U.S. patent application number 09/738190 was filed with the patent office on 2001-06-21 for synchronous double data rate dram.
This patent application is currently assigned to NEC CORPORATION. Invention is credited to Murakami, Naoki.
Application Number | 20010004335 09/738190 |
Document ID | / |
Family ID | 18450827 |
Filed Date | 2001-06-21 |
United States Patent
Application |
20010004335 |
Kind Code |
A1 |
Murakami, Naoki |
June 21, 2001 |
Synchronous double data rate dram
Abstract
A DDR-SDRM includes a data write section including a latch
section for latching first write data at a rising edge of a data
strobe signal and second write data at a falling edge of the data
strobe signal, and a write buffer section for writing the first
write data and the second write data at a time based on the signals
generated from the data strobe signal. A read operation is
conducted based on the system clock signal.
Inventors: |
Murakami, Naoki;
(Kawasaki-shi, JP) |
Correspondence
Address: |
Attention: Norman P. Soloway
HAYES, SOLOWAY, HENNESSEY,
GROSSMAN & HAGE, P.C.
175 Canal Street
Manchester
NH
03101
US
|
Assignee: |
NEC CORPORATION
|
Family ID: |
18450827 |
Appl. No.: |
09/738190 |
Filed: |
December 15, 2000 |
Current U.S.
Class: |
365/233.1 |
Current CPC
Class: |
G11C 7/1078 20130101;
G11C 7/1093 20130101; G11C 7/22 20130101; G11C 11/4096 20130101;
G11C 11/4076 20130101; G11C 11/4093 20130101; G11C 7/1072 20130101;
G11C 7/1066 20130101 |
Class at
Publication: |
365/233 |
International
Class: |
G11C 008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 16, 1999 |
JP |
11-356797 |
Claims
What is claimed is:
1. A double data rate synchronous DRAM (DDR-SDRM) comprising a
memory cell array including a plurality of memory cells, an address
signal processing section for receiving and processing an address
signal for specifying an address of one of said memory cells, said
address signal processing section operating in synchrony with a
system clock signal, a data write section for receiving write data
and writing the write data into said one of said memory cells in a
write cycle, and a data read section for responding to said system
clock signal to read data from said one of said memory cells in a
read cycle to deliver read data, said data write section operating
based on a data strobe signal transferred in synchrony with said
write data to receive said write data from outside said DDR-SDRM
and deliver said write data through I/O lines to said one of said
memory cells.
2. The DDR-SDRM as defined in claim 1, wherein said data write
section includes a data latch section for receiving said write data
from outside said DDR-SDRM and a write buffer section for writing
said write data to said one of said memory cells.
3. The DDR-SDRM as defined in claim 2, wherein said data latch
section includes at least one first latch for responding to a
rising edge of said data strobe signal and at least one second
latch for responding to a falling edge of said data strobe signal
to latch first write data and second write data, respectively, and
said write buffer section writes said first write data and said
second write data at a time based on said falling edge of said data
strobe signal.
4. The DDR-SDRM as defined in claim 3, wherein either of said at
least one first data latch and at least one second data latch
includes a plurality of data latches, and said write buffer section
includes write buffers in number corresponding to the number of
data latches in each of said at least one first data latch and at
least one second data latch.
5. The DDR-SDRM as defined in claim 3, wherein either of said at
least one first data latch and at least one second data latches
includes first latch block and second latch block each including a
specified number of data latches.
6. The DDR-SDRM as defined in claim 5, wherein data latched by said
at least one first data latch is delayed by half a clock period of
said system clock signal in said latch section.
7. The DDR-SDRM as defined in claim 5, wherein said first and
second latch bnocclock period of said system clock signal with
respect to said system clock signal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a synchronous double data
rate DRAM (DDR-SDRM) and, more particularly to an improvement of
the read/write performance of a DDR-SDRM.
[0003] 2. Description of the Related Art
[0004] In a DDR-SDRM (which may be referred to as simply SDRAM,
hereinafter), the read/write operation is controlled by a system
clock signal CLK supplied as an external clock signal, whereas
read/write data are transferred between the SDRAM and the external
circuit in synchrony with a data strobe signal DQS. The data strobe
signal DQS occurs with a specified phase delay with respect to the
clock pulse in the system clock signal CLK. The pulses in both the
system clock signal CLK and the data strobe signal DQS have the
same pulse duration.
[0005] More specifically, the read/write operation is controlled in
synchrony with the system clock signal CLK whereas the latch of the
input write data is controlled with the data strobe signal DQS
supplied form outside the DRAM. The relationship between the system
clock signal CLK and the data strobe signal DQS is specified by the
JEDEC standard. For example, it is prescribed that the phase
difference between the system clock signal CLK and the data strobe
signal DQS reside between 75% and 125% of the clock period Tck of
the system clock signal CLK.
[0006] FIG. 1 shows signal flows in a conventional DDR-SDRM during
a write operation, the DDR-SDRM including a data write section 10,
data read section 30 and an address processing section not shown in
the figure. The system clock signal CLK is supplied through a clock
buffer 11 to a command decoder 12, a write timing generator 13 and
a second-stage data latch (latch section) 18, whereas the data
strobe signal DQS is supplied to a first-stage data latch (latch
section) 17. In the DDR-SDRM, a read/write operation is controlled
in synchrony with the system clock signal CLK whereas the latch of
the input data DQ at the first-stage latch 17 is controlled by the
data strobe signal DQS.
[0007] FIG. 2 shows a timing chart of the signals in the DDR-SDRM
of FIG. 1. A write cycle is started with a first pulse P1 of the
system clock signal CLK. The write data DQ and the data strobe
signal DQS are supplied from outside the SDRM. First write data DQ0
is latched at time t0 by responding to the rising edge of the data
strobe signal DQS and second write data DQ1 is latched at time t1
by the falling edge of the same data strobe signal DQS in the
first-stage latch 17. The second-stage latch 18 latches both the
write data DQ0 and DQ1 at time t2 by responding to the rising edge
of the system clock signal CLK to deliver both the write data DQ0
and DQ1 to the write buffer (write buffer section) 14. The I/O
lines 19 includes a first pair of complementary I/O lines disposed
for bit lines having even serial numbers, and a second pair of
complementary I/O lines disposed for bit line pairs having odd
serial numbers.
[0008] The write buffer 14 delivers a differential voltage signal
corresponding to the write data latched at the rising edge of the
data strobe signal DQS through one of the pairs of complementary
I/O lines 19, and delivers at the same time a differential voltage
signal corresponding to the write data latched at the falling edge
of the data strobe signal DQS through the other of the pairs of the
I/O lines 19. The former I/O lines are called herein I/O lines (R)
whereas the latter I/O lines are called herein I/O lines (F). The
potential difference of the differential voltage signal is 2.2
volts in this example. The write operation ends at time t3b at
which the I/O lines 19 are subjected to balancing or equalizing the
pair of I/O lines 19.
[0009] A read cycle is started at time t4 with a fourth pulse P4 in
the system clock signal CLK, and a read amplifier 15 delivers a
pair of differential voltage signals through the I/O lines 19, the
differential voltage assuming 0.2 volts in the read operation. The
read data DQ2 and DQ3 and the data strobe signal DQS are fed to
outside from the DDR-SDRM. The data strobe signal DOS is generated
in the DDR-SDRM during the read operation in synchrony with the
system clock signal CLK. The time interval between t3b at which the
write operation is finished and t4 at which the read operation is
started is used as a balancing time interval, during which the I/O
lines 19 are isolated from the bit lines of the memory cell plate
16.
[0010] In the above operation, the DDR-SDRM must operates for
equalizing the pair of O/O lines 19, which had a potential
difference of 2.2 volts in the write operation, during the
balancing time interval for preparing the next read operation.
However, the time length may be too short for a safe operation,
especially if the read data in the succeeding read operation is
opposite to the write data supplied in the write operation.
[0011] Assuming that the clock period Tck of the system clock
signal CLK is 6.5 ns, and coupling operation for coupling the I/O
lines and the bit lines together consumes 4 ns, then the available
time length for the balancing of the I/O lines is equal to:
6.5-4=2.5 ns.
[0012] In this case, the available balancing time interval is too
short to assure a safe isolation of the I/O lines from the bit
lines.
SUMMARY OF THE INVENTION
[0013] In view of the above problem in the conventional technique,
it is an object of the present invention to provide a DDR-SDRM,
which is capable of solving the above problem and operates at a
higher speed and with a higher reliability.
[0014] The present invention provides a DDR-SDRM including a memory
cell array including a plurality of memory cells, an address signal
processing section for receiving and processing an address signal
for specifying an address of one of the memory cells, the address
signal processing section operating in synchrony with a system
clock signal, a data write section for receiving write data and
writing the write data into the one of the memory cells in a write
cycle, and a data read section for responding to the system clock
signal to read data from the one of the memory cells in a read
cycle to deliver read data, the data write section operating based
on a data strobe signal transferred in synchrony with the write
data to receive the write data from outside the DDR-SDRM and
deliver the write data through I/O lines to the one of the memory
cells.
[0015] In accordance with the present invention, the write
operation conducted based on the data strobe signal by the data
write section assures a sufficient balancing time interval for
balancing the I/O lines, whereby a safe read data can be obtained
during a succeeding read operation.
[0016] The above and other objects, features and advantages of the
present invention will be more apparent from the following
description, referring to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a block diagram of a conventional DDR-SDRM,
showing signal flows during a write operation thereof.
[0018] FIG. 2 is a timing chart of signals in the DDR-SDRM of FIG.
1.
[0019] FIG. 3 is a block diagram of a DDR-SDRM according to a first
embodiment of the present invention, showing signal flows during a
write operation thereof.
[0020] FIG. 4 is a timing chart of signals in the DDR-SDRM of FIG.
3.
[0021] FIG. 5 is a detailed timing chart during a write operation
of a modified first embodiment.
[0022] FIG. 6 is a timing chart of signals in a DDR-SDRM according
to a second embodiment of the present invention.
[0023] FIG. 7 is a block diagram of the address processing section
provided in the DDR-SDRM of FIG. 5.
PREFERRED EMBODIMENTS OF THE INVENTION
[0024] Now, the present invention is more specifically described
with reference to accompanying drawings, wherein similar
constituent elements are designated by similar reference numerals
throughout the drawings.
[0025] Referring to FIG. 3, a DDR-SDRM according to a first
embodiment of the present invention includes a memory cell array or
memory cell plate 16, a data write section 10, a data read section
30 wherein only a read amplifier 15 is depicted, and an address
processing section not shown in the figure.
[0026] The data write section 10 includes a plurality of input
buffers 11, a command decoder 12, a write timing generator 13, a
write buffer (buffer section) 14, and a data latch (latch section)
17. The DDR-SDRM operates for read/write processing with a burst
length of two, wherein a read/write operation is conducted for a
pair of data at a single read/write cycle to memory cells having
successive addresses. The configuration of the address processing
section is shown in FIG. 7.
[0027] The input buffers 11 in the data write section 10 receive
external signals to deliver the same to respective internal
sections. The command decoder 12 receives row address strobe (RAS),
column address strobe (CAS), write enable signal (WE) and chip
select signal (CS) to deliver decoded address signal SB in
association with these signals. The write timing generator 13
generates timing control signals for the write operation. The data
latch 17 latches a pair of write data DQ at the rising edge and the
falling edge, respectively, of the data strobe signal DQS, and
supplies the latched data DQ to the write buffer 14. The write
buffer 14 delivers, through the complementary I/O lines 19, write
data to the memory cells selected based on the address signal. The
read amplifier 15 reads data through the I/O lines 19 from the
memory cell selected based on the address signal. The I/O lines 19
include two sets of complementary I/O lines including I/O lines (R)
and I/O lines (F) for transferring the pair of write data at a
time.
[0028] The memory cell plate 16 includes a plurality of memory
cells arranged in a matrix, a plurality of word lines each disposed
for a corresponding row of the memory cells. If a memory cell is
selected, a corresponding bit line pair is connected to the
complementary I/O lines 19, and a corresponding word line is
activated.
[0029] The system clock signal CLK is delivered to the command
decoder 12, whereas the data strobe signal DQS is delivered to the
write timing generator 13 and the data latch 17. In the present
embodiment, a read operation is basically controlled in synchrony
with the system clock signal CLK, whereas a write operation and
data latch of the write data are controlled in synchrony with
signals generated based on the data strobe signal DQS.
[0030] Referring to FIG. 4, the DDR-SDRM of the present embodiment
operates with a burst length of two, and starts for a write
operation at a first clock pulse P1 of the system clock signal CLK.
The write operation is performed for a pair of memory cells
specified based on input address ADD0. The write data DQ0 and DQ1
are supplied from outside the DDR-SDRM. The data strobe signal DQS
has a phase delay Td with respect to the system clock signal CLK,
the phase delay being typically equal to the clock period Tck of
the system clock signal CLK and residing between 75% and 125% of
the clock period Tck based on the JEDEC standard.
[0031] The first external write data DQ0 is latched at the rising
edge of the data strobe signal DQS at time t0 by the data latch 17,
and the second external write data DQ1 is latched at the falling
edge of the same data strobe signal DOS at time t1 by the data
latch 17. The data latch 17 delivers both the first and second
write data DQ0 and DQ1 as internal write data DI to the write
buffer (write buffer section) 14 at time t1.
[0032] The write buffer 14 responds to the internal write data DI
to deliver differential voltage signals through respective
complementary I/O lines 19 to the memory cell plate 16. Thereafter,
the I/O lines 19 are isolated from the bit line pairs at time t3a
to end the write operation. It is to be noted that the write data
DQ0 and DQ1 are transferred from the data latch 17 to the memory
cells by responding to the data strobe signal DQS without using the
system clock signal CLK.
[0033] A read operation is started at time t4 by responding to the
rising edge of a fourth pulse P4 in the system clock signal CLK.
The read data DQ2 and DQ3 and the data strobe signal DQS are
generated in the DDR-SDRM. I/O lines 19 are coupled to the bit line
pairs at time t4, followed by delivery of the differential voltage
signal by the read amplifier 15. The first read data DQ2 is
delivered at time t5 by responding to the rising edge of the data
strobe signal DQS, and the second read data DQ3 is delivered at
time t6 by responding to the falling edge of the data strobe
signal.
[0034] Assuming that the clock period Tck, the time length
necessary for coupling the complementary I/O liens 19 and the bit
lines, and the phase delay Td of the data strobe signal DQS with
respect to the system clock signal CLK are 6.5 ns, 4 ns and 8.125
ns (=Tck.times.1.25, i.e., maximum delay), respectively, the time
t3a at which the write operation is finished occurs 1.625 ns
earlier than the time t3b at which the write operation is finished
in the conventional SDRAM. Thus, a corresponding time length can be
used for the balancing time interval in addition to the time length
obtained in the conventional device.
[0035] Referring to FIG. 5, there is shown a timing chart of a
modification of the write operation shown in FIG. 4. In this
modification, the burst length is four, wherein a write operation
is performed to four memory cells in a single write cycle. Four
data latches are provided in the data latch section 17, for
latching four write data DQ0 to DQ3 in a single write cycle. The
four write data are written into the memory cells specified by a
single external address signal which indicates the first address
ADD0 among the four addresses ADD0 to ADD3. Command signals and the
external address signal are supplied from outside the memory device
at time t11, whereby the write cycle for four memory cells is
started.
[0036] The synchronizing signal SA is generated by responding to
the rising edge of the system clock signal CLK, and supplied to the
command decoder 12. The latch signals SC are generated at the
rising edges and the falling edges of the data strobe signal DQS,
and supplied to the data latch (latch section) 17 for latching. The
timing signal SD is generated in synchrony with the falling edge of
the system clock signal CLK, and supplied to the write timing
generator 13.
[0037] The command decoder 12 responds to the synchronizing signal
SA to decode the command signal, whereby the decoded signal
indicating the write operation is supplied to the write timing
generator 13. The data latch 17 latches the first data DQ0 at the
rising edge of the first pulse P11 of the latch signal SC to hold
the same as an internal data (R) for the I/O lines (R), latches the
second data DQ1 at the falling edge of the first pulse P11 to hold
the same as an internal data (F) for the I/O lines (F), latches the
third data DQ2 at the rising edge of the second pulse P12 to hold
the same as an internal data (R) for the I/O lines (R), and latches
the fourth data DQ3 at the falling edge of the second pulse P12 to
hold the same as an internal data (F) for the I/O lines (F). These
latched data are delivered to the write buffer (write buffer
section) 14 immediately after the latching. The write timing
generator 13 supplies a write timing signal SE to the write buffer
14 in synchrony with the timing signal SD.
[0038] Referring to FIG. 7, the address processing section includes
an address buffer 21 for receiving the external address signal,
first and second latches 22 and 23 each for responding to the
synchronizing signal SA to transfer the external address, an
address latch section 24 for responding to the timing signal SD, a
counter 26 and a column decoder 26.
[0039] The address latch section 24 latches the external first
address ADD0 in synchrony with the system clock signal CLK through
the internal address bus and the first and second latches 22 and
23, and delivers the first address ADD0 through a second internal
address bus at the next clock cycle to the internal counter 25. The
internal counter 25 generates second to fourth addresses ADD1 to
ADD3 following the first address ADD0 and delivers first to fourth
addresses ADD0 to ADD3 to the address latch section 24. The column
decoder 26 then latches first and third addresses ADD0 and ADD2
through the internal address bus (R), and latches second and fourth
addresses ADD1 and ADD3 through another internal address bus (F),
by responding to the timing signal SD. The timing signal SD is
generated at the falling edge of the data strobe signal DQS, and
the write timing signal SF is generated based on the timing signal
SD. More specifically, the write timing signal SE rises at the
falling edge of the timing signal SD and falls at the rising edge
of the timing signal SD a shown in FIG. 5.
[0040] The write buffer 14 delivers, at a first pulse P21 of the
write timing signal SE, the first data DQ0 through I/O lines (R) to
the memory cell of the first address ADD0, and the second a data
DQ1 through I/O lines (F) to the memory cell of the second address
ADD1. The write buffer 14 then delivers, at a second pulse P22 of
the write timing signal SE, the third data DQ2 through I/O lines
(R) to the memory cell of the third address ADD2, and the fourth
data DQ3 through I/O lines (F) to the memory cell of the fourth
address ADD3.
[0041] In the above embodiment, since the write buffer 14 operates
for writing in synchrony with the signal generated based on the
data strobe signal DQS, a longer time length can be secured for
balancing the I/O lines 19 compared to the case wherein the write
buffer 14 operates for writing in synchrony with the system clock
signal CLK, as in the conventional DDR-SDRM. Thus, a read operation
following the write operation does not cause an error due to the
smaller balancing time interval.
[0042] Referring to Fig, 6, there is shown another timing chart of
signals in a DDR-SDRM according to a second embodiment of the
present invention. In the present embodiment, the input write data
are supplied through two sets of input sections. The DDR-SDRM has
two sets of write buffers 14, two sets of read amplifiers 15, and
two sets of data latches for each of data (R) and (F) corresponding
to the two sets of input sections for a single memory cell plate
16. Each set includes eight constituent elements, for example, for
processing 64 write data during a single write cycle. These two
sets of constituent elements are designated by U-side (upper side
in the sequential order) and L-side (lower side in the sequential
order) constituent elements in the drawing.
[0043] It is to be noted that the number of the input terminals for
write data is sixteen, wherein the lower serial numbers (0 to 7) of
the input terminals corresponds to L-side set and higher serial
numbers (8 to 16) of the input terminals corresponds to U-side set.
Thus, U-side input section receives 32 write data, designated by
DQU0 to DQU3 in FIG. 6 each including eight data, in synchrony with
pulses P31 and P32 in the data strobe signal UDQS, whereas L-side
input section receives 32 write data, designated by DQL0 to DQL3 in
FIG. 6 each including eight data, in synchrony with pulses P41 and
P42 in the data strobe signal LDQS.
[0044] In the example of FIG. 6, U-side data strobe signal UDQS
leads with respect to L-side data strobe signal LDQS by half the
clock period Tck of the system clock signal CLK. A write operation
is started at time t11 for processing write data with a burst
length of four by inputting an address signal ADD0. The first
through fourth U-side write data DQU0 to DQU3 are supplied
beginning at time t12a, with two successive pulses P31 and P32 of
the U-side data strobe signal UDQS. In addition, the first through
fourth L-side write data DQL0 to DQL3 are supplied beginning at
time t13b with two successive pulses P41 and P42 of the L-side data
strobe signal LDQS, having a delay of half the clock period Tck
with respect to the U-side write data. The U-side and L-side data
are separately processed by the two sets of processing
sections.
[0045] Latch signals USC are supplied to the U-side data latches in
synchrony with rising edges and falling edges of both the pulses
P31 and P32 of the U-side data strobe signal UDQS. Similarly, latch
signals LQS are supplied to the L-side data latches in synchrony
with rising edges and falling edges of both the pulses P41 and P42
of the L-side data strobe signal LDQS.
[0046] A phase delay detection section detects both the phase
delays of the U-side and L-side data strobe signals UDQS and LDQS
with respect to the system clock signal CLK, and determines which
delay of one of the data strobe signals UDQS and LDQS is longer
with respect to the delay of the other of the data strobe signals
UDQS and LDQS. The phase delay detection section selects the data
strobe signal having a longer phase delay Td, and delivers a timing
signal SD in synchrony with the falling edge of the selected data
strobe signal to the write timing generator 13.
[0047] Each of the first U-side data latches 17 latches the first
and third U-side data DQU0 and DQU2, while delaying the data latch
by half the clock period Tck, as a set of internal data UDQ(2R).
Each of the second U-side data latches 17 latches the second and
fourth data DQU1 and DQU3 without a delay as a set of internal data
UDQ(F). Thus, two sets of U-side data UDQ(2R) and UDQ(F) are
delivered to the write buffer 14 at the same time.
[0048] On the other hand, each of the first L-side data latches 17
latches the first and third L-side data DQL0 and DQL2 while
delaying the data latch by half the clock period Tck, as a set of
internal data LDQ(2R). Each of the second U-side data latches 17
latches the second and fourth data DQL1 and DQL3 without a delay as
a set of internal data LDQ(F). Thus, two sets of L-side data
LDQ(2R) and LDQ(F) are delivered to the write buffer 14 at the same
time, the L-side data LDQ(2R) and LDQ(F) being delayed by half the
clock cycle Tck with respect to the U-side data UDQ(2R) and
UDQ(F).
[0049] The write buffer 14 delivers data, at a first pulse P61 of
the write timing signal SE, data DQU0 and DQL0 to the memory cells
of the addresses ADDU0 and ADDL0, respectively, via U- and L-side
I/O lines (R). The write buffer also delivers data, at the first
pulse P61 of the write timing signal SE, data DQU1 and DQL1 to the
memory cells of the addresses ADDU1 and ADDL1, respectively, via
U-side and L-side I/O lines (F).
[0050] The write buffer 14 delivers data, at a second pulse P62 of
the write timing signal SE, data DQU2 and DQL2 to the memory cells
of the addresses ADDU2 and ADDL2, respectively, via U- and L-side
I/O lines (R). The write buffer also delivers data, at the second
pulse P62 of the write timing signal SE, data DQU3 and DQL3 to the
memory cells of the addresses ADDU3 and ADDL3, respectively, via
U-side and L-side I/O lines (F).
[0051] The internal addresses are generated by the internal address
counter similarly to the first embodiment.
[0052] In the second embodiment, the write operation of the first
through fourth data are started by responding to the data strobe
signal LDQS which occurs among the data strobe signals UDQS and
LDQS supplied to the DDR-SDRM. This assures a safe writing
operation, while assuring a sufficient time length for the
balancing time interval of the I/O lines to prepare a next read
operation.
[0053] In the second embodiment, if internal latches are provided
in the data latch section 17 for latching all the data at a time,
the write operation can be performed more safely.
[0054] In another alternative, if the input of the write data is
conducted at a time for all the write data, the write operation can
be performed further safely.
[0055] In the second embodiment, if a precharge command is
generated in the write operation, the potentials of the word lines
are lowered after a specified time interval since the generation of
the command to store data in the memory cell. Then, bit lines are
balanced or equalized for preparing the next write or read command
In the second embodiment, since a sufficient balancing time
interval is assured, the time interval between the precharge
command and the command for the write or read command.
[0056] Since the above embodiments are described only for examples,
the present invention is not limited to the above embodiments and
various modifications or alterations can be easily made therefrom
by those skilled in the art without departing from the scope of the
present invention.
* * * * *