U.S. patent application number 15/375390 was filed with the patent office on 2018-06-14 for read assist circuit with process, voltage and temperature tracking for a static random access memory (sram).
This patent application is currently assigned to STMicroelectronics International N.V.. The applicant listed for this patent is STMicroelectronics International N.V.. Invention is credited to Abhishek Pathak.
Application Number | 20180166127 15/375390 |
Document ID | / |
Family ID | 62455088 |
Filed Date | 2018-06-14 |
United States Patent
Application |
20180166127 |
Kind Code |
A1 |
Pathak; Abhishek |
June 14, 2018 |
READ ASSIST CIRCUIT WITH PROCESS, VOLTAGE AND TEMPERATURE TRACKING
FOR A STATIC RANDOM ACCESS MEMORY (SRAM)
Abstract
A memory circuit includes a wordline, memory cells connected to
the wordline and a wordline driver circuit. The memory circuit
further includes a read assist circuit including an n-channel
pull-down transistor having a source-drain path connected between
the wordline and a ground node. A bias circuit applies a biasing
voltage to the gate terminal of the n-channel pull-down transistor
that is modulated responsive to process, voltage and temperature
conditions in order to provide controlled word line underdrive.
Inventors: |
Pathak; Abhishek; (Nowgong
(BKD), IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
STMicroelectronics International N.V. |
Amsterdam |
|
NL |
|
|
Assignee: |
STMicroelectronics International
N.V.
Amsterdam
NL
|
Family ID: |
62455088 |
Appl. No.: |
15/375390 |
Filed: |
December 12, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G11C 7/04 20130101; G11C
11/419 20130101; G11C 11/418 20130101 |
International
Class: |
G11C 11/419 20060101
G11C011/419; G11C 11/418 20060101 G11C011/418 |
Claims
1. A circuit, comprising: a wordline configured to be coupled to a
plurality of memory cells; a pull-down transistor having a
source-drain path connected between the wordline and a ground node;
and a bias circuit configured to apply a biasing voltage to a
control terminal of the pull-down transistor to provide wordline
underdrive in response to assertion of a read assist control
signal, wherein the bias circuit comprises: a first n-channel
transistor coupled between a positive supply voltage node and the
control terminal of the pull-down transistor; a diode-connected
first p-channel transistor coupled between the control terminal of
the pull-down transistor and the ground node; and a switching
circuit configured to connect the first n-channel transistor in a
diode-connected configuration in response to assertion of the read
assist control signal.
2. The circuit of claim 1, further comprising a wordline driver
circuit including a p-channel pull-up transistor, said pull-down
transistor being an n-channel transistor.
3. The circuit of claim 1, wherein the switching circuit is further
configured to connect a control terminal of the first n-channel
transistor to the ground node in response to deassertion of the
read assist control signal.
4. The circuit of claim 1, wherein the bias circuit further
comprises: a second n-channel transistor and a diode-connected
third n-channel transistor coupled in series between a control
terminal of the first n-channel transistor and the ground node; and
a diode-connected second p-channel transistor; and a further
switching circuit configured to selectively couple the
diode-connected second p-channel transistor between the positive
supply voltage node and a control terminal of the second n-channel
transistor in response to assertion of the read assist control
signal.
5. A circuit, comprising: a wordline configured to be coupled to a
plurality of memory cells; a pull-down transistor having a
source-drain path connected between the wordline and a ground node;
and a bias circuit configured to apply a biasing voltage to a
control terminal of the pull-down transistor to provide wordline
underdrive in response to assertion of a read assist control
signal, wherein the bias circuit comprises: a first transistor
coupled between a first supply voltage node and the control
terminal of the pull-down transistor; a diode-connected second
transistor coupled between the control terminal of the pull-down
transistor and the ground node; and a switching circuit configured
to connect the first transistor in a diode-connected configuration
in response to assertion of the read assist control signal.
6. The circuit of claim 5, wherein the bias circuit further
comprises: a third transistor and diode-connected fourth transistor
coupled in series between a control terminal of the first
transistor and one of the first supply voltage and ground nodes; a
diode-connected fifth transistor; and a further switching circuit
configured to selectively couple the diode-connected fifth
transistor between one of the first supply voltage and ground nodes
and a control terminal of the third transistor in response to
assertion of the read assist control signal.
7-9. (canceled)
10. A circuit, comprising: a wordline configured to be coupled to a
plurality of memory cells; a pull-down transistor having a
source-drain path connected between the wordline and a ground node;
and a bias circuit configured to apply a biasing voltage to a
control terminal of the pull-down transistor to provide wordline
underdrive in response to assertion of a read assist control
signal, wherein the bias circuit comprises: a first transistor
coupled between a positive supply voltage node and the control
terminal of the pull-down transistor; and means for modulating a
voltage on a control terminal of said first transistor so that said
transistor operates during read assist to pull the biasing voltage
toward the ground node in response to increase in a supply voltage
at said positive supply voltage node and further permit the
process, voltage and temperature dependent biasing voltage to move
toward the positive supply voltage node in response to a decrease
in the supply voltage, wherein said means for modulating comprises:
a second transistor and diode-connected third transistor coupled in
series between the control terminal of the first transistor and the
ground node; and a diode-connected fourth transistor selectively
coupled between the positive supply voltage node and a control
terminal of the second transistor during read assist.
11-19. (canceled)
20. The circuit of claim 10, further comprising a wordline driver
circuit including a p-channel pull-up transistor, said pull-down
transistor being an n-channel transistor.
21. The circuit of claim 10, further comprising: a further wordline
configured to be coupled to a further plurality of memory cells; a
further pull-down transistor having a source-drain path connected
between the further wordline and the ground node; and wherein the
biasing voltage is applied to both the control terminal of the
pull-down transistor and a control terminal of the further
pull-down transistor.
22. The circuit of claim 1, further comprising: a further wordline
configured to be coupled to a further plurality of memory cells; a
further pull-down transistor having a source-drain path connected
between the further wordline and the ground node; and wherein the
biasing voltage is applied to both the control terminal of the
pull-down transistor and a control terminal of the further
pull-down transistor.
23. The circuit of claim 5, further comprising a wordline driver
circuit including a p-channel pull-up transistor, said pull-down
transistor being an n-channel transistor.
24. The circuit of claim 5, further comprising: a further wordline
configured to be coupled to a further plurality of memory cells; a
further pull-down transistor having a source-drain path connected
between the further wordline and the ground node; and wherein the
biasing voltage is applied to both the control terminal of the
pull-down transistor and a control terminal of the further
pull-down transistor.
25. A circuit, comprising: a wordline configured to be coupled to a
plurality of memory cells; a pull-down transistor having a
source-drain path connected between the wordline and a ground node;
and a bias circuit configured to apply a biasing voltage to a
control terminal of the pull-down transistor to provide wordline
underdrive in response to assertion of a read assist control
signal, wherein the bias circuit comprises: a first n-channel
transistor coupled between a positive supply voltage node and the
control terminal of the pull-down transistor; a second n-channel
transistor and a diode-connected third n-channel transistor coupled
in series between a control terminal of the first n-channel
transistor and the ground node; a diode-connected p-channel
transistor; and a switching circuit configured to selectively
couple the diode-connected p-channel transistor between the
positive supply voltage node and a control terminal of the second
n-channel transistor in response to assertion of the read assist
control signal.
26. The circuit of claim 25, further comprising a further switching
circuit configured to connect the first n-channel transistor in a
diode-connected configuration in response to assertion of the read
assist control signal.
27. The circuit of claim 26, wherein the switching circuit is
further configured to connect a control terminal of the first
n-channel transistor to the ground node in response to deassertion
of the read assist control signal.
28. The circuit of claim 25, further comprising: a further wordline
configured to be coupled to a further plurality of memory cells; a
further pull-down transistor having a source-drain path connected
between the further wordline and the ground node; and wherein the
biasing voltage is applied to both the control terminal of the
pull-down transistor and a control terminal of the further
pull-down transistor.
29. The circuit of claim 25, further comprising a wordline driver
circuit including a p-channel pull-up transistor, said pull-down
transistor being an n-channel transistor.
30. A circuit, comprising: a wordline configured to be coupled to a
plurality of memory cells; a pull-down transistor having a
source-drain path connected between the wordline and a ground node;
and a bias circuit configured to apply a biasing voltage to a
control terminal of the pull-down transistor to provide wordline
underdrive in response to assertion of a read assist control
signal, wherein the bias circuit comprises: a first transistor
coupled between a first supply voltage node and the control
terminal of the pull-down transistor; and a switching circuit
configured to connect the first transistor in a diode-connected
configuration in response to assertion of the read assist control
signal and connect a gate of the first transistor and the control
terminal of the pull-down transistor to the ground node in response
to deassertion of the read assist control signal.
31. The circuit of claim 30, further comprising a wordline driver
circuit including a p-channel pull-up transistor, said pull-down
transistor being an n-channel transistor.
32. The circuit of claim 30, further comprising: a further wordline
configured to be coupled to a further plurality of memory cells; a
further pull-down transistor having a source-drain path connected
between the further wordline and the ground node; and wherein the
biasing voltage is applied to both the control terminal of the
pull-down transistor and a control terminal of the further
pull-down transistor.
33. A circuit, comprising: a wordline configured to be coupled to a
plurality of memory cells; a pull-down transistor having a
source-drain path connected between the wordline and a ground node;
and a bias circuit configured to apply a biasing voltage to a
control terminal of the pull-down transistor to provide wordline
underdrive in response to assertion of a read assist control
signal, wherein the bias circuit comprises: a first n-channel
transistor coupled between a positive supply voltage node and the
control terminal of the pull-down transistor; a second n-channel
transistor coupled in between a control terminal of the first
n-channel transistor and the ground node; a diode-connected
p-channel transistor; and a switching circuit configured to
selectively couple the diode-connected p-channel transistor between
the positive supply voltage node and a control terminal of the
second n-channel transistor in response to assertion of the read
assist control signal.
34. The circuit of claim 33, further comprising a further switching
circuit configured to connect the first n-channel transistor in a
diode-connected configuration in response to assertion of the read
assist control signal.
35. The circuit of claim 34, wherein the switching circuit is
further configured to connect a control terminal of the first
n-channel transistor to the ground node in response to deassertion
of the read assist control signal.
36. The circuit of claim 33, further comprising: a further wordline
configured to be coupled to a further plurality of memory cells; a
further pull-down transistor having a source-drain path connected
between the further wordline and the ground node; and wherein the
biasing voltage is applied to both the control terminal of the
pull-down transistor and a control terminal of the further
pull-down transistor.
37. The circuit of claim 33, further comprising a wordline driver
circuit including a p-channel pull-up transistor, said pull-down
transistor being an n-channel transistor.
Description
TECHNICAL FIELD
[0001] The disclosure relates to integrated memory circuits and in
particular to a read assist circuit for a static random access
memory (SRAM).
BACKGROUND
[0002] Reference is made to FIG. 1 which shows a schematic diagram
of a standard memory circuit 10 including a plurality of memory
cells 12 that are typically arranged in an array including plural
columns and rows. Each memory cell 12 in this implementation is,
for example, a conventional six transistor (6T) static random
access memory (SRAM) cell 12. The memory circuit 10 further
includes a wordline driver 14 for each row and an address decoder
16 configured to control operation of the wordline drivers.
[0003] Each memory cell 12 includes two cross-coupled CMOS
inverters 22 and 24, each inverter including a series connected
p-channel and n-channel MOSFET transistor pair. The inputs and
outputs of the inverters 22 and 24 are coupled to form a latch
circuit having a true data storage node QT and a complement data
storage node QB. The cell 12 further includes two transfer
(passgate) transistors 26 and 28 whose gate terminals are driven by
a wordline (WL) that is coupled to an output of the wordline driver
14. Transistor 26 is source-drain connected between the true data
storage node QT and a node associated with a true bitline (BLT).
Transistor 28 is source-drain connected between the complement data
storage node QB and a node associated with a complement bitline
(BLB). The source terminals of the p-channel transistors 30 and 32
in each inverter 22 and 24 are coupled to receive a high supply
voltage (for example, Vdd) at a high supply node, while the source
terminals of the n-channel transistors 34 and 36 in each inverter
22 and 24 are coupled to receive a low supply voltage (for example,
Gnd) at a low supply node. The high supply voltage Vdd at the high
supply node and the low supply voltage Gnd at the low supply node
comprise the power supply set of voltages for the cell 12.
[0004] The wordline driver circuit 14 includes a series connected
p-channel and n-channel MOSFET transistor pair forming a logic
inverter. The wordline driver circuit 14 is also coupled to receive
the high supply voltage (Vdd) at the high supply node and is
referenced to the low supply voltage (Gnd) at the low supply node.
The input of the wordline driver circuit 14 is coupled to an output
of the address decoder 16 and the wordline (WL) for a row of cells
12 is coupled to the output of the corresponding wordline driver
circuit 14. The address decoder 16 receives an address (Addr),
decodes the received address and selectively actuates the wordline
through the wordline driver circuit 14.
[0005] The memory circuit 10 further includes a read assist circuit
40 coupled to each wordline (WL) or to pairs of wordlines. The
functionality of the read assist circuit 40 is provided for use in
read limited SRAM cells for operation at a supply voltage that is
lower than a minimum functional voltage of the technology. One
known technique for read assist is wordline lowering. In this
technique, the wordline is pulled down by the read assist circuit
40 to a voltage lower than supply voltage in order to provide
sufficient static noise margin (SNM) for the read and write
operation. It will be noted that SNM varies with process, voltage
and temperature (PVT), so it is important to track the word line
underdrive (WLUD) voltage (a voltage lower than the wordline
driver's high supply voltage Vdd) with PVT in order to achieve
better power, performance and area (PPA) figures for the memory
circuit.
[0006] The read assist circuit 40 as known in the art tracks
wordline voltage lowering with process and temperature, but fails
to track with respect to voltage. As SNM increases with increased
supply voltage, this is an important factor not accounted for in
prior art read assist circuits. Additionally, such tracking is
important because the WLUD voltage increases as well with increased
supply voltage, and thus can result in a slowdown of the read
operation and also write failure. To protect against such a
failure, there is an associated performance and power penalty and
extra circuit architecture overhead.
[0007] There is accordingly a need in the art for an improved read
assist circuit with process, voltage and temperature tracking
capability.
SUMMARY
[0008] In an embodiment, a circuit comprises: a wordline configured
to be coupled to a plurality of memory cells; a pull-down
transistor having a source-drain path connected between the
wordline and a ground node; and a bias circuit configured to apply
a process, voltage and temperature dependent biasing voltage to a
control terminal of the pull-down transistor to provide wordline
underdrive during read assist.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] For a better understanding of the embodiments, reference
will now be made by way of example only to the accompanying figures
in which:
[0010] FIG. 1 is a schematic diagram of a standard memory circuit
with read assist;
[0011] FIG. 2 is a schematic diagram of a read assist circuit with
process, voltage and temperature tracking capability;
[0012] FIG. 3 shows operation waveforms for the read assist circuit
of FIG. 2; and
[0013] FIG. 4 is a block diagram showing use of multiple read
assist circuits in a memory array.
DETAILED DESCRIPTION OF THE DRAWINGS
[0014] Reference is now made to FIG. 2 which shows a schematic
diagram of a read assist circuit 40' with process, voltage and
temperature tracking capability. The circuit 40' may, for example,
be used in place of the prior art read assist circuit 40 of FIG. 1.
Thus, the circuit 40' is coupled to a plurality of wordlines (WL)
that are coupled to the rows of a memory array including a
plurality of memory cells (such as, for example, SRAM cells). In
another way of implementation, circuit 40' can also be repeated for
the pairs of wordline. For example, for N number of wordlines the
circuit 40' is repeated M times and then N/M number of wordlines
will be coupled to each circuit 40' of FIG. 2 (as shown in FIG.
4).
[0015] The read assist circuit 40' includes a logic circuit 100
having a first input coupled to receive a decoder select signal
(PRECOM) output from the address decoder 16 and a second input
configured to receive a read assist clock signal (RACLK) output
from a clock generator circuit (not shown). The logic circuit 100
logically combines the decoder select signal and the read assist
clock signal to generate a read assist enable signal (RAEN). In an
embodiment, the logic circuit 100 comprises a logic NAND gate and
thus the read assist enable signal RAEN is asserted logic low when
read assist is enabled for operation (and, conversely, deasserted
logic high when read assist is disabled). The logic low value of
the read assist enable signal RAEN occurs when both the decoder
select signal PRECOM and the read assist clock signal RACLK are
logic high.
[0016] The read assist circuit 40' includes a plurality of
n-channel pull down transistors 102. Each pull down transistor 102
has its source terminal coupled to the low supply voltage Gnd at
the low supply node and its drain terminal coupled to one wordline
(WL). The gate terminals of the pull down transistors 102 are
coupled together at node 104 and driven by a control signal
CNT.
[0017] The read assist circuit 40' includes a voltage tracking
circuit 110 and a process and temperature tracking circuit 112.
Each circuit 110 and 112 is configured to receive the read assist
enable signal RAEN.
[0018] The voltage tracking circuit 110 includes a p-channel
transistor 120, a p-channel transistor 122 and an n-channel
transistor 124 whose source-drain paths are coupled in series
between the high supply voltage (Vdd) at the high supply node and
the low supply voltage Gnd at the low supply node. More
specifically, the source terminal of transistor 120 is coupled to
the high supply node and the drain terminal of transistor 120 is
coupled to the source terminal of transistor 122 at node 126. The
transistor 122 is configured as a diode-connected device with its
gate terminal coupled to its drain terminal at node 128. The drain
terminal of transistor 124 is also coupled to node 128, and the
source terminal of transistor 124 is coupled to the low supply
node. The gate terminals of transistors 120 and 124 are coupled to
receive the read assist enable signal RAEN. The voltage tracking
circuit 110 further includes an n-channel transistor 130 and an
n-channel transistor 132 whose source-drain paths are coupled in
series between a node 134 and the low supply node. More
specifically, the drain terminal of transistor 130 is coupled to
node 134 and the source terminal of transistor 130 is coupled to
the drain terminal of transistor 132 at node 136. The transistor
132 is configured as a diode-connected device with its gate
terminal coupled to its drain terminal at node 136. The source
terminal of transistor 132 is coupled to the low supply node. The
gate terminal of transistor 130 is coupled to node 128.
[0019] The process and temperature tracking circuit 112 includes a
p-channel transistor 150 and an n-channel transistor 152 whose
source-drain paths are coupled in series between the high supply
node and the low supply node. More specifically, the source
terminal of transistor 150 is coupled to the high supply node and
the drain terminal of transistor 150 is coupled to the drain
terminal of transistor 152 at node 134. The source terminal of
transistor 152 is coupled to the low supply node. The gate
terminals of transistors 150 and 152 are coupled to receive the
read assist enable signal RAEN. The process and temperature
tracking circuit 112 further includes an n-channel transistor 156
and a p-channel transistor 158 whose source-drain paths are coupled
in series between the high supply node and the low supply node.
More specifically, the drain terminal of transistor 156 is coupled
to the high supply node and the source terminal of transistor 156
is coupled to the source terminal of transistor 158 at node 104.
The gate terminal of transistor 156 is coupled to node 134. The
transistor 158 is configured as a diode-connected device with its
gate terminal coupled to its drain terminal at the low supply node.
The process and temperature tracking circuit 112 still further
includes an n-channel transistor 160 having a source-drain path
coupled between the node 104 and the low supply node. More
specifically, the drain terminal of transistor 160 is coupled to
the node 104 and the source terminal of transistor 160 is coupled
to the low supply node. The gate terminal of transistor 160 is
coupled to receive the read assist enable signal RAEN.
[0020] The read assist circuit 40' operates to modulate the voltage
level of the read assist control signal CNT at node 104 in order to
achieve different WLUD performance across PVT ranges. The higher
the modulated voltage of the control signal CNT, the greater the
WLUD (and vice versa).
[0021] The need for WLUD from an SNM point of view is at maximum
with respect to a fast NMOS/slow PMOS (FS) process corner.
Conversely, the need for WLUD from an SNM point of view is at
minimum with respect to a slow NMOS/fast PMOS (SF) process corner.
The transistors 156 and 158 of the process and temperature tracking
circuit 112 function to track the WLUD requirement across process
corner. At the fast NMOS process corner, the process and
temperature tracking circuit 112 causes the voltage at node 104 to
settle more toward the high supply voltage (Vdd) at the high supply
node due to the operation of n-channel transistor 156 causing an
increase in the voltage level of the control signal CNT at node 104
to provide more WLUD. With respect to the fast PMOS corner,
however, the process and temperature tracking circuit 112 causes
the voltage at node 104 to settle more toward the low supply
voltage Gnd at the low supply node due to the operation of
p-channel transistor 158 so causing a decrease in the voltage level
of the control signal CNT at node 104 to provide less WLUD.
[0022] The need for WLUD from an SNM point of view is at maximum
for a higher temperature and at minimum for a lower temperature.
The process and temperature tracking circuit 112 tracks the WLUD
requirement over temperature change. When the read assist enable
signal RAEN is logic low (i.e., read assist is enabled for
operation), transistor 150 turns on and configures transistor 156
as a diode-connected transistor with its gate terminal coupled to
its drain terminal at the high supply node. Because the transistor
156 is now configured as a diode-connected device, its threshold
voltage will vary with change in temperature. As the temperature
increases, the threshold voltage of transistor 156 decreases and
the process and temperature tracking circuit 112 causes the voltage
at node 104 to settle more toward the high supply voltage (Vdd) at
the high supply node. Conversely, with decrease in temperature, the
threshold voltage of transistor 156 increases and the process and
temperature tracking circuit 112 causes the voltage at node 104 to
settle more toward the low supply voltage Gnd at the low supply
node.
[0023] The transistor 158 is designed for two purposes. One purpose
is for process tracking. If the PMOS is at fast corner, transistor
158 adjusts the CNT signal to reduce WLUD. Conversely, if the PMOS
is at slow corner, transistor adjusts the CNT signal to increase
WLUD. Another purpose concerns temperature compensation with
transistor 158 discharging node 104 close to ground level at the
start of the read/write operation in order to avoid settling of
node 104 at the Vdd level during a previous read/write operation or
no operating condition. Since the transistor 156 is designed to
track WLUD with varying NMOS process corner, that is the reason
transistor 158 is also designed as a diode for the proper process
tracking with NMOS and PMOS process corner.
[0024] The need for WLUD from an SNM point of view is at maximum
for lower supply voltage and at minimum for higher supply voltage.
The voltage tracking circuit 110 tracks the WLUD need over voltage
change. When the read assist enable signal RAEN is logic high, the
read assist is disabled. Transistor 124 is turned on to ground node
128 and turn off transistor 130. When the read assist enable signal
RAEN transitions to logic low, the read assist is enabled for WLUD.
Transistor 150 turns on to apply the supply voltage to the source
terminal of transistor 130. Because transistor 132 is configured as
a diode-connected device, the voltage at node 136 settles at the
NMOS threshold voltage (Vtn). Because transistor 122 is configured
as a diode-connected device, the voltage at node 128 settles at
PMOS threshold voltage (Vtp) below Vdd (i.e., Vdd-Vtp). The gate
overdrive voltage of transistor 130 is accordingly equal to
Vdd-Vtp-Vtn-Vtn. With a relatively lower supply voltage, the
voltage at node 128 is lower and transistor 130 will only weakly
turn on. As a result, the voltage at node 134 will settle close to
the high supply voltage Vdd to turn transistor 156 more strongly on
and cause the voltage at node 104 to settle more toward the high
supply voltage (Vdd) at the high supply node. With a relatively
higher supply voltage, on the other hand, the voltage at node 128
is higher and transistor 130 will more strongly turn on. As a
result, the voltage at node 134 will settle further away from the
high supply voltage Vdd to turn transistor 156 less strongly on and
cause the voltage at node 104 to settle more toward the low supply
voltage Gnd at the low supply node.
[0025] FIG. 3 shows signal waveforms for operation of the circuit
40'. At time t1, the decoder select signal PRECOM is asserted logic
high indicating the selection of the desired row of memory cells in
the memory array based on the address Addr. At time t2, the read
assist clock signal RACLK is asserted logic high indicating the
start of a read or write operation on the memory array and the
output of logic circuit 100 changes to logic low and the read
assist enable signal RAEN is asserted to enable operation of the
read assist circuit 40'. At time t3, the voltage at node 134
settles to a value that tracks the supply voltage Vdd. For example,
with respect to a relatively lower supply voltage, the voltage at
node 134 will settle closer to the high supply voltage Vdd
(reference 200). Conversely, with respect to a relatively higher
supply voltage, the voltage at node 134 will settle closer to the
ground supply voltage Gnd (reference 202). The voltage level at
node 134 affects the turn on strength of transistor 156. At time
t4, the voltage at node 104 settles to a value that tracks the
supply voltage Vdd. For example, with respect to a relatively lower
supply voltage, the voltage at node 104 will settle closer to the
high supply voltage Vdd (reference 204). Conversely, with respect
to a relatively higher supply voltage, the voltage at node 104 will
settle closer to the ground supply voltage Gnd (reference 206). The
voltage level at node 104 affects the turn on strength of
transistor 102. At time t5, the address decoder enable the wordline
driver 14. At time t6, the wordline voltage settles to a value to
provide the correct WLUD with respect to supply voltage. For
example, with respect to a relatively lower supply voltage, the
transistor 102 turns on more strongly and the wordline voltage will
settle closer to the ground supply voltage Gnd (reference 208).
Conversely, with respect to a relatively higher supply voltage, the
transistor 102 turns on less strongly and the wordline voltage will
settle closer to the supply voltage Vdd (reference 210).
[0026] The WLUD voltage is applied for the duration of the read or
write operation. At time t7, the address decoder disables the
wordline driver 14. At time t8, the wordline driver terminates
application of the wordline signal. At time t9, the read assist
clock signal RACLK is deasserted (logic low) and the output of
logic circuit 100 changes to logic high and the read assist enable
signal RAEN is deasserted. This causes transistors 152 and 160 to
turn on and force the voltage at nodes 134 and 104 to ground at
times t10 and t11, respectively.
[0027] The memory is designed to operate at different supply ranges
for the optimization of power and performance based on the need. At
a relatively lower voltage range, the implementation of WLUD
provides sufficient SNM to the memory cell. However, performance
degrades due to lower WL level compared to Vdd. The memory operates
at the lower voltage range in order to save the power consumption,
and hence performance degradation is acceptable. For better
performance, the memory is operated at higher voltage range. Once a
read-assist circuit without voltage tracking is tuned for WLUD at
the lower voltage range according to the need of SNM, the same
tuning cannot track required WLUD as the memory operating voltage
increases. Furthermore, due to unnecessary greater WLUD at the
higher voltage range, the memory ends up losing performance and
power. In other words, the write and read operation has to be
performed for longer duration due to lower WL voltage than Vdd.
Implementing voltage tracking along with process and temperature
tracking in read assist circuitry helps the memory to achieve
better PPA across wide operating voltage range.
[0028] The foregoing description has provided by way of exemplary
and non-limiting examples a full and informative description of the
exemplary embodiment of this invention. However, various
modifications and adaptations may become apparent to those skilled
in the relevant arts in view of the foregoing description, when
read in conjunction with the accompanying drawings and the appended
claims. However, all such and similar modifications of the
teachings of this invention will still fall within the scope of
this invention as defined in the appended claims.
* * * * *