U.S. patent application number 11/731058 was filed with the patent office on 2007-07-26 for method and system for low power refresh of dynamic random access memories.
This patent application is currently assigned to Micron Technology, Inc.. Invention is credited to Greg A. Blodgett, Donald M. Morgan.
Application Number | 20070171753 11/731058 |
Document ID | / |
Family ID | 28790654 |
Filed Date | 2007-07-26 |
United States Patent
Application |
20070171753 |
Kind Code |
A1 |
Morgan; Donald M. ; et
al. |
July 26, 2007 |
Method and system for low power refresh of dynamic random access
memories
Abstract
A method and system for operating a DRAM device in either a high
power, full density mode or a low power, half density mode. In the
full density mode, each data bit is stored in a single memory cell,
and, in the half density mode, each data bit is stored in two
memory cells that are refreshed at the same time to permit a
relatively slow refresh rate. When transitioning from the full
density mode to the half density mode, data are copied from each
row of memory cells storing data to an adjacent row of memory
cells. The adjacent row of memory cells are made free to store data
from an adjacent row by remapping the most significant bit of the
row address to the least significant bit of the row address, and
then remapping all of the remaining bits of the row address to the
next highest order bit.
Inventors: |
Morgan; Donald M.;
(Meridian, ID) ; Blodgett; Greg A.; (Nampa,
ID) |
Correspondence
Address: |
Edward W. Bulchis, Esq.;DORSEY & WHITNEY LLP
Suite 3400
1420 Fifth Avenue
Seatle
WA
98101
US
|
Assignee: |
Micron Technology, Inc.
Boise
ID
|
Family ID: |
28790654 |
Appl. No.: |
11/731058 |
Filed: |
March 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11402479 |
Apr 11, 2006 |
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11731058 |
Mar 30, 2007 |
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10753895 |
Jan 8, 2004 |
7072237 |
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11402479 |
Apr 11, 2006 |
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10122943 |
Apr 11, 2002 |
6751143 |
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10753895 |
Jan 8, 2004 |
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Current U.S.
Class: |
365/222 |
Current CPC
Class: |
G11C 11/40622 20130101;
G11C 5/14 20130101; G11C 11/406 20130101; G11C 2211/4067 20130101;
G11C 2211/4061 20130101; G11C 11/408 20130101 |
Class at
Publication: |
365/222 |
International
Class: |
G11C 7/00 20060101
G11C007/00 |
Claims
1-39. (canceled)
40. A method of operating a DRAM device in either a first
relatively higher power mode or a second, relatively higher power
mode, comprising: when operating in the first mode, refreshing rows
of memory cells in an array one-row-at-a-time at a first rate; when
operating in the second mode, refreshing rows of memory cells in
the array multiple rows-at-a-time at a second rate that is slower
than the first rate; and when switching from operation in the first
mode to operation in the second mode, transferring data from each
row of the array in which data are stored to at least one other row
of memory cells.
41. The method of claim 40 wherein the act of transferring data
from each row of the array in which data are stored to at least one
other row of memory cells comprises transferring data from each row
of the array in which data are stored to a respective adjacent row
of memory cells.
42. The method of claim 40 wherein the act of transferring data
from each row of the array in which data are stored to at least one
other row of memory cells comprises: activating a word line for the
row in which data are stored thereby coupling each of the memory
cells in the row to one of a pair of respective complimentary digit
lines; sensing the voltage between each of the pairs of
complimentary digit lines using a respective sense amplifier that
drives the differential voltage between the complimentary digit
lines to a predetermined voltage; and while each of the sense
amplifiers is driving the predetermined voltage between the
respective pair of complimentary digit lines, activating a
respective word line for the at least one other row of memory cells
thereby coupling one of the digit lines in each of the pairs of
complimentary digit lines to the respective memory cell in the at
least one other row of memory cells.
43. The method of claim 42 wherein the act of activating a word
line for the at least one other row of memory cells thereby
coupling one of the digit lines in each of the pairs of
complimentary digit lines to the respective memory cell in the at
least one other row of memory cells comprises activating a word
line for a respective adjacent row of memory cells thereby coupling
one of the digit lines in each of the pairs of complimentary digit
lines to the respective memory cell in the respective adjacent row
of memory cells.
44. The method of claim 40 wherein the act of refreshing rows of
memory cells in the array multiple rows-at-a-time comprises
simultaneously refreshing adjacent rows of memory cells.
45. The method of claim 40 wherein the DRAM device includes a pair
of complimentary digit lines for each column of the array, and
wherein, when operating in the second mode, the act of refreshing
rows of memory cells in the array multiple rows-at-a-time comprises
simultaneously coupling a memory cell in each column of one row of
memory cells to the same digit line to which at least one memory
cell in the same column of the at least one other row of memory
cells is coupled.
46. The method of claim 40 wherein the DRAM device includes a pair
of complimentary digit lines for each column of the array, and
wherein, when operating in the second mode, the act of refreshing
rows of memory cells in the array multiple rows-at-a-time comprises
simultaneously coupling a memory cell in each column of one row of
memory cells to a different digit line from which at least one
memory cell in the same column of the at least one other row of
memory cells is coupled.
Description
TECHNICAL FIELD
[0001] This invention relates to dynamic random access memory
devices, and, more particularly, to a method and system for
allowing a memory device to be quickly and easily switched into and
out of a low power, half density, operating mode.
BACKGROUND OF THE INVENTION
[0002] As the use of electronic devices, such as personal
computers, continue to increase, it is becoming ever more important
to make such devices portable. The usefulness of portable
electronic devices, such as notebook computers, is the limited by
the limited length of time batteries are capable of powering the
device before needing to be recharged. This problem has been
addressed by attempts to increase battery life and attempts to
reduce the rate at which such electronic devices consume power.
[0003] Various techniques have been used to reduce power
consumption in electronic devices, the nature of which often
depends upon the type of power consuming electronic circuits that
are in the device. For example, electronic devices, such a notebook
computers, typically include dynamic random access memory ("DRAM")
devices that consume a substantial amount of power. As the data
storage capacity and operating speeds of DRAM devices continues to
increase, the power consumed by such devices has continued to
increase in a corresponding manner.
[0004] A variety of operations are performed in DRAM devices, each
of which affects the rate at which the DRAM device consumes power.
One operation that tends to consume power at a substantial rate is
refresh of memory cells in the DRAM device. As is well-known in the
art, DRAM memory cells, each of which essentially consists of a
capacitor, must be periodically refreshed to retain data stored in
the DRAM device. Refresh is typically performed by essentially
reading data bits from the memory cells in each row of a memory
cell array and then writing those same data bits back to the same
cells in the row. This refresh is generally performed on a
row-by-row basis at a rate needed to keep charge stored in the
memory cells from leaking excessively between refreshes. Since
refresh essentially involves reading data bits from and writing
data bits to a large number of memory cells refresh tends to be a
particularly power-hungry operation. Thus many attempts to reduce
power consumption in DRAM devices have focused on reducing the rate
at which power is consumed during refresh.
[0005] The amount of power consumed by refresh also depends on
which of several refresh modes is active. A Self Refresh mode is
normally active during periods when data are not being read from or
written to the DRAM device. Since portable electronic devices are
often inactive for substantial periods of time, the amount of power
consumed during Self Refresh can be an important factor in
determining how long the electronic device can be used between
battery charges.
[0006] One technique that has been used to reduce the amount of
power consumed by refreshing DRAM memory cells is to vary the
refresh rate as a function of temperature. As is well known in the
art, the rate at which charge leaks from a DRAM memory cell
increases with temperature. The refresh rate must be sufficiently
high to ensure that no data is lost at the highest temperature in
the specified range of operating temperatures of the DRAM device.
Yet, DRAM devices normally operate at a temperature that is
substantially lower than their maximum operating temperature.
Therefore, DRAM devices are generally refreshed at a rate that is
higher than the rate actually needed to prevent data from being
lost, and, as a result, unnecessarily consume power. To address
this problem, some commercially available DRAM devices allow the
user to program a mode register to select a lower maximum operating
temperature. The DRAM device then adjusts the refresh rate to
correspond to the maximum operating temperature selected by the
user.
[0007] Although adjusting the refresh rate as a function of
temperature does reduce the rate of power consumed by refresh, it
nevertheless still allows power to be consumed at a significant
rate for several reasons. For example, although the refresh rate
may be reduced with reduced maximum operating temperature, the
refresh may still result in refreshing a large number of memory
cells that are not actually storing data.
[0008] Another approach to reducing the rate at which power is
consumed by a refresh operation is to refresh less than all of the
memory cells in the DRAM device in attempt to refresh only those
memory cells needed to store data for a given application. As
described in U.S. Pat. No. 5,148,546 to Blodgett, a software
program being executed in a computer system containing the DRAM
devices is analyzed to determine the data storage requirements for
the program. The DRAM device then refreshed only those rows of
memory cells that are needed to store data. In another approach,
the DRAM device may be operated in a partial array self refresh
("PASR") mode. In the PASR mode, a mode register is programmed by a
user to specify a bank or portion thereof of memory cells that will
be used and thus must be refreshed. The remaining memory cells are
not used and thus need not be refreshed during at least some
refresh modes. Although these techniques for refreshing less than
all of the memory cells in a memory device can substantially reduce
the rate of power consumption, it can nevertheless require a
substantial amount of power to refresh the cells that are to be
refreshed.
[0009] Still another technique that can be used to reduce the rate
of refresh involves operating DRAM devices in a half density mode.
A DRAM device that may be operated in a half density mode is
described in U.S. Pat. No. 5,781,483 to Shore. In the half density
mode, the low order bit of each row address, which normally
designates whether the addressed row is even or odd, is ignored,
and both the odd row and adjacent even row are addressed for each
memory access. In a folded digit line architecture, activating an
odd row will couple each memory cell in the row to a respective
digit line, and activating an even row will couple each memory cell
in the row to a respective complimentary digit line. Thus, for
example, writing a "1" to an addressed row and column would result
in writing a logic "1" voltage level to the memory cell in the
addressed odd row and writing a logic "0" logic level to the memory
cell in the addressed even row. Reading from the addressed row and
column results in a logic "1" voltage level being applied to the
digit line for the addressed column and a logic "0" voltage level
being applied to the complimentary digit line for the addressed
column. Therefore, in the half density mode, a sense amplifier
coupled to the digit line and complimentary digit line for each
column receives twice the differential voltage that it normally
receives when reading a memory cell at an addressed row and
column.
[0010] The patent to Shore describes the use of the half density
mode for the purpose of allowing the DRAM device to be used despite
the presence of defective memory cells. If a memory cell in an
addressed row and column is defective, the data bit stored in that
memory cell can still be recovered from the other memory cell in
the addressed row and column. However, it has been recognized that
the half density mode can be used to reduce that rate at which
power is consumed during refresh. Although a refresh in the half
density mode requires twice as many memory cells to be refreshed
for a given amount of stored data, the required refresh rate is
less than half the required refresh rate when the DRAM device is
operating in the full density mode. The substantially lower refresh
rate required in the half density results from the increased
differential voltage that is applied to the sense amplifiers in the
half density mode, as previously explained. As a result, the memory
cells can be allowed to discharge to a greater degree between
refreshes without the data bits stored therein being lost.
Therefore, storing data in the half density mode can reduce the
rate of power consumption during refresh
[0011] In conventional DRAM devices, the density mode, i.e., either
half or full, is generally determined prior to sale of the device.
If the power consumption of the DRAM device is of concern, the half
density mode can be selected. Otherwise, the full density mode can
be selected. Yet many power management algorithms for electronic
devices containing DRAM devices, such as notebook computers, switch
to a low power mode when the electronic device is inactive and back
to a high power mode when the electronic device is active. It is
therefore necessary for electronic devices to be able to frequently
switch back and forth between low power and high power modes.
[0012] In conventional DRAM devices, it is not possible to switch
between a full density mode and a half density mode. This
limitation may be due to the difficulty in making this transition.
The difficulty of being able to rapidly switch between the full
density mode and the half density mode primarily results from two
requirements. First is the need to first free-up alternate rows of
memory cells into which data from an adjacent row of memory cells
can be transferred for half density storage. The second requirement
is the need to transfer data from the memory cells in a row storing
data to a memory cell in the adjacent row once the adjacent row has
been freed up by transferring data to another row. More
particularly, if the DRAM device is operating in the full density
mode, generally data will be stored in both even rows and odd rows
of memory cells. To switch to the half density mode would require
that the data stored in the even rows of memory cells, for example,
be transferred to empty odd rows of memory cells. It would then be
necessary to read the data stored in each odd row, and write the
read data to corresponding memory cells in the adjacent even row.
Transferring data between memory cells in this manner by
conventional read/write operations would require a great deal of
time and would therefore preclude quickly switching back and forth
between the full density mode and the half density mode. Also,
transferring approximately half of the data stored in the DRAM
device by conventional read/write operations, which would be
necessary to switch from the full density mode to the half density
mode, would itself consume a great deal of power. While more
efficient row copy schemes have been proposed for test purposes,
such as the row copy scheme described in U.S. Pat. No. 5,381,368 to
Morgan et al., these row copy schemes are generally suitable only
when the same data or a repeating pattern of data are to be written
to the entire array of memory cells. Yet switching from the full
density mode to the half density mode would require transferring
many rows of disparate data bits to respective adjacent rows after
freeing up the adjacent rows by transferring the disparate data
bits to other rows. It therefore does not seem possible to easily
transition between the half density mode and the full density
mode.
[0013] There is therefore a need for a power-saving technique that
would allow switching into and out of a half density, low refresh
rate mode without requiring time and power consuming reading and
writing of data to a second set of memory cells.
SUMMARY OF THE INVENTION
[0014] A system and method according to the invention allows a DRAM
device to be easily and quickly switched back and forth between a
full density mode consuming power at a relatively fast rate and a
half density mode consuming power at a relatively slow rate. The
row addresses applied to the DRAM device are reordered by remapping
the most significant bit of each row address to the least
significant bit of the row address during all operating modes. As a
result, all of the odd (or even) rows of the DRAM array are
populated with data before any of the even (or odd) rows are
populated with data. As long as the data stored in the DRAM device
uses less than half of the capacity of the DRAM device, data will
then be stored only in alternate rows, and the row adjacent each
row in which data are stored will be free to store data. When the
DRAM device is to be switched from the full density mode to the
half density mode, data stored in each row is simply transferred to
the adjacent row. Thereafter when operating in the half density
mode, the row corresponding to each row address and the adjacent
row are accessed at the same time. Although the data stored in each
row can be transferred to the adjacent row by a variety of
techniques, it is preferably transferred by transferring the data
from each row to the adjacent row during the first refresh of the
row. More particularly, when a row is first refreshed after the
DRAM device has been switched to the half density mode, the sense
amplifiers are left active so that the voltage levels corresponding
to the data stored in the memory cells being refreshed are
maintained on the respective digit line pairs. The adjacent row is
then activated thereby transferring the voltage on the digit lines
to the memory cells in the adjacent row. Once the data have been
transferred to the adjacent rows during refresh at the full density
refresh rate, the refresh rate can be significantly reduced during
operation in the half density mode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a memory map showing the addressing scheme of a
DRAM device operating in a conventional manner.
[0016] FIG. 2 is a memory map showing the addressing scheme of a
DRAM device operating according to one embodiment of the invention
by reordering row addresses.
[0017] FIG. 3 is a specific example of a memory map using the row
addressing scheme shown in the memory map of FIG. 2.
[0018] FIG. 4 is a block diagram and schematic of one embodiment of
a system and method for allowing a DRAM device to be operated in a
low power, half density mode in a manner that allows switching back
and forth to a high power, full density mode.
[0019] FIG. 5 is a block diagram and schematic of one embodiment of
a memory array topography that may be used in the DRAM device of
FIG. 4.
[0020] FIG. 6 is a block diagram and schematic of another
embodiment of a memory array topography that may be used in the
DRAM device of FIG. 4.
[0021] FIG. 7 is a flowchart showing the operation of the DRAM
device of FIG. 4 when transitioning from the high power, full
density mode to the low power, half density mode.
[0022] FIG. 8 is a flowchart showing the operation of the DRAM
device of FIG. 4 in the low power, half density mode.
[0023] FIG. 9 is a flowchart showing the operation of the DRAM
device of FIG. 4 when transitioning from the low power, half
density mode to the high power, full density mode, and the
continued operation in the high power, full density mode.
[0024] FIG. 10 is a block diagram of one embodiment of a computer
system using the DRAM device of FIG. 4.
DETAILED DESCRIPTION OF THE INVENTION
[0025] A memory map 10 for a conventional DRAM device (not shown)
is shown in FIG. 1. The memory map 10 illustrates how the physical
locations in an array of memory cells (not shown) in the DRAM
device correspond to row and column addresses that may be applied
to the DRAM device. The row and column addresses are normally
binary numbers that are decoded by the DRAM to select the
corresponding row or column. It is assumed for purposes of
illustration that the array of memory cells that will be discussed
with reference to FIG. 1 consists of M+1 rows and N+1 columns. The
memory cell that is in the upper left hand corner is selected by a
row address of 0 and a column address of 0. The memory cell in the
upper right hand corner is selected by a row address of 0 and a
column address of N. The memory cell in the lower left hand corner
is selected by a row address of M and a column address of 0. The
memory cell in the lower right hand corner is selected by a row
address of M and a column address of N. As shown in FIG. 1, the
rows are physically arranged in sequential order, and so are the
row addresses. Therefore, the selected rows identically correspond
to the row addresses.
[0026] Data are often written to the rows of memory cells in a DRAM
array in numerical order. As a result, data written to the DRAM
device first populates the memory cells in row 0, then the memory
cells in row 1, then the memory cells in row 2, etc. The presence
of valid data in adjacent rows is the primary reason why it would
be very time consuming to switch from the full density mode to the
half density mode, as previously explained. Since data will
generally be stored in row 1, it would not be possible to simply
transfer the data from row 0 to row 1. Instead, that data stored in
the memory cells in row 1 must first be read from row 1 and then
written to unused memory cells in another row. Only then can the
data in row 0 be transferred to row 1. As mentioned earlier,
transferring a large block of data in this manner is time consuming
and requires a relatively large amount of power.
[0027] A memory map 20 showing the organization of memory cells in
a DRAM device according to one embodiment of the invention is shown
in FIG. 2. In this embodiment of a DRAM device, the row addresses
are remapped so that the lowest numbered row addresses select all
of the even rows, and the highest numbered row addresses select all
of the odd rows. Thus, the even rows are activated responsive to
row addresses ("RA") from RA=0 to RA=((M+1)/2)-1, and the odd rows
are activated responsive to row addresses from RA=(M+1)/2 to RA=M.
Significantly, data written to the DRAM device normally first
populates the memory cells in row 0, then the memory cells in row
2, then the memory cells in row 4, etc. Data would not be written
to any of the odd rows in the array until more than half the memory
capacity of the array was being used. Therefore, as long as less
than half the capacity of the array is being used, data would be
stored only in the even rows, and the adjacent odd rows would be
available to store a redundant copy of the data stored in the even
rows.
[0028] One technique for organizing a DRAM array as shown by the
memory map 20 of FIG. 2 is to reorder the bits of the row address
by mapping the most significant bit ("MSB") of the row address to
the least significant bit ("LSB") of the row address, and then
mapping all of the remaining bits of the row address to the next
highest order bit. Thus, a row address containing bits "N, N-1, N-2
. . . 2, 1, 0" would be remapped to a row address of "N-1, N-2 . .
. 2, 1, 0, N" where N is the MSB and 0 is the LSB of the original
row address. For example, consider an array containing 64 rows
(actual DRAM arrays would, of course, generally include many more
rows of memory cells). A memory map 30 for such a DRAM array is
shown in FIG. 3. The row addresses applied to the DRAM device would
be mapped as shown in Table 1: TABLE-US-00001 TABLE 1 Original
Original Mapped Mapped Row Addr. Row Addr. Row Addr. Row Addr.
(Binary) (Decimal) (Binary) (Decimal) 0, 0, 0, 0, 0, 0 0 0, 0, 0,
0, 0, 0 0 0, 0, 0, 0, 0, 1 1 0, 0, 0, 0, 1, 0 2 0, 0, 0, 0, 1, 0 2
0, 0, 0, 1, 0, 0 4 0, 0, 0, 0, 1, 1 3 0, 0, 0, 1, 1, 0 6 0, 1, 1,
1, 1, 0 30 1, 1, 1, 1, 0, 0 60 0, 1, 1, 1, 1, 1 31 1, 1, 1, 1, 1, 0
62 1, 0, 0, 0, 0, 0 32 0, 0, 0, 0, 0, 1 1 1, 0, 0, 0, 0, 1 33 0, 0,
0, 0, 1, 1 3 1, 1, 1, 1, 1, 0 62 1, 1, 1, 1, 0, 1 61 1, 1, 1, 1, 1,
1 63 1, 1, 1, 1, 1, 1 63
[0029] As shown in Table 1, consecutive row addresses are mapped to
addresses for consecutive even rows until row address 31, which is
mapped to row address 62. At this point, half of the rows have been
mapped. Row address 32, the next row address in chronological
sequence, is mapped to row 1. Thus, it is not until row 32 has been
addressed that data are stored in any odd row. Thereafter,
consecutive row addresses are mapped to consecutive odd rows until
row 63 is mapped to row address 63.
[0030] Organizing the memory as explained with reference to FIGS. 2
and 3 provides several advantages. First, as previously explained,
data are not stored in the memory cells of the odd rows until at
least half the memory capacity of the DRAM array has been used. At
this point, of course, the half density mode could not be used
because the capacity of the DRAM array in the half density mode is
only half the capacity in the normal mode. The absence of data in
the memory cells of the odd rows leaves the odd rows available to
store data from the adjacent even rows, which can be done quickly
and efficiently in a row copy procedure that will be explained
below. The second advantage of the above-described memory storage
organization is that it is not necessary to alter the memory
storage organization when switching from the half density mode to
the full density mode. More specifically, the data stored in the
DRAM array is accessible at the same addresses in the full density
mode as it was in the half density mode. Therefore, there is no
need to transfer data from one row of memory cells to another when
transitioning from the half density mode to the full density
mode.
[0031] One embodiment of a DRAM device 40 according to one
embodiment of the invention is shown in FIG. 4. The DRAM device 40
is shown in simplified form, it being understood that DRAM devices
typically include a large number of other components, which have
been omitted from FIG. 4 in the interests of brevity and clarity.
The DRAM device 40 includes a DRAM memory array 44 of conventional
design. The array 44 includes memory cells arranged in rows and
columns. The rows of memory cells are activated by a respective
word line, two of which 46, 48 are shown in FIG. 4, although it
will be understood that the memory array 44 will normally include a
much larger number of word lines. The word line 46 is an
even-numbered word line, which, like all of the other even-numbered
word lines, is coupled to a respective even row decoder 50.
Similarly, the word line 48 is an odd-numbered word line that is
connected to a respective odd row decoder 52 along with all of the
other odd-numbered word lines. The row decoders 50, 52 receive
either external row addresses through an address bus 56 or internal
row addresses from a row address counter 58 of conventional design.
The row decoders 50, 52 activate each of a large number of word
lines for respective rows of the array 44. The row decoders 50, 52
also map the most significant bit of the row address to the least
significant bit of the row address, and they also map all of the
remaining bits of the row address to the next highest order bit, as
previously explained. However, it will be understood that this
mapping or reordering function can alternatively be accomplished by
other components in the DRAM device 40.
[0032] Specific memory cells in an active row are selected by a
column decoder 66 responsive to either an external column address
received through the address bus 56 or internal column addresses
received from a column address counter 68. The column address
counter 68 is used in a burst mode to sequentially access several
columns starting from a column designated by an externally applied
column address. Data from memory cells selected by row and column
addresses are coupled between the memory array 44 and a data bus 70
by an Input/Output Control circuit 72.
[0033] One embodiment of the memory array 44 is shown in greater
detail in FIG. 5. The even numbered word lines 46a,b are coupled to
the gates of respective access transistors 74a,b, and the
odd-numbered word lines 48a,b are coupled to the gates of
respective access transistors 76a,b. The access transistors 74a,
76b are coupled between a digit line 80 and a respective memory
cell capacitor 82, 84. The opposite plate of the memory cell
capacitors 82, 84 normally constitute a common "cell plate" that is
biased at 0.5 V.sub.CC. Similarly, the access transistors 74b, 76a
are coupled between a complementary digit line 86 and a respective
memory cell capacitor 90, 92. The digit lines 80, 86 are coupled to
a sense amplifier 96, which outputs data on a data line 98. One
sense amplifier 96 is provided for each column of memory cells in
the array 44, and the sense amplifiers 96 may be included in the
Input/Output Control circuit 72.
[0034] In operation in the normal mode, data bits are written to
the memory cell capacitors 82, 84, 90, 92 by causing one of the row
decoders 50, 52 to actuate one of the word lines 46, 48 and then
driving one of the digit lines 80, 86 to either 0 volts or
V.sub.CC. The voltage on one of the digit lines 80, 86 is thereby
transferred to one of the memory cell capacitors 82, 84, 90, 92.
Data bits are read from the memory cell capacitors 82, 84, 90, 92
by equilibrating the digit lines 80, 86 to 0.5 V.sub.CC, then
causing one of the row decoders 50, 52 to actuate one of the word
lines 46, 48, and then enabling the sense amplifier 96. The charge
of the memory cell capacitor, which is at either 0 volts or
V.sub.CC (or some voltage between 0 volts and V.sub.CC if the
memory cell has not been refreshed recently), is then coupled
through one of the access transistors 74, 76 to one of the digit
lines 80, 86. The capacitor then charges or discharges the digit
line 80, 86 below or above 0.5 V.sub.CC. The other digit line 80,
86 that is not coupled to a memory cell capacitor will remain at
the 0.5 V.sub.CC voltage to which it was originally set during
equilibration. The sense amplifier 96 responds to the increase or
decrease in voltage coupled to one of the digit lines 80, 86 by
driving the digit lines 80, 86 to opposite voltages (0 volts and
V.sub.CC) and outputs a corresponding data bit on the data line
98.
[0035] As previously explained, charge can leak from the memory
cell capacitors 82, 84, 90, 92 so that the data bits stored therein
become unreadable unless they are refreshed at a fairly frequent
interval. During refresh, each of the word lines 46, 48 is
sequentially activated and the sense amplifier 96 for each column
is energized to recharge or discharge the memory cell capacitors
82, 84, 90, 92. Because of the large number of memory cells in a
conventional DRAM array 44, refreshing in the memory cells can
require substantial current.
[0036] In operation in the low power, half density mode, data bits
are stored in the memory cell capacitors 82, 84, 90, 92 in the same
manner as described for the normal operating mode. However, in
order for the DRAM device 40 to be operable in the half density
mode, the data stored in the DRAM device 40 must occupy less than
half of its capacity. Under the circumstances, data will be stored
only in the memory cells coupled to the even-numbered word lines.
The data bit stored in each memory cell in each of the
even-numbered rows is written to the memory cell in the same column
of the adjacent odd-numbered row. Thus, for example, if the memory
cell capacitor 82 has been charged to V.sub.CC indicative of a
binary "1" data bit stored in the memory cell capacitor 82, the
memory cell capacitor 92 in the adjacent odd-numbered row will be
discharged to 0 volts. Charging the memory cell capacitor 92 to 0
volts is also indicative of a binary "1" data bit stored in the
memory cell capacitor 92 since the memory cell capacitor 92 is
coupled to the complementary digit line 86. When data are read from
the memory array 44, the even-numbered word line 46a and the
odd-numbered word line 48a are activated at the same time. The
charge on the memory cell capacitor 82, which is at V.sub.CC, is
then coupled through the access transistor 74a to the digit line
80, and the lack of charge on the memory cell capacitor 92, which
is at 0 volts, is then coupled through the access transistors 76a
to the complimentary digit line 86. The differential voltage
applied to the sense amplifier 96 will thus be twice the voltage
applied to the sense amplifier during a read operation in the
normal operating mode. As a result of this increased differential
voltage applied to the sense amplifier 96, the charge on the memory
cell capacitors 82, 92 can be permitted to change to a greater
extent without a loss of data. The time between refreshing the
memory cell capacitors 82, 92 can therefore be substantially
increased. Alternatively, data could be stored in the full density
mode only in the memory cells coupled to the odd-numbered word
lines, and, in transitioning to the half density mode, the data
could be transferred to the memory cells coupled to the
even-numbered memory cells.
[0037] Another embodiment of a memory array 44'' is shown in FIG. 6
in which the component shown therein have been designated using the
same reference numerals that were used to designate those same
components in FIG. 5. The array 44'' of FIG. 6 differs from the
array 44' of FIG. 5 by coupling the memory cells in adjacent rows
to the same digit line 80, 86 rather than two different
complementary digit lines, as shown in FIG. 5. The memory array
44'' functions in substantially the same manner as the memory array
44' in the normal, full-density mode. However, in the low power,
half-density mode, since both memory cell capacitors 82, 92 are
coupled to the same digit line 80, the change in voltage when the
capacitors 82, 92 are coupled to the digit line 80 is twice the
change in voltage when a single capacitor 82, 92 is coupled to the
digit line 80 in the normal operating mode. As a result, the charge
on the memory cell capacitors 82, 92 can be permitted to change by
a greater amount than in the full density operating mode. The time
between refreshes can therefore be significantly longer in the low
power, half-density mode.
[0038] Returning, now, to FIG. 4, in operation during the high
power, full density mode, row addresses are applied to the row
decoders 50, 52 to access a specific row of memory cells, and a
column address is applied to the column decoder 66 to select one or
more columns of data, which are routed to or from the memory array
44 through the Input/Output Control circuit 72 and data bus 70. The
operation of the DRAM device 40 is controlled by a command decoder
104 that decodes conventional high-order memory commands to
generate a number of control signals. These control signals include
a load mode register "LMR" signal that causes mode bits on the
address bus 56 to be loaded into a mode register 110, and "AREF"
and "SREF" control signals that cause the DRAM device 40 to operate
in either an Auto Refresh or a Self Refresh mode, as will be
explained in greater detail below. The command decoder 104 also
generates control signals that are applied to an array control
circuit 112. The array control circuit 112, in turn, generates
control signals with the proper timing to perform a number
functions in the memory array 44. These control signals include an
"Activate Row" signal that activates a row of memory cells selected
by a row address as decoded by a row decoder 50, 52, a "Fire Sense
amplifiers" signal that energizes sense amplifiers 96 (FIGS. 5 and
6), an "Activate Column" signal that causes data bits to be coupled
from respective memory cells in an addressed column, and a
"Read/Write" signal that determines whether a memory access will
write data to or read data from the memory array 44. Other signals
may also be generated, but a description of these signals have been
omitted in the interest of brevity.
[0039] As previously explained, it is necessary to periodically
refresh the memory cells in the array 44. The memory cells may be
refreshed in the active mode by the command decoder 104 decoding an
Auto Refresh command applied to the DRAM device 40. The command
decoder 104 then causes the Array Control circuit 112 to generate
appropriate control signals to refresh the memory cells in the
array 44 one row at a time. The rows are selected for refresh by
respective row addresses generated by the Row Address Counter 58
responsive to the AREF control signal generated by the command
decoder 104. In subsequent AREF cycles, the Counter 58 is
incremented once for each Auto Refresh command to generate
respective row addresses for each row of memory cells that causes
the row decoders 50, 52 to activate respective word lines.
[0040] The memory cells in the array 44 may be refreshed by the
command decoder 104 applying the SREF control signal to a Self
Refresh Control circuit 116, which, in turn, causes an internal
timer to periodically increment the Row Address Counter 58. The Row
Address Counter 58 then generates respective row addresses for each
row of memory cells. Once the DRAM device 40 is placed in the Self
Refresh mode responsive to a decoded SREF signal, the Self Refresh
Control circuit 116 will remain in the Self Refresh mode until it
is taken out of that mode responsive to an appropriate memory
command being applied to the command decoder 104. In the Self
Refresh mode, the Self Refresh Control circuit 116 supplies a
signal to the Array Control circuit 112 to cause the circuit 112 to
generate control signals to activate a row of memory cells
corresponding to the row address generated by the Counter 58 and to
energize a sense amplifier for each column of memory cells. The
Self Refresh mode is thus similar to the Auto Refresh mode except
that, in the Self Refresh mode, the command signal to begin each
refresh cycle is generated internally by the Self Refresh Control
circuit 116 rather than by an external Auto Refresh command. The
ability of the DRAM device 40 to remain in the Self Refresh mode
without any external input is the primary reason that the Self
Refresh mode is typically used when the DRAM device 40 is inactive.
When the DRAM device 40 is inactive, many of the circuits in the
DRAM device 40 are also often deenergized to reduce the power
consumed by the DRAM device 40.
[0041] The Self Refresh Control circuit 116 is also coupled to a
Row Address Counter 118 that is used in transitioning to the low
power, half density mode in accordance with an embodiment of the
invention. More specifically, when transitioning to the half
density mode, the counter 118 is reset and then increments
responsive to each refresh as data from each even-numbered row of
memory cells are copied to adjacent odd-numbered row of memory
cells. The Row Address Counter 118 thus keeps track of the number
of even-numbered rows that have been copied to adjacent
odd-numbered rows to determine when the transition to the half
density mode is complete. When all of the data stored in the
even-numbered rows have been copied to the odd-numbered rows, the
Row Address Counter 118 outputs a COPY DONE signal to the Self
Refresh Control circuit 116. The manner in which the circuit 116
transitions to the low power, half density mode will now be
explained with reference to the flowchart of FIG. 7. The low power,
half density mode is entered at 140 responsive to the DRAM device
40 becoming inactive if the Self Refresh Control circuit 116 (FIG.
4) was previously enabled by a PASR signal from the mode register
110. The mode register 110 generates the PASR signal responsive to
the command decoder 104 receiving a load mode register command
while a bit pattern is placed on the address bus 56 that selects
operation in the low power mode. The command decoder 104 then
generates an LMR signal that loads the bit pattern into the Mode
Register 110.
[0042] Returning to FIG. 7, a count Y for the Row Address Counter
118 is set to zero at 150, and an even-numbered row N corresponding
to a count from the Counter 58 is activated at 152 by causing the
Array Control circuit 112 to generate an Activate Row signal. The
count of the counter will be at whatever count was reached at the
end of the prior refresh. However, for the present example, it will
be assumed that the Counter 58 initially generates a count for Row
0. Activating row 0 causes the memory cells in row 0 to be coupled
to respective digit lines 80 or 86 (FIGS. 5 and 6). The voltage
applied to one of the digit lines by a memory cell capacitor in
each column is sensed by a respective sense amplifier 96 after the
sense amplifier 96 is energized at 154. Respective sense amplifiers
96 for each column then drive the pairs of digit lines 80, 86 for
each column to opposite voltages (0 volts and V.sub.CC)
corresponding to the data bits stored in the memory cell in that
column of the active row. The adjacent row N+1, which, in the
present example, is initially row 1, is then activated at 158,
thereby charging or discharging the memory cell capacitors for each
column in the adjacent row to a voltage value corresponding to the
data bits stored in the respective column for row N. Both rows N
and N+1 are then deactivated at 160 and the sense amplifiers 96 are
deenergized at 162. At this time, the data that was stored in row 0
is now also stored in row 1. The above copy procedure is performed
each time a row of memory cells is to be refreshed, which, for the
Self Refresh mode, is determined by an internal timer (not
shown).
[0043] A determination is made at 166 as to whether the final row
of the memory array 40 has been reached, which, as previously
explained, is indicated by the Row Address Counter 118 generating
the COPY DONE signal. Initially, of course, the final row will not
have been reached so that the Row Address Counter 58 is incremented
by two rows at 168. The process then returns and repeats steps
152-162 to copy the data from each even row to the adjacent odd
row. Data from an even row is ultimately written to the final odd
row of the memory array 44, and a determination is then made at 166
that Y=Y.sub.MAX responsive to the Row Address Counter 118 applying
the COPY DONE signal to the Self Refresh Control circuit 116. The
Refresh Control circuit 116 then causes the operation of the DRAM
device 40 to exit at 170 to a process that maintains the DRAM
device 40 in the low power, half density mode, as shown in FIG. 8.
Other methods of generating the COPY DONE signal may be used
eliminating the need for Row Address Counter 118. The starting
address from Row Address Counter 58 could be stored when the copy
begins and compared to the new address at each new refresh cycle.
When the new address is the same as the stored beginning address
then COPY DONE could be asserted. Yet another usable method is to
monitor the most significant bit of Row Address Counter 118, in
this case bit 0, and after three transitions low to high or high to
low, generate the COPY DONE signal. This method may perform a few
extra copy cycles but requires very little circuitry.
[0044] With reference to FIG. 8, the continued operation in the low
power, half density mode is entered at 180. A time constant T in an
internal timer is then set to t.sub.1 at 182, and the status of the
internal timer is checked at 184. The internal timer will not
initially be timed-out. The operation will remain in a loop at 184,
until the timer has timed out. The initial time constant t.sub.1 is
set to a relatively large value so that the internal timer will
time-out after a delay corresponding to the permissible refresh
rate of the DRAM device 40 in the low power, half density mode, as
previously explained. When a determination is made at 184 that the
timer has timed out, the memory cells in the array 44 are refreshed
in the low power, half density mode.
[0045] Rows N (assumed in the present example to be initially row
0) and N+1 (assumed in the present example to be initially row 1)
are then activated at 192 by causing the Array Control circuit 112
to generate an Activate Row signal while the Row Address Counter 58
is outputting the Row address for row N. However, in this mode, the
least significant bit of the row address is ignored by the Row
Address Counter 58 (FIG. 4) so both row N and row N+1 are
activated. The sense amplifiers 96 (FIGS. 5 and 6) are then
activated at 194 to drive the digit lines 80, 86 for each column to
opposite voltages (0 volts and V.sub.CC) corresponding to the data
bits stored in the memory cell in that column of rows N and N+1.
These voltages on the digit lines 80, 86 then return the memory
cell capacitors in rows N and N+1 to their original charge level.
Both rows N and N+1 are then deactivated at 196 and the sense
amplifiers 96 are deenergized at 198.
[0046] A determination is then made at 200 as to whether the DRAM
device 40 is becoming active so that it should no longer operate in
the low power, half density mode. If a determination is made at 200
that the DRAM device 40 should transition to the high power, full
density mode, the half density procedure will exit at 202. The
operation of the DRAM device 40 will than transition to the high
power, full density mode as shown in FIG. 9. However, assuming that
a determination is made at 200 that the DRAM device 40 is to
continue operating in the low power mode, the Row Address Counter
60 is incremented by two rows at 206. The process then returns to
184 to await the timing-out of the internal timer for the next
refresh cycle, at which time steps 192-198 are repeated.
[0047] The principle difference between the transition to the low
power, half density mode shown in FIG. 7 and continuous operation
in the low power, half density mode shown in FIG. 8 is that, when
continuing to operating in the low power mode, even and odd rows
are activated at the same time rather than sequentially as in
transitioning into the low power mode.
[0048] With reference to FIG. 9, the transition from the low power,
half density mode to the high power, full density mode is initiated
at 210. The transition may be initiated by applying appropriate
mode bits to the high power, full density mode to the address bus
56 and generating the load mode register "LMR" signal to cause the
mode bits to be loaded into the mode register 110. Alternatively,
the electronic equipment containing the DRAM device may become
active, thus requiring that the DRAM device be accessed. In
transitioning to the high power, full density mode, the time T of
internal timer is set at 212 to a value t.sub.2 that is
substantially shorter than the value t.sub.1 to which the time T
was initially set in the half density mode. The time value t.sub.2
is commensurate with the significantly higher refresh rate required
in the full density mode. The operation remains in a loop at 214,
until the timer times out, as previously explained. The DRAM device
40 then undergoes a refresh cycle.
[0049] In contrast to the refresh procedures in the half density
mode, in the full density mode, only a single row N is activated at
222. The sense amplifiers are then energized at 226. After the
charge on the memory cell capacitor has been restored to its
original value, the row N is deactivated at 228, and the sense
amplifiers are deenergized at 230. A determination is made at 232
whether the DRAM device 40 has become inactive so that operation
should transition to the low power, half density mode. If so, the
procedure exits at 236 to the procedure shown in FIG. 7, as
previously explained. Otherwise, the Row Address Counter 58 is
incremented by one row at 242, and the procedure returns to 214 to
wait until the next refresh is to begin. Alternatively, each
refresh cycle can be initiated by an Auto Refresh command.
[0050] It will therefore be apparent that the DRAM device 40 can
seamlessly transition back to-and-fourth between the high power,
full density mode and the low power, half density mode without
requiring cumbersome relocation of data in the odd rows.
[0051] A computer system 250 using the DRAM device 40 of FIG. 4 is
shown in FIG. 10. The computer system 250 includes a processor 252
for performing various computing functions, such as executing
specific software to perform specific calculations or tasks. The
processor 252 includes a processor bus 254 that normally includes
an address bus, a control bus, and a data bus. In addition, the
computer system 250 includes one or more input devices 264, such as
a keyboard or a mouse, coupled to the processor 252 to allow an
operator to interface with the computer system 250. Typically, the
computer system 250 also includes one or more output devices 266
coupled to the processor 252, such output devices typically being a
printer or a video terminal. One or more data storage devices 268
are also typically coupled to the processor 252 to allow the
processor 252 to store data or retrieve data from internal or
external storage media (not shown). Examples of typical storage
devices 268 include hard and floppy disks, tape cassettes, and
compact disk read-only memories (CD-ROMs). The processor 252 is
also typically coupled to cache memory 270, which is usually static
random access memory ("SRAM") and to the DRAM device 40, which may
be a synchronous DRAM ("SDRAM") or another variety of DRAM, through
a memory controller 280. The memory controller 280 normally
includes a control bus 282 and an address bus 284 that is coupled
to the DRAM device 40. A data bus 290 of the DRAM device 40 may be
coupled to the processor bus 254 either directly (as shown),
through the memory controller 280, or by some other means.
[0052] Although the present invention has been described with
reference to a preferred embodiment, the invention is not limited
to this preferred embodiment. For example, instead of storing data
in two rows in the low power mode, data can alternatively be stored
in 4, 8 or more rows by copying the data that is stored in the full
density to mode to 3, 7 or more rows of memory cells, and
reordering addresses accordingly. For example, for a quarter
density mode, the two most significant row address bits can be
reordered to be the two least significant row address bits inside
the memory device. Then as the memory is written to sequentially,
only every fourth row will be written internally if only one forth
of the memory capacity is used. When transitioning to a quarter
density mode, the valid row of data can be copied to the next three
empty rows by first turning on the valid row, then turning on the
empty rows as described previously. Thereafter, all four rows can
be simultaneously turned on to enhance the signal applied to the
senseamps and therefore improve the refresh characteristics.
Conventionally memory cells are grouped into sub arrays of cells
where each sub array has associated wordline drivers and senseamps
where the row address MSB will select between groups of memory sub
arrays. In the preferred embodiment of the present invention, the
row address MSB is mapped to the internal row address LSB to allow
for a fast row copy operation when transitioning to a low power
partial density mode. Alternatively, the row address MSB could be
remapped to some other row address within the sub array address
space other than the low LSB. Therefore, the invention is limited
only by the appended claims, which include within their scope all
equivalent devices or methods which operate according to the
principles of the invention as described.
* * * * *