address does not
match the address of any cached data, but there exists an empty
cache row (a cache row that does not hold valid data). The data in
the addressed row are then read from the memory cell array onto the
sense lines, and also loaded into the empty cache row, as
follows.
[0160] Referring to FIG. 15, if row address X.sub.0 does not match
the row address held in any tag memory 29.sub.j, and cache row
210.sub.1 is empty, then the tag circuit 28 drives control line
SWc1 to the active state. The word line WL0 corresponding to row
address X.sub.0 is also activated. Transfer circuit 8 is turned on
by control line TG, so the data DA1 in row X.sub.0 are read out of
their memory cells via the bit lines BLi and {overscore (BL)}i and
amplified, thus appearing on the sense lines Si and {overscore
(S)}i. Control line SWa is active, so the sense lines are coupled
to the column data lines CDi and {overscore (CD)}i, on which data
DA1 also appear. Since control line SWc1 is active, the data DA1
are loaded into the first cache row 210.sub.1.
[0161] Next, an external read/write control signal (not shown) is
placed in the read state, and a column address Y.sub.i is input.
This activates the corresponding read column line RCLi,
transferring data DA1 onto the pair of read data lines RDB and
{overscore (RDB)} as explained above. Since the NMOS transistors
221 and 222 that drive the read data lines RDB and {overscore
(RDB)} act as amplifiers, data DA1 can be read onto the read data
lines RDB and {overscore (RDB)} even before amplification by the
sense amplifier 16 is completed.
[0162] The read/write signal is now placed in the write state to
write to the same column address Y.sub.i. This causes the write
control line WCLi in the i-th column to go high, transferring data
DA2 from the pair of write data lines WDB and {overscore (WDB)} to
the pair of column data lines CDi and {overscore (CD)}i, thence to
cache cell 12.sub.i1 in cache row 210.sub.1 and the pair of sense
lines Si and {overscore (S)}i. Control line TG remains active, so
data DA2 are transferred from the sense lines Si and {overscore
(S)}i to the bit lines BLi and {overscore (BL)}i and copied back
into the corresponding memory cell. At most one pair of bit lines,
namely BLi and {overscore (BL)}i in the i-th column, has to be
charged or discharged in this copy-back operation, so it is
completed quickly. As soon as BLi and {overscore (BL)}i are fully
charged and discharged, the next cycle can begin.
[0163] Control lines TG, SWa, and SWc1 and word line WL0 are left
active at the end of this cycle, so that if row address X.sub.0 is
input again, access can continue immediately using the data
remaining in the sense amplifiers, sense lines Si and {overscore
(S)}i, and column data lines CDi and {overscore (CD)}i.
[0164] Although the same column address Y.sub.i was accessed for
both read and write above, the read and write column addresses can
of course be different. Nor is the cycle limited to one read access
and one write access; multiple read accesses and multiple write
accesses to different columns in the same row are possible. There
is no restriction on the order of-accesses; read may precede write,
or vice versa.
[0165] (2) Cache Hit Cycle
[0166] A cache hit cycle occurs when a new row address is input and
it matches one of the row addresses held in the tag circuit. The
sense lines are then disconnected from the bit lines, and the data
for the addressed row are recalled from the cache. At the end of
the cycle, the bit lines are coupled to the sense lines again to
write any updated data back to the memory cells.
[0167] Assume, for example, that row address X.sub.1 matches the
address held in tag memory 29.sub.2, indicating that the data of
this row are currently stored in cache row 210.sub.2. Control line
TG is deactivated to disconnect the sense lines {overscore (S)}i
and Si from the bit lines BLi and {overscore (BL)}i2L, and the tag
circuit 28 deactivates cache control line SWc1, disconnecting cache
row 210.sub.1 from the column data lines CDi and {overscore (CD)}i.
The data for row address X.sub.0 are now held in the first cache
row 210.sub.1, and are still present on the bit lines BLi and
{overscore (BL)}i and their connected memory cells.
[0168] Next the sense lines Si and {overscore (S)}i and column data
lines CDi and {overscore (CD)}i are equalized, then cache control
line SWc2 is activated by the tag circuit 28, transferring the data
from cache row 210.sub.2 onto the column data lines CDi and
{overscore (CD)}i. Control line SWa remains active, so these data
are also transferred to the sense lines Si and {overscore (S)}i and
amplified by the sense amplifiers 16.
[0169] As soon as the data for row address X.sub.1 have been
recalled in this way from the cache, read and write access can
begin. Column address Y.sub.i is input, and data DA3 are read onto
the read data lines RDB and {overscore (RDB)}; then data DA4 are
written from the write data lines WDB and {overscore (WDB)} onto
the column data lines CDi and {overscore (CD)}i and sense lines Si
and {overscore (S)}i in the i-th column.
[0170] While the data are being recalled from the cache, word line
WL0 is deactivated and the bit lines are precharged. The data for
row address X.sub.0, including the updated data DA2 in the i-th
column, thus remain held in both the memory cells 4 of row X.sub.0
in the memory cell array 2, and the cache cells 12.sub.i1 of the
first cache row 210.sub.1. Next, control line TG is activated and
word line WL1 is activated, to copy the data of row address X.sub.1
from the sense lines Si and {overscore (S)}i back to the memory
cells 4 on word line WL1. This copy-back includes the newly written
data DA4.
[0171] Access in a cache hit cycle takes place quickly because the
data are recalled from the cache via the column data lines CDi and
{overscore (CD)}i, which have a relatively small capacitance to be
charged or discharged, instead of via the bit lines BLi and
{overscore (BL)}i, which have a larger capacitance and resistance.
Time is also saved by precharging the memory cell array 2 while the
cached data are being recalled and read.
[0172] (3) Cache Replace Cycle
[0173] A cache replace cycle occurs when the input row address does
not match the row address held in any tag memory, and there is no
empty cache row. One cache row is selected for replacement. The
addressed row of data is read from the memory cell array, accessed,
and written into the selected cache row. No copy-back is necessary
at the start of a cache replace cycle, because the cache contents
and memory-cell contents are consistent at the end of the preceding
cycle.
[0174] The cache row to be replaced can be selected according to
various algorithms, such as the well-known least-recently-used
(RLU) rule. The present invention is not restricted to any
particular replacement algorithm. Here it will be assumed that
cache row 210.sub.1 is replaced.
[0175] In FIG. 15, input row address X.sub.2 does not match any
current tag address, and no empty cache row is available. The first
part of this cache replace cycle resembles a cache hit cycle.
Control line SWa remains active, but control lines TG and SWc2 are
deactivated, leaving the data for the row address X.sub.1 in the
previous cycle present on the bit lines BLi and {overscore (BL)}i
and in cache row 210.sub.2. Next, the sense lines Si and {overscore
(S)}i and column data lines CDi and {overscore (CD)}i are
equalized.
[0176] Control line SWc1 is now activated, coupling the cache row
210.sub.1 to be replaced to the column data lines CDi and
{overscore (CD)}i. Equalization of the sense lines Si and
{overscore (S)}i is continued, and the column data lines CDi and
{overscore (CD)}i are also equalized, so the existing data in cache
row 210.sub.1 are lost. In the meantime the memory cell array 2 is
precharged, leaving the data for row address X.sub.1 stored in the
memory cells 4 of that row.
[0177] When this precharge is completed, control line TG is
activated, the word line WL2 designated by the new row address
X.sub.2 is activated, the sense amplifiers 16 are enabled, and
equalization of the sense lines Si and {overscore (S)}i and column
data lines CDi and {overscore (CD)}i is discontinued. The data for
row X.sub.2 are thereby read onto the bit lines BLi and {overscore
(BL)}i, transferred to the sense lines Si and {overscore (S)}i,
amplified by the sense amplifiers 16, transferred to the column
data lines CDi and {overscore (CD)}i, and loaded into cache row
210.sub.1.
[0178] Read or write access now proceeds as in a cache hit cycle or
load cycle. In the drawing, data DA5 are read from the i-th column
onto the data bus.
[0179] (4) Refresh During Cache Hit Cycle
[0180] During a cache hit cycle on one row address, it is possible
to refresh a different row in the memory cell array. After the
cached data have been recalled onto the column data lines and
amplified, the column data lines are disconnected from the sense
lines. While read and write access to the column data lines
continues, the sense amplifiers can be used to refresh an arbitrary
row in the memory cell array. At the end of the cycle, the data on
the column data lines are copied back to the addressed row in the
memory cell array.
[0181] Referring to FIG. 16, input row address X.sub.1 matches the
address held in tag circuit 292, so a cache hit cycle is initiated.
Just as in the cache hit cycle in FIG. 15, control line TG is
deactivated and the memory cell array 2 is precharged. Control line
SWc1 is also deactivated by the tag circuit 28. As a result, the
data (e.g. DAS) left on the column data lines CDi and {overscore
(CD)}i at the end of the preceding cycle are saved in the memory
cells 4 coupled to word line WL2 and the cache cells 12.sub.i1 in
cache row 1. Concurrently with these operations, the sense lines Si
and {overscore (S)}i and column data lines CDi and {overscore
(CD)}i are equalized; then the control line SWc2 of the cache row
210.sub.2 that was hit by the new row address X.sub.1 is activated
by the tag circuit 28, the sense amplifiers 16 are enabled, and
data are recalled from cache row 2102 onto the column data lines
CDi and {overscore (CD)}i and sense lines Si and {overscore
(S)}i.
[0182] In preparation for refreshing a row in the memory cell
array, control line SWa is now deactivated, disconnecting the sense
lines Si and {overscore (S)}i from the column data lines CDi and
{overscore (CD)}i. Read and write accesses continue to be carried
out in the usual way, by input of column addresses, except that in
write access, the written data are not amplified by the sense
amplifiers. Thus when data DA6 are written in the i-th column, for
example, the resulting swing on the column data lines CDi and
{overscore (CD)}i is less than in FIG. 15. This does not prevent
subsequent read access from taking place, because the reduced swing
is still adequate to drive the NMOS transistors 221 and 222 and
place the new data on the read data lines RDB and {overscore
(RDB)}. After the reading of data DA4 and writing of data DA6,
further column addresses are input and read and write access
continues in the same or other columns.
[0183] While this access to the column data lines CDi and
{overscore (CD)}i is taking place, the sense amplifiers 16 are
disabled and the sense lines Si and {overscore (S)}i are equalized.
Next, after the sense lines Si and {overscore (S)}i and bit lines
BLi and {overscore (BL)}i have both been equalized, control line TG
is reactivated, turning on the transfer circuit 8. Simultaneously,
a word line to be refreshed is selected by an internal refresh
counter and activated. This word line is unrelated to the current
input row address X.sub.1; in the drawing, word line WL3 is
refreshed.
[0184] After word line WL3 has been activated, the sense amplifiers
16 are enabled again, and the data in the memory cells 4 coupled to
word line WL3 are amplified on the bit lines BLi and {overscore
(BL)}i and sense lines Si and {overscore (S)}i. When amplification
is complete, control line TG is deactivated to disconnect the bit
lines BLi and {overscore (BL)}i from the sense lines Si and
{overscore (S)}i, and the memory cell array 2 is precharged,
leaving refreshed data in the memory cells 4 coupled to word line
WL3.
[0185] After control line TG has been deactivated and the transfer
circuit 8 has been turned off, the sense amplifiers 16 are disabled
and the sense lines Si and {overscore (S)}i are equalized. Then
control line SWa is activated again, coupling the column data lines
CDi and {overscore (CD)}i to the sense lines Si and {overscore
(S)}i, and the sense amplifiers are enabled. Data such as DA6 on
the column data lines CDi and {overscore (CD)}i are now amplified
to obtain a full potential swing.
[0186] When the memory cell array 2 has been precharged, control
line TG is activated again, transferring the amplified data such as
DA6 to the bit lines BLi and {overscore (BL)}i. Then word line WL1
is activated to copy the amplified data back to the memory cells in
row X.sub.1 in the memory cell array.
[0187] If a refresh cycle cannot be scheduled to coincide with a
cache hit cycle, then a conventional refresh operation is carried
out, and external access is delayed.
[0188] The advantages of placing the cache cells in the column
switching circuits can be summarized as follows.
[0189] First, refresh cycles can be executed within cache hit
cycles. Refresh can then be carried out without impairing response
speed, and the average data rate can be improved.
[0190] Second, precharging of the memory cell array 2 and the
recall of data onto the column data lines CDi and {overscore (CD)}i
can be carried out concurrently, by disconnecting the bit lines
from the sense lines. The enables a shortening of the minimum cycle
time (the time from input of one row address until input of the
next row address can be accepted).
[0191] FIG. 17 is a circuit drawing of part of another novel DRAM.
Elements which are the same as elements in FIG. 14 have the same
reference numerals.
[0192] The only difference between FIGS. 14 and 17 is that in the
DRAM in FIG. 17, a write buffer 300 is provided between the sense
lines Si and {overscore (S)}i. The write buffer 300 comprises a
switching element 301 such as an NMOS transistor coupled to sense
line Si, a switching element 302 such as another NMOS transistor
coupled to sense line {overscore (S)}i, and a storage element 303
such as a capacitor coupled between these switching elements 301
and 302. Switching elements 301 and 302 are controlled by a common
control line SWb, which extends in the row direction and controls
the write buffers 300 in all columns simultaneously.
[0193] The write buffers require only a small amount of additional
circuit space, and enable cache access and copy-back can be
executed concurrently. Copy-back operations can furthermore be
advantageously deferred until made necessary by cache
replacement.
[0194] FIGS. 18 and 19 illustrate the operation of this DRAM in a
cache load cycle, cache hit cycle, cache replace cycle, and
combined refresh and cache hit cycle. A feature of these operations
is that the cache and memory cell array are allowed to become
inconsistent.
[0195] (1) Cache Load Cycle
[0196] The main difference between the cache load cycle in FIG. 18
and the one in FIG. 15 is that in FIG. 18, control line TG is
activated only long enough to transfer the data of row X.sub.0 from
the bit lines BLi and {overscore (BL)}i to the sense lines Si and
{overscore (S)}i. As soon as this transfer is completed, control
line TG is deactivated again, disconnecting the bit lines BLi and
{overscore (BL)}i from the sense lines Si and {overscore (S)}i, and
the memory cell array 2 is precharged. At the point when control
line TG is deactivated and the transfer circuit 8 is turned off,
amplification of the signals on the bit lines BLi and {overscore
(BL)}i and sense lines Si and {overscore (S)}i has barely begun, so
the potential swing on the bit lines BLi and {overscore (BL)}i is
still small. Equalization of the bit lines BLi and {overscore
(BL)}i is accordingly completed in a very short time, with
relatively little current dissipation. The current drawn by the
sense amplifiers 16 is also greatly reduced, since they do not have
to charge and discharge the bit lines BLi and {overscore
(BL)}i.
[0197] Control signal SWa is active throughout this cycle, so the
data transferred to the sense lines Si and {overscore (S)}i are
also transferred to the column data lines CDi and {overscore
(CD)}i. At an appropriate point, control line SWc1 is activated to
store these data in the empty cache row 210.sub.1. In addition,
column addresses Y.sub.i are input and read and write accesses take
place as in FIG. 15. As in FIG. 15, read or write access can begin
even before the data have been fully amplified, and write access
can be further speeded up because it is not necessary to charge or
discharge the bit lines BLi and {overscore (BL)}i.
[0198] In FIG. 18, precharging of the memory cell array 2 is
completed even before the end of the first read access (that is,
before the read column line RCLi goes low). The memory is
accordingly ready to begin a new cycle at this point; a new row
address could now be input, so the minimum cycle time is very
short.
[0199] At the end of the cache load cycle in FIG. 18, the data for
row address X.sub.0, including the newly written data DA2, are
present on the sense lines Si and {overscore (S)}i and column data
lines CDi and {overscore (CD)}i, and in cache row 210.sub.1 but are
not held in the memory cell array 2.
[0200] (2) Cache Hit Cycle
[0201] The cache hit cycle in FIG. 18 is identical to the one in
FIG. 15, except that control line TG is inactive, the transfer
circuit 8 is in the off state throughout the cycle, and there is no
activity in the memory cell array 2. Read and write access are
confined completely to the column data lines CDi and {overscore
(CD)}i and sense lines Si and {overscore (S)}i. Write access can be
speeded up, current dissipation reduced, and the minimum cycle time
shortened, because there is no need to charge or discharge any pair
of bit lines BLi and {overscore (BL)}i or activate or deactivate
any word lines.
[0202] At the end of the cache hit cycle in FIG. 18, data for row
addresses X.sub.0 and X.sub.1 are held in cache rows 210.sub.1 and
210.sub.2, and the corresponding rows in the memory cell array 2
are both invalid.
[0203] (3) Cache Replace Cycle
[0204] The cache replace cycle in FIG. 18 differs from the one in
FIG. 15. In FIG. 18, a cache row is selected for replacement and
its data are saved into the write buffers 300. Then the addressed
row of data is read out of the memory cell array 2 onto the column
data lines CDi and {overscore (CD)}i and accessed there. While
access to the data on the column data lines is in progress, the
replaced data are copied back from the write buffers to the memory
cell array.
[0205] At the beginning of the cycle, it is recognized that the
input row address X.sub.2 does not correspond to the row address in
any tag memory, and that no cache row is empty, so cache row
210.sub.1, for example, is selected for replacement. Control line
SWc2 is deactivated, the sense amplifiers are disabled, and the
column data lines CDi and {overscore (CD)}i and sense lines Si and
{overscore (S)}i are equalized. Control line SWb is activated, so
that the write buffers 300 are also equalized.
[0206] Next, control line SWc1 is activated. The data (e.g. DA2)
that will be replaced are read from cache row 210.sub.1 onto the
column data lines CDi and {overscore (CD)}i and sense lines Si and
{overscore (S)}i, amplified by the sense amplifiers, and
transferred into the write buffers 300. After a short time, control
line SWb is switched off again, leaving the data stored in the
write buffers. The sense lines Si and {overscore (S)}i and column
data lines CDi and {overscore (CD)}i are now equalized once
again.
[0207] While the data of row address X.sub.0 are being transferred
from cache row 210.sub.1 into the write buffers 300 in this way,
the newly addressed word line WL2 is activated in the memory cell
array 2, and the data stored in the memory cells 4 coupled to this
word line (the data with row address X.sub.2) are transferred onto
the bit lines BLi and {overscore (BL)}i. When the sense lines Si
and {overscore (S)}i have been equalized, control line TG is
activated and the transfer circuit 8 is turned on briefly,
transferring the data (e.g. DA5) for row address X.sub.2 from the
bit lines BLi and {overscore (BL)}i to the sense lines Si and
{overscore (S)}i. Then control line TG is deactivated again and the
memory cell array 2 is precharged. Since the potential swing on the
bit lines BLi and {overscore (BL)}i was small, the precharge time
is short.
[0208] After the deactivation of control line TG, the sense
amplifiers 16 continue to amplify the data (DA5) on the sense lines
Si and {overscore (S)}i and column data lines CDi and {overscore
(CD)}i. When amplification is completed, control line SWa is
deactivated, disconnecting the column data lines CDi and {overscore
(CD)}i from the sense lines Si and {overscore (S)}i.
[0209] Read and write access can now take place using the column
data lines CDi and {overscore (CD)}i alone. In the drawing, column
address Y.sub.i is input, data DA5 are read from the column data
lines CDi and {overscore (CD)}i in the i-th column onto the read
data lines RDB and {overscore (RDB)}, then data DA6 are written
from the write data lines WDB and {overscore (WDB)} onto the same
pair of column data lines CDi and {overscore (CD)}i. Next other
column addresses Y.sub.i' and Y.sub.i" are input and read access,
for example, is carried out in other columns.
[0210] While this access is taking place, the sense lines Si and
{overscore (S)}i are equalized again; then control line SWb is
activated and the data DA2 that were stored in the write buffers
300 are transferred onto the sense lines Si and {overscore (S)}i
and amplified. At the same time, word line WL0 is activated in the
memory cell array 2. When amplification is complete, control line
TG is activated, the transfer circuit 8 is turned on, and data
(e.g. DA2) are transferred from the sense lines Si and {overscore
(S)}i via the bit lines BLi and {overscore (BL)}i to the memory
cells 4 at row address X.sub.0. When this copy-back is completed,
control line TG is deactivated again and the memory cell array 2 is
precharged.
[0211] After the control line TG is deactivated and transfer
circuit 8 is turned off, the sense lines Si and {overscore (S)}i
are equalized again, then control line SWa is activated to
reconnect the sense lines Si and {overscore (S)}i to the column
data lines CDi and {overscore (CD)}i. The sense amplifiers 16 now
amplify the data on the column data lines CDi and {overscore
(CD)}i, including the newly written data DA6 in the i-th
column.
[0212] At the end of the cache replace cycle in FIG. 18, data for
row addresses X.sub.1 and X.sub.2 are held in cache rows 210.sub.2
and 210.sub.1, respectively, while data for row address X.sub.0 are
held in the memory cell array 2. The data for row address X.sub.2
also remain amplified and available on the sense lines Si and
{overscore (S)}i and column data lines CDi and {overscore (CD)}i,
ready for immediate access if the same row address X.sub.2 is input
again.
[0213] Compared with FIG. 15, the cache replace cycle in FIG. 18
enables a quicker transfer of the required data from the memory
cell array to the sense lines, because the newly addressed word
line (WL2) can be activated at the beginning of the cycle, without
waiting for the memory cell array to be precharged.
[0214] (4) Refresh Cycle During a Cache Hit Cycle
[0215] Since there is no interchange of data between the cache and
memory cell array in a cache hit cycle, refreshing the memory cell
array during a cache hit cycle is a simple matter. After the
required data have been recalled, the sense lines are disconnected
from the column data lines and coupled to the bit lines, and the
sense amplifiers are used to refresh an arbitrary row of memory
cells.
[0216] Referring to FIG. 19, this cycle is shown as a type-1
refresh cycle. When row address X.sub.1 is input, a hit is
recognized on cache row 210.sub.2, so control line SWc1 is
deactivated (storing data DA6 in cache row 210.sub.1), and the
sense lines Si and {overscore (S)}i and column data lines CDi and
{overscore (CD)}i are equalized. Then control line SWc2 is
activated, recalling the data for row address X.sub.1 (e.g. DA4)
onto the column data lines CDi and {overscore (CD)}i. Control line
SWa, however, is deactivated, so the recalled data are not
amplified. The unamplified signals are still adequate for read
access, so when column address Y.sub.i is input, data DA4 are read
from the i-th column onto the read data lines RDB and {overscore
(RDB)}. This is followed by a write access that replaces data DA4
with data DA9, then a further access to a different column
Y.sub.i".
[0217] Meanwhile, when the cache hit was first recognized, a
refresh address was generated and the corresponding word line (e.g.
WL3) was activated in the memory cell array 2. After control line
SWa is deactivated to disconnect the sense lines Si and {overscore
(S)}i from the column data lines CDi and {overscore (CD)}i, the
sense lines Si and {overscore (S)}i and sense amplifiers 16 are
equalized, then control line TC is activated to turn on the
transfer circuit 8, coupling the sense lines Si and {overscore
(S)}i to the bit lines BLi and {overscore (BL)}i, so that the sense
amplifiers can amplify and refresh the data (e.g. DA11) in the
memory cells 4 coupled to word line WL3. When the amplification is
complete, control line TG is deactivated and the memory cell array
2 is precharged, leaving refreshed data in these 59 memory cells
4.
[0218] After control line TG is deactivated, the sense lines Si and
{overscore (S)}i are equalized again, then control line SWa is
activated to recouple the sense lines Si and {overscore (S)}i to
the column data lines CDi and {overscore (CD)}i. This enables the
sense amplifiers to amplify the data DA9 on the column data lines
CDi and {overscore (CD)}i, and the subsequently written data
DA10.
[0219] Compared with the refresh cycle in FIG. 16, the one in FIG.
19 is finished earlier, because word line WL3 can be activated at
the beginning of the cache hit cycle, without waiting for the
memory cell array 2 to be precharged.
[0220] From FIGS. 18 and 19, the write buffers 300 can be seen to
provide the following advantages.
[0221] First, every cycle begins with the memory cell array already
precharged. Accordingly, when the memory cell array must be
accessed to read or refresh data, the required word line can be
activated immediately. Access times and cycle times can accordingly
be shortened.
[0222] Second, the cycle time in a cache hit can be further
shortened, and operating current reduced, by leaving the memory
cell array 2 disconnected throughout the cycle. No word lines need
be activated, and no precharging need be carried out.
[0223] Third, when data are transferred from the memory cell array
2 to the sense lines, cycle times can be shortened and power
dissipation greatly reduced by turning off the transfer circuit 8
before amplification of the data is completed, so that the
potential swing on the bit lines BLi and {overscore (BL)}i remains
small.
[0224] FIG. 20 is a circuit diagram showing a transfer control
circuit for controlling the transfer circuit 8.
[0225] In this transfer control circuit, a control circuit 310
receives, among other input signals, the sense amplifier enable
signal SAE, and generates two control signals T.sub.0 and T.sub.1.
Signal To controls the transfer circuit 8 in operations in which
data must be transferred from the sense lines to the bit lines;
these operations include copy-back and refresh. Signal T.sub.1
controls the transfer circuit 8 in operations in which data are
transferred unidirectionally from the bit lines to the sense lines;
that is, operations in which it is not necessary to transfer
amplified data back to the bit lines. In the present DRAM, these
can be characterized as operations in which data are transferred
from the memory cell array 2 to the cache.
[0226] A logic circuit 312 combines SAE with T.sub.1 to generate a
signal T.sub.2 that is active when T1 is active and SAE is
inactive, and is inactive at other times. In FIG. 20 this logic
circuit 312 comprises an inverter 314 that inverts SAE, and an AND
gate 316 that takes the logical AND of T.sub.1 and the output of
the inverter 314.
[0227] A copy-back detector 318 detects copy-back and refresh
operations and controls a switch 320 that outputs the TG signal
that controls the transfer circuit 8. One input to the switch 320
is T.sub.0. The other input is the output T.sub.2 of the logic
circuit 312; that is, the output of the AND gate 316.
[0228] When the copy-back detector 318 does not detect a copy-back
or refresh operation, it sets the switch 320 as shown in the
drawing. Accordingly, after signals T.sub.1, T.sub.2, and TG are
activated to turn on the transfer circuit 8, as soon as SAE goes
high and the sense amplifiers are enabled, T.sub.2 and TG are
deactivated again by the inverted SAE signal, thereby minimizing
amplification of the potential difference on the bit lines.
[0229] When the copy-back detector 318 detects either a copy-back
or a refresh, it sets the switch 320 to the opposite position, so
that TG is identical to To, and is not deactivated when the sense
amplifiers are enabled.
[0230] FIG. 21 illustrates a refresh control circuit that can be
used in the DRAM in FIG. 17. The purpose of this circuit is to
combine refresh and copy-back operations.
[0231] In this refresh control circuit, a multiplexer 410 selects
either a row address X indicating data to be accessed or a refresh
address X.sub.f indicating data to be refreshed, and supplies the
selected address to the tag circuit 28.
[0232] Part of this refresh control circuit is disposed in the
DRAM's central control circuit 420, which comprises logic circuits
and various timing generators that direct operations of the entire
DRAM. One of these timing generators is a refresh timing generator
421 that generates a refresh signal REF and two different sets of
refresh timing signals RT1 and RT2. Refresh timing signals RT1 are
adapted to control the type-1 refresh cycle shown in FIG. 19.
Refresh timing signals RT2 are adapted for a type-2 refresh cycle,
which will be described below.
[0233] The signals REF, RT1, and RT2 are supplied to a refresh
selection circuit 430, which also receives the cache control
signals SWcj (j=1, . . . , n) from the tag circuit 28. The refresh
signal REF and cache control signals SWcj are supplied to a logic
circuit 431 comprising an n-input OR gate 432 that receives the
cache control signals SWcj, and a two-input AND gate 433 that
receives REF and the output of the OR gate 432. The output signal C
of the AND gate 433 controls a switch 434 that selects either RT1
or RT2 for supply to circuits (not shown) that direct execution of
the type-1 and type-2 refresh cycles.
[0234] FIG. 21 is somewhat simplified; for example, it does not
show additional circuits for distinguishing between a type-1
refresh that coincides with a cache hit cycle and a refresh cycle
that does not coincide with a cache hit.
[0235] Next, the operation of the refresh control circuit will be
described.
[0236] When a refresh address X.sub.f is generated, it is fed
through the multiplexer 410 to the tag circuit 28. At the same
time, the refresh timing generator 421 begins generating both sets
of refresh timing signals RT1 and RT2 and activates the refresh
signal REF.
[0237] If the refresh address X.sub.f does not match the row
address held in any tag memory, the outputs of the OR and AND gates
432 and 433 in the logic circuit 431 are both low, the switch 432
selects refresh timing signals RT1, and a type-1 refresh operation
is carried out as already described. If the refresh address X.sub.f
matches the address in one of the tag circuits, however, the
outputs of the OR and AND gates 432 and 433 in the logic circuit
are both high, the switch 432 selects the second set of refresh
timing signals RT2, and a type-2 refresh is carried out as
described below. Here it will be assumed that the refresh address
X.sub.f matches row address X.sub.0, which is held in the tag
memory 291 for cache row 210.sub.1.
[0238] Referring again to FIG. 19, in a type-2 refresh operation,
the word line WL0 of the memory cells to be refreshed is activated,
and the data DA6 held in the corresponding cache row 210.sub.1 are
transferred to the sense lines Si and {overscore (S)}i and
amplified by the sense amplifiers 16. This incidentally refreshes
the cache row 210.sub.1. After amplification is complete, control
line TG is activated and the transfer circuit 8 is turned on,
transferring the data DA6 to the bit lines BLi and {overscore
(BL)}i. That is, a copy-back is performed.
[0239] The type-2 refresh in FIG. 19 coincides with a cache hit
cycle on row address X.sub.1, corresponding to cache row 210.sub.2.
Accordingly, after data DA6 have been transferred from the internal
column data lines CDi and {overscore (CD)}i to the sense lines Si
and {overscore (S)}i, control signal SWa is deactivated to
disconnect the column data lines CDi and {overscore (CD)}i from the
sense lines Si and {overscore (S)}i, cache control line SWc1 is
deactivated, the column data lines CDi and {overscore (CD)}i are
equalizes then cache control line SWc2 is activated, and data DA9
in cache row 210.sub.2 are placed on the column data lines CDi and
{overscore (CD)}i.
[0240] Subsequent operations in a type-2 refresh are similar to a
type-1 refresh. Read and write accesses can be carried out on the
data on the column data lines CDi and {overscore (CD)}i; in FIG.
19, data DA9 are read from the i-th column, then data DA10 are
written in the i-th column. At the end of the cycle, control line
TG is deactivated, the transfer circuit 8 is turned off, the memory
cell array 2 is precharged, control line SWa is activated again,
and the sense amplifiers 16 are used to amplify the data on the
column data lines CDi and {overscore (CD)}i.
[0241] Use of the type-2 refresh cycle has the following advantage.
Since data stored in the cache cells 12.sub.ij are not kept up to
date in the memory cell array 2, the cache cells 12.sub.ij must be
refreshed to prevent data loss. The refresh control circuit in FIG.
21 assures that this will take place. When a refresh address
X.sub.f is generated, if the corresponding row of data is not
currently stored in the cache, the memory cells 4 are refreshed,
e.g. by a type-1 refresh. If the corresponding row of data is
currently stored in the cache, a type-2 refresh is carried out, the
cache cells are refreshed, and the data are also copied back to the
memory cell array.
[0242] As a result, the cache cells do not have to be designed to
tolerate longer intervals between refreshes than do the memory
cells. Nor is it necessary to increase the refresh rate to provide
additional refreshes for the cache cells.
[0243] A further advantage occurs if the DRAM remains in standby
mode for an extended time. Refreshing continues in standby under
control of the central control circuit 420, with type-2 refreshes
being carried out on data stored in the cache. After all row
addresses have been refreshed, all data in the cache cells will
have been copied back to the memory cells. At this point further
refreshing of the cache cells is unnecessary. The tag circuit 28
can now be reset by flagging all cache rows as empty, and current
can be conserved by deactivating control line SWa, so that the
column data lines CDi and {overscore (CD)}i do not have to be
charged and discharged.
[0244] To reduce standby current dissipation, the memory cells 4
are preferably designed to tolerate a relatively slow refresh rate
in the standby state. Another advantage of the refresh control
circuit of FIG. 21 is that the cache cells do not have to be
designed for this slow refresh rate. Refresh cycles can be executed
at a faster rate while the DRAM is active, and until all row
addresses have been refreshed once in standby; then the refresh
rate can be reduced. The cache design can thus be simplified: for
example, instead of setting the NMOS transistor substrate well
potential to a negative value, it can be set at ground level (Vss),
and the substrate well can be shared with the sense amplifiers 16
and other elements, thereby reducing dimensions.
[0245] In addition, the size of the switching transistors in the
cache cells can be reduced, so less power is required to drive
them. Significant power can be saved in this way, because to enable
a Vcc potential to be stored in the cache cells, the cache control
lines SWc1-SWcn are driven at an active level higher than Vcc.
[0246] To enable a Vcc potential to be stored in the memory cells,
the word lines and transfer gate lines (e.g. TG) must also be
driven at an active level higher than Vcc. With the scheme
illustrated in FIGS. 17 to 19, however, when data are transferred
from the memory cell array to the cache, it is not necessary to
drive the word lines and transfer gate line TG at this higher
level, because the transfer circuit 8 is turned off before
amplification of the data is completed. Power can accordingly be
saved by providing three-level drivers for the word lines and
TG.
[0247] FIG. 22 illustrates one example of a three-level driver
circuit that can be used for driving the word lines and TG in the
DRAM of FIG. 17.
[0248] This driver circuit has an input terminal 450 that inputs,
for example, a decoded row address signal to activate a word line,
or a control signal to activate the TG control line. A copy-back
detector 451 outputs a copy-back mode signal CBM that is active
(high) when a copy-back or refresh is performed. The input signal
from input terminal 450 and the CBM signal are combined by AND
gates 452 and 453 and inverters 454 and 455 to control switches
456, 457, 458, and 459. Switch 456 is coupled to a source of a
first (boosted) potential V.sub.1 such as Vcc+V.sub.tn. Switch 456
is coupled to a source of a second potential V.sub.2 such as Vcc.
Both switches 456 and 457 are coupled to an output terminal 460,
which is connected to a word line or to control line TG. Switches
458 and 459 are coupled in series between the output terminal 460
and ground (Vss).
[0249] This driver circuit operates as follows.
[0250] When the copy-back detector 451 detects a copy-back or
refresh operation, it activates the CBM signal, so that the output
of AND gate 452 is high or low according to the level of input
terminal 450, while the output of AND gate 453 is low. Accordingly,
signal {overscore (IN)}.sub.2 in FIG. 22 is low while {overscore
(IN)}.sub.2 is high, and switch 457 is off while switch 459 is on.
If the input terminal 450 is high, signal {overscore (IN)}.sub.1
will be high and {overscore (IN)}.sub.1 will be low, switch 456
will be on and switch 458 will be off, and potential V.sub.1 will
be coupled to the output terminal 460. If the input terminal 450 is
low, signal {overscore (IN)}.sub.1 will be low and {overscore
(IN)}.sub.1 will be high, switch 456 will be off and switch 458
will be on, and the output terminal 460 will be coupled to
ground.
[0251] When the copy-back detector 451 does not detect a copy-back
or refresh, CBM is deactivated (low), making the output of AND gate
452 low while the output of AND gate 453 depends on the logic level
of the input terminal 450. Operations analogous to the above cause
the output terminal 460 to go to V.sub.2 when the input level is
high, and to ground when the input level is low.
[0252] If this driver is used, then the word lines and TG line will
be driven at a level V.sub.1 higher than Vcc only when a copy-back
or refresh is carried out. At other times they will be driven at
the lower V.sub.2 level, thereby conserving power.
[0253] FIG. 23 shows one preferred arrangement of cache cells,
switching elements, and signal lines in the column switching
circuit 200.sub.i of FIG. 14 or 17.
[0254] In this column switching circuit 200.sub.i, cache cells
12.sub.i1-12.sub.i4 are formed in a cache element area 480. To the
right of this area 480 are to be found read data line {overscore
(RDB)}, write data line {overscore (WDB)}, and the NMOS transistors
219, 221, 223, and 226 by which the column data lines CDi and
{overscore (CD)}i are interfaced to these data lines {overscore
(RDB)} and {overscore (WDB)} and sense line Si. To the left of this
area 480 are to be found read data line RDB, write data line WDB,
and the NMOS transistors 220, 222, 224, and 225 by which the column
data lines CDi and {overscore (CD)}i are interfaced to RDB, WDB,
and sense line {overscore (S)}i. The write column line RCLi and
read column line WCLi extend across the area 480, being routed
between the column data lines CDi and {overscore (CD)}i.
[0255] This arrangement is symmetrical and permits a dense layout
of signal lines and circuit elements, regardless of the dimensions
of the circuit elements. It is suitable for high levels of
integration.
[0256] FIG. 24 shows another preferred layout. Write data lines WDB
and {overscore (WDB)} are now disposed to the left of the cache
element area 480, together with the transistors 225 and 226 that
couple them to the column data lines CDi and {overscore (CD)}i.
Read data lines RDB and {overscore (RDB)} are disposed to the right
of the cache element area 480, together with the transistors 219,
220, 221, and 222 that couple them to the column data lines CDi and
{overscore (CD)}i, and transistors 223 and 224 that couple the
column data lines CDi and {overscore (CD)}i to the sense lines Si
and {overscore (S)}i.
[0257] This layout is also suited for high integration. It
furthermore prevents interference between the read and write data
lines, and by placing complementary bus lines side-by-side,
provides good noise immunity.
[0258] FIG. 25 shows a layout in which one row 501 of sense
amplifiers and column switching circuits is disposed on the left of
the memory cell array 2, and another similar row 502 is disposed on
the right of the same the memory cell array 2. Pairs of
complementary bit lines are coupled alternately to the left row 501
and right row 502. The circuit area 600 occupied by one sense
amplifier and one column switching circuit accordingly corresponds
to a span 503 of four bit lines, as indicated.
[0259] FIG. 26 shows this circuit area 600 in detail. As in FIGS.
23 and 24, cache cells 12.sub.i1-12.sub.i4 are disposed between
column data lines CDi and {overscore (CD)}i, which are disposed
between sense lines Si and {overscore (S)}i. In addition, dummy
cells 610.sub.i1-610.sub.i4 are disposed in the cache element area
480 between the column data lines CDi and {overscore (CD)}i and
sense lines Si and {overscore (S)}i.
[0260] Each of the cache cells 12.sub.i1-12.sub.i4 comprises a
switching element and a storage element, as in FIG. 2 for example,
the switching element coupling the storage element to one of the
column data lines CDi or {overscore (CD)}i. The dummy cells
610.sub.i1-610.sub.i4 comprise similar switching elements 612 and
storage elements 614, with the same dimensions and shapes as the
corresponding elements in the cache cells 12.sub.i1-12.sub.i4, but
the switching elements 612 are coupled only to the storage elements
614, and are not electrically coupled to the column data lines CDi
and {overscore (CD)}i or sense lines Si and {overscore (S)}i.
[0261] For example, if the switching elements 612 in the dummy
cells 610.sub.i1-610.sub.i4 are NMOS transistors, their drains may
be coupled to the storage elements 614, and their sources to the
sources of the NMOS-transistor switching elements 612 in the
adjacent dummy cells, with no contact holes for coupling the source
lines to anything else. Except for this lack of contact holes for
the dummy cells 610.sub.i1-610.sub.i4, the structure and pitch of
the cache cells 12.sub.i1-12.sub.i4 and dummy cells
610.sub.i1-610.sub.i4 closely mimics the structure and pitch of the
memory cells in the memory cell array 2. It therefore becomes easy
to set fine-pattern fabrication conditions, and production yields
can be improved.
[0262] FIGS. 23, 24, and 26 show only four cache cells
12.sub.i1-12.sub.i4 and dummy cells 610.sub.i1-610.sub.i4, but of
course this is not a restriction; there may be an arbitrary number
of each. To simplify FIG. 26, the data bus 21 is indicated by a
single line DB, and the transistors that interface the data bus 21
to the column data lines are shown collected into a single block
620, but this should be understood as representing the arrangement
in FIG. 23 or 24, or another suitable arrangement.
[0263] The present invention is not restricted to the above
embodiments. For example, the NAND gates in FIGS. 1 and 5 may be
replaced by circuits, similar to the one in FIG. 21, that refresh
the cache when the corresponding rows of memory cells are
refreshed. The circuit configurations in FIGS. 11, 20, 21, and 22
may be altered in various ways, signal polarities may be reversed,
and other modifications which will be apparent to those skilled in
the art can be made without departing from the scope of the
invention as claimed below. Moreover, many aspects of the invention
apply not only to DRAM devices but to other monolithic
semiconductor memory devices that can usefully employ a built-in
cache.
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