U.S. patent application number 11/932643 was filed with the patent office on 2008-10-30 for method of selectively powering memory device.
Invention is credited to Shyam Balasubramanian, Ajay Bhatia, Daniel Fung, Yolin Lih, Jun Liu, Dennis Wendell.
Application Number | 20080266995 11/932643 |
Document ID | / |
Family ID | 39676026 |
Filed Date | 2008-10-30 |
United States Patent
Application |
20080266995 |
Kind Code |
A1 |
Lih; Yolin ; et al. |
October 30, 2008 |
METHOD OF SELECTIVELY POWERING MEMORY DEVICE
Abstract
An approach to selectively powering a memory device is provided
to improve the writeability of static random access memory (SRAM)
cells without adversely compromising their stability. For example,
various methods are provided to permit the voltage or current of a
power supply line connected with one side of an SRAM cell to drop
during write operations. This drop weakens one side of the SRAM
cell and reduces the drive-fight between transistors of the SRAM
cell and external write circuitry. As a result, the minimum voltage
for writing new logic states into the SRAM cell is reduced to
permit overall lower operating voltages for the SRAM cell and
related circuitry. By continuing to maintain a second side of the
SRAM cell at the reference voltage or current, the SRAM cell can
successfully switch to a newly written logic state.
Inventors: |
Lih; Yolin; (San Jose,
CA) ; Wendell; Dennis; (Sunnyvale, CA) ; Liu;
Jun; (Cupertino, CA) ; Fung; Daniel; (Fremont,
CA) ; Bhatia; Ajay; (Santa Clara, CA) ;
Balasubramanian; Shyam; (Santa Clara, CA) |
Correspondence
Address: |
BROOKS KUSHMAN P.C. / SUN / STK
1000 TOWN CENTER, TWENTY-SECOND FLOOR
SOUTHFIELD
MI
48075-1238
US
|
Family ID: |
39676026 |
Appl. No.: |
11/932643 |
Filed: |
October 31, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60888006 |
Feb 2, 2007 |
|
|
|
Current U.S.
Class: |
365/226 |
Current CPC
Class: |
G11C 11/417 20130101;
G11C 11/419 20130101; G11C 5/147 20130101 |
Class at
Publication: |
365/226 |
International
Class: |
G11C 5/14 20060101
G11C005/14 |
Claims
1. A method of operating a static random access memory (SRAM) cell
comprising first and second cross-coupled logic gates, wherein a
first power supply line is connected with the first cross-coupled
logic gate, wherein a second power supply line is connected with
the second cross-coupled logic gate, the method comprising:
maintaining the first power supply line at a first power level
during a first read operation; permitting the first power supply
line to transition from the first power level to a second power
level during a first write operation to store a first logic state
in the SRAM cell; maintaining the second power supply line at the
first power level during the first read operation; and maintaining
the second power supply line at the first power level during the
first write operation.
2. The method of claim 1, wherein the first power level corresponds
to a reference voltage and the second power level corresponds to a
voltage less than the reference voltage.
3. The method of claim 1, wherein the first power level corresponds
to a reference current and the second power level corresponds to a
current less than the reference current.
4. The method of claim 1, further comprising maintaining the first
power supply line at the first power level during a second write
operation to store a second logic state in the SRAM cell.
5. The method of claim 4, further comprising maintaining the second
power supply line at the first power level during the second write
operation.
6. The method of claim 4, further comprising permitting the second
power supply line to transition from the first power level to the
second power level during the second write operation.
7. A method of operating a static random access memory (SRAM) cell
comprising first and second cross-coupled logic gates, wherein
first and second power supply lines are connected with the first
cross-coupled logic gate, the method comprising: maintaining the
first power supply line at a first power level during a first read
operation; permitting the first power supply line to transition
from the first power level to a second power level during a first
write operation to store a first logic state in the SRAM cell;
maintaining the second power supply line at a third power level
during the first read operation; and maintaining the second power
supply line at the third power level during the first write
operation.
8. The method of claim 7, wherein the first power level corresponds
to a first reference voltage and the third power level corresponds
to a second reference voltage less than the first reference
voltage.
9. The method of claim 7, wherein the first power level corresponds
to a reference voltage and the second power level corresponds to a
voltage less than the reference voltage.
10. The method of claim 7, wherein the first power level
corresponds to a first reference current and the third power level
corresponds to a second reference current less than the first
reference current.
11. The method of claim 7, wherein the first power level
corresponds to a reference current and the second power level
corresponds to a current less than the reference current.
12. The method of claim 7, further comprising maintaining the first
power supply line at the first power level during a second write
operation to store a second logic state in the SRAM cell.
13. The method of claim 12, further comprising maintaining the
second power supply line at the third power level during the second
write operation.
14. The method of claim 12, further comprising permitting the
second power supply line to transition from the third power level
to a fourth power level during the second write operation.
15. The method of claim 14, wherein the third power level
corresponds to a reference voltage and the fourth power level
corresponds to a voltage less than the reference voltage.
16. The method of claim 15, wherein the third power level
corresponds to a reference current and the fourth power level
corresponds to a current less than the reference current.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 60/888,006 filed on Feb. 2, 2007 and
entitled "Split Power Switch for Memory Cells", which is
incorporated herein by reference.
BACKGROUND
[0002] As is well known, static random access memory (SRAM) cells
can be implemented using cross-coupled logic gates to maintain
logic states corresponding to various associated data values. In
this regard, it is generally desirable for SRAM cells to hold their
stored logic states despite possible changes in voltage,
temperature, or other operating conditions. It is also desirable
for SRAM cells to permit changes in their logic states in response
to write operations. Unfortunately, existing SRAM cell designs
often fail to provide high degrees of both stability and
writeability.
[0003] For example, in one approach to improve stability and
writeability, the minimum and maximum operating voltage of an SRAM
cell may be limited. However, such implementations can become
impractical due to possible changes in voltage caused by
environmental or other operating conditions.
[0004] In an approach to improve stability, an SRAM cell may be
implemented with robust cross-coupled logic gates that are
resilient to outside disturbances. However, if the SRAM cell is too
robust, it can become difficult for the SRAM cell to switch to a
newly written logic state. For example, if the SRAM cell's PMOS
transistors are too strong, they may prevent one of the SRAM cell's
internal nodes from being pulled down to an appropriate voltage
corresponding to a newly written logic state. This can negatively
affect the writeability of the SRAM cell.
[0005] In an approach to improve writeability, the cross-coupled
logic gates of an SRAM cell may be weakened. Nevertheless, the
logic states stored by the weakened SRAM cell may inadvertently
change in response to variations in operating conditions, thereby
compromising stability. Such a weakened SRAM cell can also impair
writeability if the SRAM cell is unable to regenerate newly written
logic states between its cross-coupled logic gates. For example, if
the SRAM cell's PMOS transistors are too weak, they may be unable
to pull up one of the SRAM cell's internal nodes to an appropriate
voltage in response to a newly written logic state.
[0006] In another approach to improve writeability, a single power
switch may be connected with a column of SRAM cells to reduce the
voltage provided to all cross-coupled portions of the SRAM cells
during write operations in response to write enable and column
select signals. Although this approach may permit the voltage of
one node of an SRAM cell to be easily pulled down, the reduced
voltage on both cross-coupled logic gates can inhibit the SRAM
cell's ability to adequately pull up the voltage of a second node
of the SRAM cell in order to regenerate the newly written logic
state between its cross-coupled logic gates. Other efforts to
improve writeability, such as increasing the size or strength of
external circuit elements connected to word lines, lowering
threshold voltages of transistors of the SRAM cell, increasing the
word line voltage, or weakening the access transistors connected to
the word lines can also negatively impact stability. Moreover, as
SRAM operating voltages are reduced, variations in operating
conditions and SRAM components can more easily impact the operation
of SRAM cells which can have a correspondingly greater effect on
stability and writeability.
SUMMARY
[0007] Various implementations of a split power switch and methods
of operation are provided that may be used to improve the
writeability characteristics of memory cells such as SRAM cells
without adversely compromising their stability. In one
implementation, a method of operating a static random access memory
(SRAM) cell is provided. The SRAM cell includes first and second
cross-coupled logic gates. A first power supply line is connected
with the first cross-coupled logic gate and a second power supply
line is connected with the second cross-coupled logic gate. The
method includes maintaining the first power supply line at a first
power level during a first read operation. The method also includes
permitting the first power supply line to transition from the first
power level to a second power level during a first write operation
to store a first logic state in the SRAM cell. The method further
includes maintaining the second power supply line at the first
power level during the first read operation. In addition, the
method includes maintaining the second power supply line at the
first power level during the first write operation.
[0008] Additional implementations of various split power switch
circuits, SRAM cells, and other embodiments are further set forth
herein. These and other features and advantages of the invention
will be more readily apparent from the detailed description of the
embodiments set forth below taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE FIGURES
[0009] FIG. 1 illustrates a conceptual block diagram of a plurality
of SRAM cells connected with a split power switch in accordance
with an embodiment of the invention.
[0010] FIG. 2 illustrates a circuit to implement an SRAM cell in
accordance with an embodiment of the invention
[0011] FIGS. 3-6 illustrate circuits to implement various split
power switches in accordance with embodiments of the invention.
[0012] FIG. 7 illustrates a circuit to provide a write recovery
signal to a split power switch in accordance with an embodiment of
the invention.
[0013] FIG. 8 illustrates an example of a memory device including
an SRAM cell in accordance with an embodiment of the invention.
[0014] FIG. 9 illustrates another circuit to implement an SRAM cell
in accordance with an embodiment of the invention
[0015] FIG. 10 illustrates another circuit to implement a split
power switch in accordance with an embodiment of the invention.
DETAILED DESCRIPTION
[0016] In accordance with various embodiments further described
herein, a split power switch is provided to improve the
writeability characteristics of memory cells such as SRAM cells
without adversely compromising their stability. In particular,
various split power switch circuits described herein permit the
voltage or current of a power supply line connected with one side
of an SRAM cell to drop during write operations. This drop weakens
one side of the SRAM cell and reduces the drive-fight between
transistors of the SRAM cell and external write circuitry. As a
result, the minimum voltage for writing new logic states into the
SRAM cell is reduced to permit overall lower operating voltages for
the SRAM cell and related circuitry. By continuing to maintain a
second side of the SRAM cell at the reference voltage or current,
the SRAM cell can successfully switch to a newly written logic
state. Referring now to the drawings wherein the showings are for
purposes of illustrating embodiments of the invention only, and not
for purposes of limiting the same, FIG. 1 illustrates a conceptual
block diagram of a plurality of SRAM cells 110 (conceptually
illustrated in FIG. 1 in block form) connected with a split power
switch 140 (also conceptually illustrated in FIG. 1 in block form)
as part of a memory device 100 in accordance with an embodiment of
the invention. Several possible circuit implementations of split
power switch 140 are illustrated in FIGS. 3-6 and FIG. 10 in
accordance with various embodiments of the invention as will be
further described herein.
[0017] As shown in FIG. 1, SRAM cells 110 are connected with power
supply lines 125 and 135. In one embodiment, each of SRAM cells 110
may be implemented with a pair of cross-coupled logic gates, such
as inverters. Although three SRAM cells 110 are illustrated in FIG.
1, any desired number of SRAM cells 110 may be implemented for use
with various embodiments disclosed herein. For purposes of example,
SRAM cells 110 are illustrated in FIG. 1 as a single column of
memory cells connected to a single split power switch 140. However,
other orientations may be used as may be desired in particular
implementations. In this regard, additional groups of SRAM cells
110, power supply lines 125 and 135, and split power switches 140
may be used to provide a memory cell array arranged in a plurality
of columns. For example, in one embodiment, each column of SRAM
cells 110 may be implemented with associated power supply lines 125
and 135 and a single split power switch 140 as shown in FIG. 1. In
another embodiment, multiple columns or subcolumns of SRAM cells
110 may be implemented to share associated power supply lines 125
and 135 and a single split power switch 140.
[0018] In yet another embodiment, multiple split power switches 140
may be used by a single column of SRAM cells 110. For example, a
first set of SRAM cells 110 of the column may share a first split
power switch 140 and a first set of power supply lines 125 and 135,
and a second set of SRAM cells 110 of the column may share a second
split power switch 140 and a second set of power supply lines 125
and 135. In this embodiment, all SRAM cells 110 of the column may
optionally be implemented to share additional circuitry such as,
for example, bit lines, data lines, and/or other read or write
circuitry further described herein. The use of multiple split power
switches 140 in this manner can permit quicker adjustment of power
levels provided to each set of SRAM cells 110 through power supply
lines 125 and 135.
[0019] In the embodiment shown in FIG. 1, each of power supply
lines 125 and 135 is connected with one of the cross-coupled logic
gates of each of SRAM cells 110. As also shown in FIG. 1, power
supply lines 125 and 135 are connected with split power switch 140
which is also connected with a reference voltage 120 (labeled Vdd)
which may be a positive voltage in this embodiment. In various
embodiments, other power sources such as different reference
voltages or reference currents may be used in place of reference
voltage 120. As also shown in FIG. 1, bit lines 270 and 280 are
connected with each of SRAM cells 110-1 through 110-N. In this
regard, SRAM cells 110-1 through 110-N may share bitlines 270 and
280. However, in other embodiments, bit lines may not be shared by
all of SRAM cells 110-1 through 110-N. For example, in one
embodiment, a first set of bit lines may be provided to SRAM cells
110-1 and 110-2, and a second set of bit lines may be provided to
SRAM cell 110-N. In such an embodiment, SRAM cells 110-1 and 110-2
may use bit lines separate from those used by SRAM cell 110-N. In
other embodiments, any desired combination of shared and/or
unshared bit lines may be used.
[0020] In accordance with various embodiments further described
herein, split power switch 140 selectively adjusts the voltages
provided to power supply lines 125 and 135 (and therefore adjust
the voltages provided by power supply lines 125 and 135 to all of
SRAM cells 110) in response to one or more control signals 150. In
particular, split power switch 140 may be configured to provide
reference voltage 120 or a reference current to each of power
supply lines 125 and 135 during read operations performed using
SRAM cells 110. Split power switch 140 may be further implemented
to permit the voltage or current provided to one of power supply
lines 125 or 135 to fall during appropriate write operations
performed on SRAM cells 110, while still maintaining a voltage or
current approximately equal to reference voltage 120 or a reference
current on the other one of power supply lines 125 or 135.
[0021] In one embodiment, memory device 100 is designed by
initially selecting a device size for cross-coupled logic gates of
SRAM cells 110 while power supply lines 125 and 135 are connected
directly to a desired maximum value of reference voltage 120. Then,
power supply lines 125 and 135 are connected to reference voltage
120 through split power switch 140. Split power switch 140 is then
be sized to provide a desired reduced voltage to power supply lines
125 and 135.
[0022] FIG. 2 illustrates a circuit 200 to implement an SRAM cell,
such as any of SRAM cells 110 of FIG. 1, in accordance with an
embodiment of the invention. In this regard, circuit 200 is
connected with split power switch 140 through power supply lines
125 and 135. As shown in FIG. 2, circuit 200 includes a pair of
cross-coupled inverters 225 and 245 implemented by transistors
210/220 and 230/240, respectively. Transistor 210 is connected with
power supply line 135. Similarly, transistor 230 is connected with
power supply line 125. Each of transistors 220 and 240 are
connected with a reference voltage 295 (labeled Vss) which may
correspond, for example, to ground. FIG. 2 further illustrates that
n-well and p-well voltages 205 and 215 (labeled vnw and vpw,
respectively) of the transistors of circuit 200 need not be
connected with reference voltage 120 (previously described in FIG.
1) or reference voltage 295.
[0023] Access transistors 250 and 260 are connected with word line
290 as well as bit lines 270 and 280, and nodes 255 and 265,
respectively. Accordingly, word line 290 selectively connects bit
lines 270 and 280 with nodes 255 and 265 through access transistors
250 and 260, respectively.
[0024] While circuit 200 is storing a given logic state, split
power switch 140 maintains the voltage of each of power supply
lines 125 and 135 approximately equal to reference voltage 120. For
example, if circuit 200 is storing a first logic state (e.g.,
corresponding to a data value of 0), node 255 may be set to a logic
low voltage while node 265 is set to a logic high voltage.
Accordingly, transistors 220 and 230 will be turned on, and
transistors 210 and 240 will be turned off. In this case, because
the voltage of power supply line 125 is approximately equal to
reference voltage 120, transistor 230 pulls up the voltage of node
265 to maintain the first logic state.
[0025] Similarly, if circuit 200 is storing a second logic state
(e.g., corresponding to a data value of 1), node 265 may be set to
a logic low voltage while node 255 is set to a logic high voltage.
Accordingly, transistors 210 and 240 turn on, and transistors 220
and 230 turn off. In this second case, because the voltage of power
supply line 135 is also approximately equal to reference voltage
120, transistor 210 pulls up the voltage of node 255 to maintain
the second logic state.
[0026] Split power switch 140 continues to maintain the voltage of
each of power supply lines 125 and 135 approximately equal to
reference voltage 120 during read operations. For example, during a
read operation, bit lines 270 and 280 may be precharged and word
line 290 may be set to a logic high voltage to turn on access
transistors 250 and 260. Accordingly, inverters 225 and 245 drive
bit lines 270 and 280 with appropriate voltages corresponding to
the logic state stored by circuit 200.
[0027] For a read operation in which circuit 200 is storing a first
logic state, transistors 220 and 230 turn on, and transistors 210
and 240 turn off. Accordingly, transistor 220 pulls down the
voltage of bit line 270, and transistor 230 pulls up the voltage of
bit line 280. Because the voltage of power supply line 125 is
approximately equal to reference voltage 120, transistor 230
operates with sufficient current to pull up the voltage of node 265
in order to drive bit line 280.
[0028] Similarly, for a read operation in which circuit 200 is
storing a second logic state, transistors 210 and 240 turn on, and
transistors 220 and 230 turn off. Accordingly, transistor 240 pulls
down the voltage of bit line 280, and transistor 210 pulls up the
voltage of bit line 270. Again, because the voltage of power supply
line 135 is approximately equal to reference voltage 120,
transistor 210 operates with sufficient current to pull up the
voltage of node 255 in order to drive bit line 270.
[0029] However, during a write operation to switch circuit 200 from
a first logic state to a second logic state, split power switch 140
permits the voltage of power supply line 125 to fall below
reference voltage 120 while continuing to maintain the voltage of
power supply line 135 approximately equal to reference voltage 120.
In this example, nodes 255 and 265 are initially set to logic low
and high voltages, respectively, transistors 220 and 230 turn on,
and transistors 210 and 240 turn off while circuit 200 initial
stores the first logic state. During the write operation to store
the second logic state into circuit 200, bit lines 270 and 280 are
driven high and low, respectively, by appropriate write circuitry
(not shown), and word line 290 is driven high to turn on access
transistors 250 and 260. Accordingly, bit line 280 pulls node 265
down from a logic high voltage to a logic low voltage. As described
above, because transistor 230 is turned on, it will continue to
attempt to pull up the voltage of node 265. However, by permitting
the voltage of power supply line 125 to fall below reference
voltage 120 during the write operation, transistor 230 operates
with less current, thereby improving the ability of bit line 280 to
overcome transistor 230 and pull down node 265.
[0030] When node 265 is pulled below the threshold voltage of
transistor 210, transistor 210 turns on and pulls up the voltage of
node 255. By maintaining the voltage of power supply line 135
approximately equal to reference voltage 120 during the write
operation, transistor 210 operates with sufficient current to pull
up the voltage of node 255 in order to change circuit 200 to the
second logic state.
[0031] During a second write operation to switch circuit 200 from
the second logic state to the first logic state, split power switch
140 permits the voltage of power supply line 135 to fall below
reference voltage 120 while continuing to maintain the voltage of
power supply line 125 approximately equal to reference voltage 120.
In this case, transistor 210 operates with less current, thereby
improving the ability of bit line 270 to overcome transistor 210
and pull down node 255. By maintaining the voltage of power supply
line 125 approximately equal to reference voltage 120 during the
second write operation, transistor 230 operates with sufficient
current to pull up the voltage of node 265 in order to change
circuit 200 back to the first logic state.
[0032] Advantageously, by selectively reducing the power provided
to power supply line 125 or 135 during write operations, the switch
points of inverters 225 and 245 are not degraded. Transistor 210 or
230 of the side of the SRAM cell being written to continues to
operate in a linear mode (e.g., operating similar to a resistor)
during a write operation while power is reduced to its associated
power supply line 125 or 135. However, because the power provided
to other side of the SRAM cell is not reduced during the write
operation, the switch point of the inverter on the other side is
not degraded. As a result, sufficient write current is provided
through access transistor 250 or 260 to cause a voltage drop
through transistor 210 or 230 sufficient to lower the voltage of
node 255 or 265 below the switch point of the inverter to properly
regenerate a newly written logic state through circuit 200.
[0033] FIG. 3 illustrates a circuit 300 to implement split power
switch 140 of FIG. 1 in accordance with an embodiment of the
invention. As shown in FIG. 3, circuit 300 may be connected with
one or more of SRAM cells 110 (which may be implemented, for
example, by one or more circuits 200) through power supply lines
125 and 135.
[0034] Circuit 300 is implemented to receive a plurality of signals
including a write recovery signal 305 (labeled write_delay), write
data signals 310A-B (labeled write_data and write_data_bar), a
column select signal 320 (labeled col_sel), and a write enable
signal 325 (labeled write enable). Signals 305, 310A-B, 320, and
325 may be used to selectively adjust the voltages provided to
power supply lines 125 and 135 as further described herein. Circuit
300 includes logic 390 which may be implemented, for example, with
NAND gates 330A-B and inverters 340A-B. NAND gates 330A-B are
connected with signals 310A-B, 320, and 325. Inverters 340A-B are
connected with the outputs of NAND gates 330A-B as well as the
gates of main switch transistors 370A-B. In this regard, the
operation of main switch transistors 370A-B is determined by logic
390 in response to signals 310A-B, 320, and 325. Specifically, main
switch transistors 370A-B turn on when inverters 340A-B provide
logic low output values, and turn off when inverters 340A-B provide
logic high output values.
[0035] Main switch transistors 370A and 370B are connected with
power supply lines 125 and 135, respectively, as well as with
reference voltage 120. Accordingly, each of main switch transistors
370A and 370B selectively provide reference voltage 120 to power
supply lines 125 and 135, respectively, in response to signals
310A-B, 320, and 325. In the particular implementation shown in
FIG. 3, inverter 340A provides a logic low output at all times
except when write data signal 310A, column select signal 320, and
write enable 325 all exhibit logic high values. Similarly, inverter
340B provides a logic low output value at all times except when
write data signal 310B, column select signal 320, and write enable
325 all exhibit logic high values. Accordingly, by adjusting
signals 31A-B, 320, and 325, one or both of main switch transistors
370A-B turn on in response to signals 310A-B, 320, and 325.
However, because write data signals 310A-B are implemented to
provide differential data input values, at least one of main switch
transistors 370A-B remains turned on.
[0036] Circuit 300 also includes damper transistors 350A-C. The
gates and drains of damper transistors 350A-B are connected with
reference voltage 120. Accordingly, damper transistor 350A remains
turned on and maintains a minimum voltage at power supply line 125
approximately equal to reference voltage 120 minus the threshold
voltage of damper transistor 350A. Similarly, damper transistor
350B remains turned on and maintains a minimum voltage at power
supply line 135 approximately equal to reference voltage 120 minus
the threshold voltage of damper transistor 350B. Clamper transistor
350C includes a gate connected with reference voltage 120, a source
connected with power supply line 125, and a drain connected with
power supply line 135. Therefore, damper transistor 350C remains
turned on and maintains a voltage difference between power supply
lines 125 and 135 no greater than the threshold voltage of damper
transistor 350C.
[0037] Accordingly, damper transistors 350A-C maintain minimum
voltages at power supply lines 125 and 135 in the event that one of
main switch transistors 370A or 370B turns off. Moreover, although
three damper transistors 350A-C are illustrated in FIG. 3, it is
contemplated that either damper transistor 350C or both of damper
transistors 350A-B may be omitted in other embodiments.
[0038] Circuit 300 also includes weak keeper transistors 360A-C,
each of which includes a gate connected with reference voltage 295
which is connected to ground in this embodiment. As a result, weak
keeper transistors 360A and 360B remain turned on to provide weak
current supplies which pull up the voltage of power supply lines
125 and 135, respectively. Weak keeper transistor 360C also remains
turned on to provide a weak current flow between power supply lines
125 and 135. Accordingly, weak keeper transistors 360A-C adjust the
voltage of power supply lines 125 and 135 in response to leakage
currents of circuit 200. Although three weak keeper transistors
360A-C are illustrated in FIG. 3, it is contemplated that either
weak keeper transistor 360C or both of weak keeper transistors
360A-B may be omitted in other embodiments.
[0039] Circuit 300 also includes write recovery transistors 380A-C
having gates connected with signal 305. Accordingly, write recovery
transistors 380A-C selectively turn on and off in response to
signal 305. In the embodiment of FIG. 3, signal 305 normally
provides a logic high value and is only set to a logic low value
following a write operation as will be further described
herein.
[0040] The operation of circuit 300 will now be described with
reference to FIGS. 2 and 3. As identified above, the operation of
main switch transistors 370A-B is determined by logic 390 in
response to signals 310A-B, 320, and 325. In this regard, column
select signal 320 provides a logic high value during read and write
operations performed on circuit 200, and a logic low value at all
other times. Write enable signal 325 is set to a logic high value
only during write operations performed on circuit 200. Write data
signals 310A-B provide differential data input values to be written
into circuit 200. For example, if a first logic state is to be
written into circuit 200, then data signal 310A is set to a logic
high value, and data signal 310B is set to a logic low value.
Similarly, if a second logic state is to be written, then data
signal 310A is set to a logic low value, and data signal 310B is
set to a logic high value. As previously described with respect to
split power switch 140, circuit 300 may be configured to maintain
the voltage of each of power supply lines 125 and 135 approximately
equal to reference voltage 120 except during write operations. In
this regard, main switch transistor 370A remains turned on at all
times except during write operations where data signal 310A
provides a logic high value. Similarly, main switch transistor 370B
remains turned on at all times except during write operations where
data signal 310B provides a logic high value. Accordingly, while
data values are maintained by SRAM cells 110 or read from SRAM
cells 110, main switch transistors 370A-B of circuit 300 remain
turned on. As a result, main switch transistors 370A-B maintain
each of power supply lines 125 and 135 approximately equal to
reference voltage 120.
[0041] However, during a write operation, column select signal 320
and write enable signal 325 provide logic high values. If the write
operation calls for a first logic state to be written into SRAM
cell 110, then write data signal 310A provides a logic high value
and write data signal 310B provides a logic low value. As a result,
inverter 340A provides a logic high value to the gate of main
switch transistor 370A, thereby turning off main switch transistor
370A. However, because write data signal 310B remains low, inverter
340B continues to provide a logic low value to the gate of main
switch transistor 370B, thereby keeping main switch transistor 370B
turned on.
[0042] While transistor 370A is turned off, the voltage of power
supply line 125 is permitted to drop. During this time, damper
transistors 350A and 350C to maintain a minimum voltage at power
supply line 125 approximately equal to reference voltage 120 minus
their associated threshold voltages. As previously described with
respect to FIG. 2, this drop in voltage improves the ability of bit
line 280 to overcome transistor 230 and pull down node 265 during
the write operation. In addition, by maintaining the voltage of
power supply line 135 approximately equal to reference voltage 120
during the write operation, transistor 210 operates with sufficient
current to pull up the voltage of node 255 in order to change
circuit 200 to the desired logic state.
[0043] The voltage of power supply line 135 may be similarly
adjusted during a second write operation that calls for a second
logic state to be written into circuit 200. In this case, write
data signal 310B is set to a logic high value and write data signal
310A is set to a logic low value. Here, transistor 370B turns off
and the voltage of power supply line 135 drops down to a minimum
voltage at power supply line 125 maintained by damper transistors
350B-C. As also previously described with respect to FIG. 2, this
drop in voltage improves the ability of bit line 270 to overcome
transistor 210 and pull down node 255 during the second write
operation. In addition, by maintaining the voltage of power supply
line 125 approximately equal to reference voltage 120 during the
second write operation, transistor 230 operates with sufficient
current to pull up the voltage of node 265 in order to change
circuit 200 to the desired logic state.
[0044] As previously described, signal 305 normally provides a
logic high value. However, signal 305 may be switched to a logic
low value after a predetermined time period (for example,
corresponding to the duration of a write operation) to pull power
supply line 125 or 135 back up to reference voltage 120 in the
event that signals 310A-B, 320, and 325 cause one of main switch
transistors 370A-B to remain turned off following the write
operation. For example, in one embodiment, after a write operation
is completed (i.e., after circuit 200 has changed logic states),
signal 305 may be set to a logic low value. In various embodiments,
signal 305 may be implemented by a delay line that receives signals
from word line 290, or by appropriate logic that mimics the
predicted operation of circuit 200 during write operations.
[0045] FIG. 4A illustrates a circuit 400A to implement split power
switch 140 of FIG. 1 in accordance with another embodiment of the
invention. As shown in FIG. 4A, circuit 400A may be connected with
one or more of SRAM cells 110 (which may be implemented, for
example, by one or more circuits 200) through power supply lines
125 and 135.
[0046] Circuit 400A is implemented to receive a plurality of
signals including write recovery signals 410 and 440 (labeled ym
and yse), and main switch signals 420 and 430 (labeled bl and bib).
Signals 410, 420, 430, and 440 may be used to selectively adjust
the voltages provided to power supply lines 125 and 135 as further
described herein. Circuit 400A includes main switch transistors
470A and 470B which are connected with power supply lines 125 and
135, respectively, as well as with reference voltage 120. In
addition, the gates of main switch transistors 470A and 470B are
connected with signals 420 and 430, respectively. Accordingly, each
of main switch transistors 470A and 470B selectively provide
reference voltage 120 to power supply lines 125 and 135 in response
to signals 420 and 430, respectively.
[0047] Signals 420 and 430 may be operated through appropriate
control circuitry (not shown) in a manner similar to logic 390 of
FIG. 3 in order to provide logic low values at all times except
during write operations at which time only one of signals 420 or
430 provides a logic high value. As a result, both of main switch
transistors 470A and 470B remain turned on except during write
operations, at which time one turns off to permit the voltage of
one of power supply lines 125 or 135 to drop.
[0048] Circuit 400A also includes damper transistors 450A-B similar
to damper transistors 350A-B of circuit 300 previously described
above. As shown in FIG. 4A, the gates and drains of damper
transistors 450A-B are connected with reference voltage 120.
Accordingly, damper transistor 450A remains turned on and maintains
a minimum voltage at power supply line 125 approximately equal to
reference voltage 120 minus the threshold voltage of damper
transistor 450A. Similarly, damper transistor 450B also remains
turned on and maintains a minimum voltage at power supply line 135
approximately equal to reference voltage 120 minus the threshold
voltage of damper transistor 450B. Accordingly, damper transistors
450A-B maintain minimum voltages at power supply lines 125 and 135
in the event that one of main switch transistors 470A or 470B turns
off in response to signals 420 or 430.
[0049] Circuit 400A also includes weak keeper transistor 460 having
its gate connected with reference voltage 295 which is connected to
ground in this embodiment. As a result, weak keeper transistor 460
also remains turned on to provide a weak current flow between power
supply lines 125 and 135 similar to weak keeper transistor 360C of
circuit 300 previously described above. Accordingly, weak keeper
transistor 460 adjusts the voltage of power supply lines 125 and
135 in response to leakage currents of circuit 200.
[0050] Circuit 400A also includes write recovery transistors 480A-B
and 480C-D having gates connected with signals 410 and 440,
respectively. Signals 410 and 440 may be operated and/or
implemented in a manner similar to signal 305 of circuit 300 of
FIG. 3. In this regard, signals 410 and 440 normally provide logic
high values and are set to logic low values following a write
operation as will be further described herein.
[0051] The operation of circuit 400A will now be described with
reference to FIGS. 2 and 4A. As identified above, the operation of
main switch transistors 470A-B is determined by signals 420 and
430, respectively. In this regard, signal 420 keeps main switch
transistor 470A turned on at all times except during write
operations to store a first logic state in circuit 200. Similarly,
signal 430 keeps main switch transistor 470B turned on at all times
except during write operations to store a second logic state in
circuit 200. Accordingly, while a data value is maintained by
circuit 200, or read from circuit 200, main switch transistors
470A-B remain turned on to keep each of power supply lines 125 and
135 approximately equal to reference voltage 120. However, during
write operations one of main switch transistors 470A-B turn
off.
[0052] If transistor 470A is turned off by signal 420 during a
first write operation, the voltage of power supply line 125 will be
permitted to drop. During this time, damper transistor 450A
maintains a minimum voltage at power supply line 125 approximately
equal to reference voltage 120 minus its associated threshold
voltage. As previously described with respect to FIG. 2, this drop
in voltage can improve the ability of bit line 280 to overcome
transistor 230 and pull down node 265 during the write operation.
In addition, by maintaining the voltage of power supply line 135
approximately equal to reference voltage 120 during the write
operation, transistor 210 operates with sufficient current to pull
up the voltage of node 255 in order to change circuit 200 to the
desired logic state.
[0053] On the other hand, if transistor 470B is turned off by
signal 430 during a second write operation, the voltage of power
supply line 135 drops down to a minimum voltage at power supply
line 135 maintained by clamper transistor 450B. As also previously
described with respect to FIG. 2, this drop in voltage improves the
ability of bit line 270 to overcome transistor 210 and pull down
node 255 during the second write operation. In addition, by
maintaining the voltage of power supply line 125 approximately
equal to reference voltage 120 during the second write operation,
transistor 230 operates with sufficient current to pull up the
voltage of node 265 in order to change circuit 200 to the desired
logic state.
[0054] Following a write operation, signals 410 and 440 are set to
logic low values to turn on write recovery transistors 480A-D which
are used to pull power supply line 125 or 135 back up to reference
voltage 120 in the event that signal 420 or 430 causes one of main
switch transistors 470A-B to remain turned off following the write
operation.
[0055] FIG. 4B illustrates a circuit 400B to implement split power
switch 140 of FIG. 1 in accordance with another embodiment of the
invention. As shown in FIG. 4B, circuit 400B may be connected with
one or more of SRAM cells 110 (which may be implemented, for
example, by one or more circuits 200) through power supply lines
125 and 135.
[0056] As also shown in FIG. 4B, circuit 400B includes various
components of circuit 400A which operate in circuit 400B in the
manner previously described herein. However, in circuit 400B,
transistor 495A is used in place of main switch transistor 470A,
write recovery transistor 480A, and write recovery transistor 480C
of circuit 400A. Also in circuit 400B, transistor 495B is used in
place of main switch transistor 470B, write recovery transistor
480B, and write recovery transistor 480D of circuit 400A.
[0057] Circuit 400B is implemented to receive a plurality of
signals including write data signals 410A-B (labeled blb and b),
and a write enable signal 425 (labeled wyb). Signals 410A-B and 425
are used to selectively adjust the voltages provided to power
supply lines 125 and 135 as further described herein.
[0058] Circuit 400B includes logic 490 which may be implemented,
for example, with NOR gates 430A-B. NOR gates 430A-B are connected
with signals 410A-B and 425 as well as with the gates of
transistors 495A-B. In this regard, the operation of transistors
495A-B is determined by logic 490 in response to signals 410A-B and
425. Specifically, transistors 495A-B turn on when NOR gates 430A-B
provide logic low output values, and turn off when NOR gates 430A-B
provide logic high output values.
[0059] Transistors 495A and 495B are connected with power supply
lines 125 and 135, respectively, as well as with reference voltage
120. Accordingly, each of transistors 495A and 495B selectively
provides reference voltage 120 to power supply lines 125 and 135,
respectively, in response to signals 410A-B and 425.
[0060] In the particular implementation shown in FIG. 4B, NOR gate
430A provides a logic low output at all times except when write
data signal 410A and write enable signal 425 both exhibit logic low
values. Similarly, NOR gate 430B provides a logic low output value
at all times except when write data signal 410B and write enable
signal 425 both exhibit logic low values. Accordingly, by adjusting
signals 410A-B and 425, one or both of transistors 495A-B turn on
in response to signals 410A-B and 425. However, because write data
signals 410A-B are implemented to provide differential data values,
at least one of transistors 495A-B remains turned on. As a result,
both of transistors 495A and 495B remain turned on except during
write operations, at which time one turns off to permit the voltage
of one of power supply lines 125 or 135 to drop.
[0061] The operation of circuit 400B will now be described with
reference to FIGS. 2 and 4B. As identified above, the operation of
transistors 495A-B is determined by logic 490 in response to
signals 410A-B and 425. In this regard, write enable signal 425
provides a logic low value during write operations performed on
circuit 200. Write data signals 410A-B provide differential data
input values to be written into circuit 200. For example, if a
first logic state is to be written into circuit 200, then data
signal 410A is set to a logic low value, and data signal 410B is
set to a logic high value. Similarly, if a second logic state is to
be written, then data signal 410A is set to a logic high value, and
data signal 410B is set to a logic low value.
[0062] Transistor 495A remains turned on at all times except during
write operations where write enable signal 425 and data signal 410A
provide logic low values. Similarly, transistor 495B remains turned
on at all times except during write operations where write enable
signal 425 and data signal 410B provide a logic low values.
Accordingly, while data values are maintained by SRAM cells 110 or
read from SRAM cells 110, transistors 495A-B of circuit 400 remain
turned on. As a result, transistors 495A-B provide each of power
supply lines 125 and 135 with reference voltage 120 or a reference
current. If transistor 495A is turned off by logic 490 in response
to signal 410A and 425 during a first write operation, the voltage
of power supply line 125 will be permitted to drop. During this
time, weak keeper transistor 460 remains turned on to provide a
weak current flow between power supply lines 125 and 135. Also
during this time, damper transistor 450A maintains a minimum
voltage at power supply line 125 approximately equal to reference
voltage 120 minus its associated threshold voltage. As previously
described with respect to FIG. 2, this drop in voltage can improve
the ability of bit line 280 to overcome transistor 230 and pull
down node 265 during the write operation. In addition, by
maintaining the voltage of power supply line 135 approximately
equal to reference voltage 120 during the write operation,
transistor 210 operates with sufficient current to pull up the
voltage of node 255 in order to change circuit 200 to the desired
logic state. On the other hand, if transistor 470B is turned off by
logic 490 in response to signals 410B and 425 during a second write
operation, the voltage of power supply line 135 drops down to a
minimum voltage at power supply line 135 maintained by damper
transistor 450B, and weak keeper transistor 460 remains turned on
to provide a weak current flow between power supply lines 125 and
135. As also previously described with respect to FIG. 2, this drop
in voltage improves the ability of bit line 270 to overcome
transistor 210 and pull down node 255 during the second write
operation. In addition, by maintaining the voltage of power supply
line 125 approximately equal to reference voltage 120 during the
second write operation, transistor 230 operates with sufficient
current to pull up the voltage of node 265 in order to change
circuit 200 to the desired logic state.
[0063] FIG. 4C illustrates a circuit 400C to implement split power
switch 140 of FIG. 1 in accordance with another embodiment of the
invention. As shown in FIG. 4C, circuit 400C may be connected with
one or more of SRAM cells 110 (which may be implemented, for
example, by one or more circuits 200) through power supply lines
125 and 135.
[0064] As also shown in FIG. 4C, circuit 400C includes various
components of circuit 400B which operate in circuit 400C in the
manner previously described herein. However, in circuit 400C,
damper transistor 450C is used in place of damper transistors
450A-B. As shown in FIG. 4C, damper transistor 450C is connected
with reference voltage 120, power supply line 125, and power supply
line 135. Accordingly, damper transistor 450C remains turned on and
prevents the voltage difference between power supply lines 125 and
135 from exceeding a minimum voltage corresponding to the threshold
voltage of damper transistor 450C during write operations when one
of transistors 495A-B is turned off.
[0065] FIG. 5 illustrates a circuit 500 to implement split power
switch 140 of FIG. 1 in accordance with an embodiment of the
invention. As shown in FIG. 5, circuit 500 may be connected with
one or more of SRAM cells 110 (which may be implemented, for
example, by one or more circuits 200) through power supply lines
125 and 135.
[0066] Circuit 500 is implemented to receive a plurality of signals
including write data signals 510A-B (labeled blb and b), and a
write enable signal 525 (labeled wyb). Signals 510A-B and 525 are
used to selectively adjust the voltages provided to power supply
lines 125 and 135 as further described herein.
[0067] Circuit 500 includes logic 590 which may be implemented, for
example, with NOR gates 530A-B. NOR gates 530A-B are connected with
signals 510A-B and 525 as well as with the gates of main switch
transistors 570A-B. In this regard, the operation of main switch
transistors 570A-B is determined by logic 590 in response to
signals 510A-B and 525. Specifically, main switch transistors
570A-B turn on when NOR gates 530A-B provide logic low output
values, and turn off when NOR gates 530A-B provide logic high
output values.
[0068] Main switch transistors 570A and 570B are connected with
power supply lines 125 and 135, respectively, as well as with
reference voltage 120. Accordingly, each of main switch transistors
570A and 570B selectively provides reference voltage 120 to power
supply lines 125 and 135, respectively, in response to signals
510A-B and 525.
[0069] In the particular implementation shown in FIG. 5, NOR gate
530A provides a logic low output at all times except when write
data signal 510A and write enable signal 525 both exhibit logic low
values. Similarly, NOR gate 530B provides a logic low output value
at all times except when write data signal 510B and write enable
signal 525 both exhibit logic low values. Accordingly, by adjusting
signals 510A-B and 525, one or both of main switch transistors
570A-B turn on in response to signals 510A-B and 525. However,
because write data signals 510A-B are implemented to provide
differential data values, at least one of main switch transistors
570A-B remains turned on.
[0070] Circuit 500 also includes weak keeper transistors 560A-B,
each of which includes a gate connected with reference voltage 295
which is connected to ground in this embodiment. As a result, weak
keeper transistors 560A and 560B remain turned on to provide a weak
current flow between power supply lines 125 and 135. Accordingly,
weak keeper transistors 560A-B adjust the current provided to power
supply lines 125 and 135, and weakly pull up the voltage of power
supply line 125 or 135 in the event that main switch transistor
570A or 570B, respectively, is turned off. Although two weak keeper
transistors 560A-B are illustrated in FIG. 5, it is contemplated
that greater or lesser numbers of weak keeper transistors may be
provided in other embodiments. In one embodiment, each of weak
keeper transistors 560A-B is sized to approximately correspond to
sizes of individual PMOS transistors of circuit 200.
[0071] The operation of circuit 500 will now be described with
reference to FIGS. 2 and 5. As identified above, the operation of
main switch transistors 570A-B is determined by logic 590 in
response to signals 510A-B and 525. In this regard, write enable
signal 525 provides a logic low value during write operations
performed on circuit 200. Write data signals 510A-B provide
differential data input values to be written into circuit 200. For
example, if a first logic state is to be written into circuit 200,
then data signal 510A is set to a logic low value, and data signal
510B is set to a logic high value. Similarly, if a second logic
state is to be written, then data signal 510A is set to a logic
high value, and data signal 510B is set to a logic low value.
[0072] Main switch transistor 570A remains turned on at all times
except during write operations where write enable signal 525 and
data signal 510A provide logic low values. Similarly, main switch
transistor 570B remains turned on at all times except during write
operations where write enable signal 525 and data signal 510B
provide logic low values. Accordingly, while data values are
maintained by SRAM cells 110 or read from SRAM cells 110, main
switch transistors 570A-B of circuit 500 remain turned on. As a
result, main switch transistors 570A-B provide each of power supply
lines 125 and 135 with reference voltage 120 or a reference
current.
[0073] However, during a write operation, write enable signal 525
provides a logic low value. If the write operation calls for a
first logic state to be written into SRAM cell 110, then write data
signal 510A provides a logic low value and write data signal 510B
provides a logic high value. As a result, NOR gate 530A provides a
logic high value to the gate of main switch transistor 570A,
thereby turning off main switch transistor 570A. However, because
write data signal 510B remains high, NOR gate 530B continues to
provide a logic low value to the gate of main switch transistor
570B, thereby keeping main switch transistor 570B turned on.
[0074] While transistor 570A is turned off, the current provided to
power supply line 125 is reduced to a minimum current provided by
weak keeper transistors 560A-B. This reduction in current improves
the ability of bit line 280 to overcome transistor 230 and pull
down node 265 during the write operation performed on circuit 200.
In addition, by continuing to provide a greater current to power
supply line 135 during the write operation, transistor 210 operates
with sufficient current to pull up the voltage of node 255 in order
to change circuit 200 to the desired logic state.
[0075] The current provided to power supply line 135 may be
similarly adjusted during a second write operation that calls for a
second logic state to be written into circuit 200. In this case,
write enable signal 525 and write data signal 510B are set to a
logic low values and write data signal 510A is set to a logic low
value. Here, transistor 570B turns off and the current provided to
power supply line 135 drops to the minimum current provided by weak
keeper transistors 560A-B. This drop in current improves the
ability of bit line 270 to overcome transistor 210 and pull down
node 255 during the second write operation. In addition, by
continuing to provide greater current to power supply line 125
during the second write operation, transistor 230 operates with
sufficient current to pull up the voltage of node 265 in order to
change circuit 200 to the desired logic state.
[0076] FIG. 6 illustrates a circuit 600 to implement split power
switch 140 of FIG. 1 in accordance with an embodiment of the
invention. As shown in FIG. 6, circuit 600 may be connected with
one or more of SRAM cells 110 (which may be implemented, for
example, by one or more circuits 200) through power supply lines
125 and 135. Circuit 600 includes write data signals 610A-B
(labeled blb and b), a write enable signal 625 (labeled wyb), logic
690, NOR gates 630A-B, and main switch transistors 670A-B
implemented in a manner as previously described with regard to
corresponding portions of circuit 500 of FIG. 5. Signals 610A-B and
625 may be used to selectively adjust the voltages provided to
power supply lines 125 and 135 as further described herein.
[0077] Circuit 600 also includes damper transistor 650 which has a
gate connected with reference voltage 120, a source connected with
power supply line 125, and a drain connected with power supply line
135. Therefore, damper transistor 650 remains turned on and
maintains a voltage difference between power supply lines 125 and
135 no greater than the threshold voltage of damper transistor 650.
Accordingly, damper transistor 650 maintains minimum voltages at
power supply lines 125 and 135 in the event that one of main switch
transistors 670A or 670B is turned off. It is contemplated that
other numbers of damper transistors may be provided in other
embodiments.
[0078] Circuit 600 also includes weak keeper transistors 660A-D,
each of which includes a gate connected with reference voltage 295
which is connected to ground in this embodiment. As shown, weak
keeper transistors 660A-B are connected with reference voltage 120,
and weak keeper transistors 660C-D are connected with power supply
lines 125 and 135. Accordingly, weak keeper transistors 660A-D
remain turned on to provide weak current supplies which pull up the
voltage of power supply lines 125 and 135, respectively in response
to leakage currents of circuit 200. It is contemplated that other
numbers of weak keeper transistors may be provided in other
embodiments. In one embodiment, each of weak keeper transistors
660A-D are sized to approximately correspond to sizes of individual
PMOS transistors of circuit 200.
[0079] The operation of circuit 600 will now be described with
reference to FIGS. 2 and 6. The operation of main switch
transistors 670A-B is determined by logic 690 in response to
signals 610A-B and 625 as similarly described above in relation to
circuit 500 of FIG. 5. Accordingly, circuit 600 may be configured
to maintain the voltage of each of power supply lines 125 and 135
approximately equal to reference voltage 120 except during write
operations. During a write operation, write enable signal 625
provides a logic low value. If the write operation calls for a
first logic state to be written into SRAM cell 110, then write data
signal 610A provides a logic low value and write data signal 610B
provides a logic high value. As a result, NOR gate 630A provides a
logic high value to the gate of main switch transistor 670A,
thereby turning off main switch transistor 670A. However, because
write data signal 610B remains high, NOR gate 630B continues to
provide a logic low value to the gate of main switch transistor
670B, thereby keeping main switch transistor 670B turned on.
[0080] While transistor 670A is turned off, the voltage of power
supply line 125 will be permitted to drop. During this time, damper
transistor 650 maintains a minimum voltage at power supply line 125
approximately equal to reference voltage 120 minus the threshold
voltage of damper transistor 650. This drop in voltage improves the
ability of bit line 280 to overcome transistor 230 and pull down
node 265 during the write operation. In addition, by maintaining
the voltage of power supply line 135 approximately equal to
reference voltage 120 during the write operation, transistor 210
operates with sufficient current to pull up the voltage of node 255
in order to change circuit 200 to the desired logic state.
[0081] The voltage of power supply line 135 may be similarly
adjusted during a second write operation that calls for a second
logic state to be written into circuit 200. In this case, write
enable signal 625 and write data signal 610B are set to logic low
values and write data signal 610A is set to a logic high value.
Here, transistor 670B turns off and the voltage of power supply
line 135 will be permitted to drop down to a minimum voltage at
power supply line 125 maintained by damper transistor 650. This
drop in voltage improves the ability of bit line 270 to overcome
transistor 210 and pull down node 255 during the second write
operation. In addition, by maintaining the voltage of power supply
line 125 approximately equal to reference voltage 120 during the
second write operation, transistor 230 operates with sufficient
current to pull up the voltage of node 265 in order to change
circuit 200 to the desired logic state.
[0082] FIG. 7 illustrates a circuit 700 to provide a write recovery
signal to a split power switch in accordance with an embodiment of
the invention. Circuit 700 includes a delay element block 720
implemented by appropriate circuitry to delay an input signal to
provide a delayed signal. Circuit 700 also includes logic 730
implemented in the illustrated embodiment by an inverter 740 and a
NAND gate 750.
[0083] Delay element block 720 and NAND gate 750 each receive an
input signal 710 (labeled write_enable_bar) which may be
implemented, for example, as an inverted version of write enable
signal 325 described herein. Delay element block 720 delays input
signal 710 to provide a delayed signal 770. Delayed signal 770 is
inverted by inverter 740 to provide an inverted delayed signal 780
to NAND gate 750. Accordingly, inverted delayed signal 780
corresponds, for example, to a delayed version of write enable
signal 325. NAND gate operates on signals 710 and 780 to provide a
write recovery signal 760 (labeled vdd_gater_on) that may be used
to implement any of write recovery signals 305, 410, or 440
described herein.
[0084] Embodiments incorporating various features disclosed herein
may be implemented in embedded or standalone SRAM memory devices,
caches, register files, multi-port memories, translation lookaside
buffers (TLBS), content-addressable memories (CAMS), ternary CAMS
(TCAMS), or other appropriate devices to operate at lower voltages
as compared to traditional six transistor SRAM cells. Such features
can be particularly advantageous for mobile, portable, or ultra-low
voltage devices in which lower supply voltages may advantageously
permit longer battery life and/or use time.
[0085] For example, FIG. 8 illustrates an example of a memory
device that may be implemented with one or more of the various SRAM
cell circuits disclosed herein. In this regard, illustrates a CAM
memory device 800 including an SRAM cell 810 and a match comparator
860 in block form. For example, in one embodiment, SRAM cell 810
may be implemented by circuit 200 of FIG. 2. As shown, SRAM cell
810 includes a read/write port 820 implemented by appropriate read
and write circuitry. For example, in one embodiment, read/write
port 820 is implemented by access transistors 250 and 260 of
circuit 200. Logic states stored by SRAM cell 810 are provided to
match comparator 860 over complementary data output lines 840 and
850 which may, for example, be connected with bit lines 270 and
280, respectively of circuit 200.
[0086] Match comparator 860 is implemented with appropriate
circuitry known in the art to compare a data value received at an
input port 880 (labeled match_data) with logic states received from
data output line 840 and/or 850. Match comparator 860 provides an
appropriate data signal through output port 870 (labeled match) to
indicate the existence of a match or non-match between the data
value received at input port 880 and the logic state stored by SRAM
cell 810. Other implementations and applications of SRAM cell
circuits in accordance with various embodiments described herein
are also contemplated.
[0087] FIG. 9 illustrates another circuit 900 to implement an SRAM
cell in accordance with an embodiment of the invention. FIG. 10
illustrates another circuit 1000 to implement a split power switch
in accordance with an embodiment of the invention. Similar to
circuit 200 described herein, circuit 900 is connected with power
supply lines 125 and 135. However, in circuit 900, power supply
lines 125 and 135 are connected with circuit 1000 of FIG. 10.
[0088] As shown in FIG. 9, circuit 900 includes a pair of
cross-coupled inverters 925 and 945 implemented by transistors
910/920 and 930/940, respectively, which may be used to store a
first logic state or a second logic state corresponding to voltages
maintained at nodes 925 and 965.
[0089] Transistor 910 is connected with reference voltage 120
(labeled Vdd), and transistor 920 is connected with reference
voltage 295 (labeled Vss). Transistor 930 is connected with power
supply line 125 (labeled gated_vdd in this embodiment), and
transistor 940 is connected with power supply line 135 (labeled
gated_vss in this embodiment).
[0090] Circuit 900 includes a plurality of read ports 950 which are
implemented as single ended read ports in the embodiment of FIG. 9.
In particular, eight read ports 950 are connected with node 992 of
circuit 900. However, any desired number of single ended or
differential read ports may be provided in other embodiments.
[0091] In the embodiment shown in FIG. 9, read ports 950 are
implemented by a plurality of transistors 952 (labeled MN9[7:0]),
which are connected with transistor 954. In this regard, although
transistors 952 are illustrated by a single transistor in FIG. 9,
eight of transistors 952 (corresponding to MN9[0] through MN9[7])
are provided in circuit 900. Each of transistors 952 is connected
with transistor 954. In another embodiment (not shown), a plurality
of transistors 954 may be provided, with each of transistors 954
connected with a corresponding one of transistors 952.
[0092] As shown in FIG. 9, the gate of transistor 954 is connected
with node 992 of circuit 900. Accordingly, transistor 954 turns on
if node 992 is set to a logic high voltage greater than the
threshold voltage of transistor 954. Similarly, transistor 954
turns off if node 992 is set to a logic low voltage lower than the
threshold voltage of transistor 954.
[0093] Each of transistors 952 is connected with a corresponding
one of read word lines 956 (labeled rwl[7:0]) and a corresponding
one of read data lines 958 (labeled rbl[7:0]) which are used to
read logic states stored by circuit 900. For example, if one of
read word lines 956 is set to a logic high value, its associated
transistor 952 turns on. If node 992 is set to a logic high value,
transistor 954 also turns on. In this case, the combined operation
of transistors 952 and 954 pull the associated read data line 958
down to a logic low value that is detected as a first logic state
by appropriate read circuitry, such as a sense amplifier (not
shown) connected with the associated read data line 958. If node
992 is set to a logic low value, transistor 954 turns off. As a
result, the associated read data line 958 is not pulled down which
is detected as a second logic state.
[0094] Circuit 900 is also implemented with a single ended write
port 960. However, any desired number of single ended or
differential read ports may be provided in other embodiments. In
the embodiment shown in FIG. 9, write port 960 is implemented by a
transistor 962. As shown in FIG. 9, transistor 962 is connected
with node 965, a write word line 966, and a write data line 968
which provides data values corresponding to logic states to be
written into circuit 900 during write operations. Accordingly,
transistor 962 pulls node 965 down to a logic low voltage if write
word line 966 and write data line 968 are set to logic high values.
At other times, when write operations are not being performed,
write data line 968 may be set to a logic low value.
[0095] Although circuit 900 includes read ports 950 and write port
960, circuit 900 may alternatively be implemented with one or more
bidirectional read/write ports. For example, in one embodiment,
circuit 900 may include a single bidirectional read/write port
implemented by an appropriate pass gate. In another embodiment,
circuit 900 may include multiple bidirectional read/write ports
implemented by multiple pass gates.
[0096] While circuit 900 is storing a given logic state, split
power switch 1000 maintains the voltage of each of power supply
lines 125 and 135 approximately equal to reference voltages 120 and
295, respectively. For example, if circuit 900 is storing a first
logic state (e.g., corresponding to a data value of 0), node 955
may be set to a logic low voltage while node 965 is set to a logic
high voltage. Accordingly, transistors 920 and 930 will be turned
on, and transistors 910 and 940 will be turned off. In this case,
because the voltage of power supply line 125 is approximately equal
to reference voltage 120, transistor 930 pulls up the voltage of
node 965 to maintain the first logic state.
[0097] Similarly, if circuit 900 is storing a second logic state
(e.g., corresponding to a data value of 1), node 965 may be set to
a logic low voltage while node 955 is set to a logic high voltage.
Accordingly, transistors 910 and 940 turn on, and transistors 920
and 930 turn off. In this second case, because the voltage of power
supply line 135 is also equal to reference voltage 295, transistor
940 pulls down the voltage of node 965 to maintain the second logic
state.
[0098] Split power switch 1000 continues to maintain the voltage of
each of power supply lines 125 and 135 approximately equal to
reference voltages 120 and 295, respectively, during read
operations.
[0099] However, during a write operation to switch circuit 900 from
a first logic state to a second logic state, split power switch
1000 permits the voltage of power supply line 125 to fall below
reference voltage 120 while continuing to maintain the voltage of
power supply line 135 approximately equal to reference voltage 295.
In this example, nodes 955 and 965 are initially set to logic low
and high voltages, respectively, transistors 920 and 930 turn on,
and transistors 910 and 940 turn off while circuit 900 initial
stores the first logic state. During the write operation to store
the second logic state into circuit 900, write word line 966 and
write data line 968 are driven high and low, respectively by
appropriate write circuitry (not shown). Accordingly, write data
line 968 pulls node 965 down from a logic high voltage to a logic
low voltage. As described above, because transistor 930 is turned
on, it will continue to attempt to pull up the voltage of node 965.
However, by permitting the voltage of power supply line 125 to fall
below reference voltage 120 during the write operation, transistor
930 operates with less current, thereby improving the ability of
write data line 968 to overcome transistor 930 and pull down node
965.
[0100] When node 965 is pulled below the threshold voltage of
transistor 910, transistor 910 turns on and pulls up the voltage of
node 955. By maintaining the voltage of power supply line 135
approximately equal to reference voltage 295 during the write
operation, transistor 940 operates with sufficient current to pull
down the voltage of node 965 in order to change circuit 900 to the
second logic state.
[0101] During a second write operation to switch circuit 900 from
the second logic state to the first logic state, write word line
966 and write data line 968 are both driven high by appropriate
write circuitry (not shown). Accordingly, write data line 968 pulls
node 965 up from a logic low voltage to a logic high voltage.
[0102] Also during this second write operation, split power switch
1000 permits the voltage of power supply line 135 to rise above
reference voltage 135 while continuing to maintain the voltage of
power supply line 125 approximately equal to reference voltage 120.
In this case, transistor 940 operates with less current, thereby
improving the ability of write data line 968 to overcome transistor
940 and pull up node 965. Optionally, in another embodiment, the
voltage of power supply line 125 may be maintained approximately
equal to reference voltage 120 during both the first and second
write operations.
[0103] Referring now to FIG. 10, circuit 1000 is implemented to
receive a plurality of signals including a write data signal 1010
(labeled write_data) and a write enable signal 1025 (labeled
write_enable). Signals 1010 and 1025 are used to selectively adjust
the voltages provided to power supply lines 125 and 135 through
main switch transistors 1070A and 1070B, respectively, as further
described herein.
[0104] Circuit 1000 includes logic 1090 which may be implemented,
for example, with a NOR gate 1030, a NAND gate 1040, and an
inverter 1050. NOR gate 1030 is connected with write data signal
1010, an inverted version of write enable signal 1025 (e.g.,
inverted by inverter 1050), and the gate of main switch transistor
1070A. NAND gate 1040 is connected with write data signal 1010,
write enable signal 1025, and the gate of main switch transistor
1070B.
[0105] In this regard, the operation of main switch transistors
1070A-B is determined by logic 1090 in response to signals 1010 and
1025. For example, if write enable signal 1025 provides a logic low
value, both of main switch transistors 1070A-B turn on. If write
enable signal 1025 provides a logic high value and write data
signal provides a logic low value, main switch transistor 1070A
turns off and main switch transistor 1070A turns on. If write
enable signal 1025 and write data signal provides both provide
logic high values, main switch transistor 1070B turns off and main
switch transistor 1070A turns on.
[0106] Main switch transistors 1070A and 1070B are connected with
power supply lines 125 and 135, respectively, as well as with
reference voltages 120 and 295, respectively. Accordingly, main
switch transistors 1070A and 1070B selectively provide reference
voltages 120 and 295 to power supply lines 125 and 135,
respectively, in response to signals 1010 and 1025.
[0107] The operation of circuit 1000 will now be described with
reference to FIGS. 9 and 10. As identified above, the operation of
main switch transistors 1070A-B is determined by logic 1090 in
response to signals 1010 and 1025. In this regard, write enable
signal 1025 provides a logic high value during write operations
performed on circuit 900. Write data signal 1010 provides data
input values to be written into circuit 900. For example, if a
first logic state is to be written into circuit 900, then write
data signal 1010 is set to a logic low value. Similarly, if a
second logic state is to be written, then write data signal 1010 is
set to a logic high value.
[0108] Main switch transistor 1070A remains turned on at all times
except during write operations where write data signal 1010
provides a logic low value and write enable signal 1025 provides a
logic high value. Similarly, main switch transistor 1070B remains
turned on at all times except during write operations where write
data signal 1010 provides a logic high value and write enable
signal 1025 provides a logic high value. Accordingly, while data
values are maintained by circuit 900 or read from circuit 900, main
switch transistors 1070A-B of circuit 1000 remain turned on. As a
result, main switch transistors 1070A-B provide power supply lines
125 and 135 with reference voltages 120 and 295, respectively or a
an appropriate reference current.
[0109] However, during a write operation, write enable signal 1025
provides a logic high value. If the write operation calls for a
first logic state to be written into circuit 900, then write data
signal 1010 provides a logic high value. As a result, NAND gate
1040 provides a logic low value to the gate of main switch
transistor 1070B, thereby turning off main switch transistor 1070B.
However, NOR gate 1030 continues to provide a logic high value to
the gate of main switch transistor 1070A, thereby keeping main
switch transistor 1070A turned on.
[0110] While transistor 1070B is turned off and transistor 1070A is
turned on, split power switch 1000 permits the voltage of power
supply line 135 to rise above reference voltage 295 while
continuing to maintain the voltage of power supply line 125
approximately equal to reference voltage 120. In this case,
transistor 940 operates with reduced current, thereby improving the
ability of write data line 968 to overcome transistor 940 and pull
up node 965 as previously described.
[0111] If the write operation calls for a second logic state to be
written into circuit 900, then write data signal 1010 provides a
logic low value. As a result, NOR gate 1030 provides a logic low
value to the gate of main switch transistor 1070A, thereby turning
off main switch transistor 1070A. However, NAND gate 1040 continues
to provide a logic high value to the gate of main switch transistor
1070B, thereby keeping main switch transistor 1070B turned on.
[0112] While transistor 1070A is turned off and transistor 1070B is
turned on, split power switch 1000 permits the voltage of power
supply line 125 to fall below reference voltage 120 while
continuing to maintain the voltage of power supply line 135
approximately equal to reference voltage 295. In this case,
transistor 930 operates with reduced current, thereby improving the
ability of write data line 968 to overcome transistor 930 and pull
down node 965 as previously described. By maintaining the voltage
of power supply line 135 approximately equal to reference voltage
295 during this write operation, transistor 940 operates with
sufficient current to pull down the voltage of node 965 as also
previously described.
[0113] Other embodiments are also contemplated. For example, a
split power switch in accordance with various embodiments described
herein may be used with SRAM cells providing multiple bidirectional
or unidirectional read or write ports. In addition, although a
positive reference voltage 120 has been described herein, a
negative reference voltage is also contemplated. Moreover, although
the weakening of one side of an SRAM cell has been described in
relation to permitting the voltage of one of power supply lines 125
or 135 to drop, split power switch 140 may alternatively be
implemented to weaken one of power supply lines 125 or 135 by
reducing current or floating one of power supply lines 125 or 135.
Advantageously, various embodiments of split power switch 140
described herein can also continue to provide reliable voltage
operation ranges for connected SRAM cells despite possible
variations in individual circuit components.
[0114] As known by one of ordinary skill in the art, this
invention, including any logic circuit or transistor circuit, may
be modeled, generated, or both by computer based on a description
of the hardware expressed in the syntax and the semantics of a
hardware description language (HDL). Such HDL descriptions are
often stored on a computer readable medium. Applicable HDLs include
those at the layout, circuit netlist, register transfer, and/or
schematic capture levels. Examples of HDLs include, but are not
limited to: GDS II and OASIS (layout level); various SPICE
languages, and IBIS (circuit netlist level); Verilog and VHDL
(register transfer level); and Virtuoso custom design language and
Design Architecture-IC custom design language (schematic capture
level). HDL descriptions may also be used for a variety of
purposes, including but not limited to layout, behavior, logic and
circuit design verification, modeling, and/or simulation.
[0115] The foregoing disclosure is not intended to limit the
invention to the precise forms or particular fields of use
disclosed. It is contemplated that various alternate embodiments
and/or modifications to the invention, whether explicitly described
or implied herein, are possible in light of the disclosure. For
example, although various embodiments have been described using
particular transistors to perform various switching operations,
such transistors are non-limiting examples of various types of
switches that may be used to perform such operations. Accordingly,
other transistors and other types of switches may be used where
appropriate.
[0116] Having thus described embodiments of the invention, persons
of ordinary skill in the art will recognize that changes may be
made in form and detail without departing from the scope of the
invention. Thus the invention is limited only by the claims.
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