DUAL RAIL SINGLE-ENDED READ DATA PATHS FOR STATIC RANDOM ACCESS
MEMORIES
Field of the Invention
[0001] The invention generally relates to field of read data paths for memory devices.
Background
[0002] Sub-micron Integrated Circuit (IC) designs often utilize a number of different
power supplies to reduce power consumption and to improve performance. For example, in sub-
micron Static Random Access Memories (SRAM) devices, lower voltage supplies may be
utilized for the memory cells to improve the dynamic power consumption, while peripheral
circuits to the memory cells may utilize high voltage supplies for interfacing the SRAM device
to external circuits. Level shifters are used to translate the logic level range of the SRAM cells
to the logic level range of the peripheral circuits. However, level shifters in the data path
between the SRAM cells and the peripheral circuits may introduce delays in reading data from
the SRAM cells, which degrades the performance of the SRAM device.
Summary
[0003] Single-ended read circuits for SRAM devices are disclosed for high performance
sub-micron designs. One embodiment is an SRAM device. The SRAM device includes a
memory cell array and a bit line traversing the memory cell array for reading data from memory
cells of the memory cell array. The SRAM device further includes a read circuit coupled to the
bit line for translating data stored in a memory cell from a cell voltage of the memory cells to a
peripheral voltage of an output of the SRAM device. The cell voltage and the peripheral voltage
share a common ground. The read circuit comprises a data path circuit that is configured to
toggle a first internal signal between a logical zero of the peripheral voltage and a logical one of
the peripheral voltage based on read data on the bit line and a second internal signal. The data
path circuit is further configured to toggle a third internal signal between a logical zero of the
cell voltage and a logical one of the cell voltage based on an enable signal and the read data on
the bit line. The read circuit further comprises a level shifter that is configured to toggle the
second internal signal between a logical zero of the peripheral voltage and a logical one of the
peripheral voltage based on the read data on the bit line and a reset signal. The read circuit

further comprises an output driver that is configured to toggle the output of the SRAM device
between a logical zero of the peripheral voltage and a logical one of the peripheral voltage based
on the first internal signal.
[0004] The various embodiments disclosed herein may be implemented in a variety of
ways as a matter of design choice. For example, some embodiments herein are implemented in
hardware whereas other embodiments may include processes that are operable to construct
and/or operate the hardware. Other exemplary embodiments are described below.
Brief Description of the Figures
[0005] Some embodiments of the present invention are now described, by way of
example only, and with reference to the accompanying drawings. The same reference number
represents the same element or the same type of element on all drawings.
[0006] FIG. 1 is a block diagram of an enhanced SRAM architecture in an exemplary
embodiment.
[0007] FIG. 2 is a block diagram of a single-ended SRAM read circuit for translating
data stored in a memory cell from a cell voltage of the memory cell to a peripheral voltage in an
exemplary embodiment.
[0008] FIG. 3 is a schematic diagram of the read circuit of FIG. 2 in an exemplary
embodiment.
[0009] FIG. 4 is a schematic diagram of data path circuit 202 of FIG. 3 in another
exemplary embodiment.
[0010] FIG. 5 is a timing diagram for the circuits of FIGS. 3-4 in an exemplary
embodiment.
[0011] FIG. 6 is a block diagram of a single-ended SRAM read circuit for translating
data stored in a memory cell from a cell voltage of the memory cell to a peripheral voltage in
another exemplary embodiment.

[0012] FIG. 7 is a schematic diagram of the read circuit of FIG. 6 in an exemplary
embodiment.
Detailed Description of the Figures
[0013] The figures and the following description illustrate specific exemplary
embodiments of the invention. It will thus be appreciated that those skilled in the art will be able
to devise various arrangements that, although not explicitly described or shown herein, embody
the principles of the invention and are included within the scope of the invention. Furthermore,
any examples described herein are intended to aid in understanding the principles of the
invention and are to be construed as being without limitation to such specifically recited
examples and conditions. As a result, the invention is not limited to the specific embodiments or
examples described below.
[0014] FIG. 1 is a block diagram of an enhanced SRAM architecture 100 in an
exemplary embodiment. Architecture 100 is a simplified block diagram view that will be used to
discuss the inventive aspects of the SRAM devices disclosed herein. But, architecture 100 is not
intended to limit the implementation to any particular embodiment. Those skilled in the art will
understand that additional components not shown or described in FIG. 1, such as drivers, latches,
decoders, sense amps, etc. may be used to implement architecture 100 in various configurations
as a matter of design choice.
[0015] Architecture 100 in this embodiment includes an array 102 of memory cells 104-
105. Memory ells 104-105 are disposed in array 102 at the intersections of column bit lines and
row word lines within array 102. For instance, memory cell 104-1 is disposed in array 102
where bit line 106-1 intersects word line 108-1. To access memory cell 104-1, word line 108-1
is asserted utilizing a row decoder circuit 112, and bit data stored at memory cell 104-1 is read
out by bit line 106 utilizing a read circuit 116-1. In this embodiment, memory cells 104-105
operate at a different voltage range than peripheral I/O 110. For example, memory cells 104-105
may operate at a lower voltage than peripheral I/O 110 in order to improve the dynamic power of
architecture 100. Thus, part of the read activity of memory cells 104-105 involves translating bit
data stored by memory cells 104-105 from the voltages used to store bit data in the memory cells
(referred to herein as cell voltage) to the voltages used to generate an OUTPUT 122 from read

circuit 116 for the bit data (referred to herein as peripheral voltage). In this embodiment,
architecture 100 utilizes a single-ended read data path from cells 104-105, and therefore, only
one bit line 106 is used to read data from a particular memory cell.
[0016] In this embodiment, read circuit 116-1 is coupled with bit line 106-1, and read
circuit 116-N is coupled with bit line 106-N. However, in other embodiments, multiple bit lines
106 may be multiplexed together before routing to read circuit 116. For instance, some subset of
the total number of bit lines 106 may be multiplexed together (e.g., 8 bit lines) before routing the
output of the multiplexer to a particular instance of read circuit 116. Out of the n-multiplexed bit
lines, one bit line is selected based on pre-decode signals. Although not shown in FIG. 1,
memory cells 104-105 may utilize a bistable latch circuit to store a bit of data (e.g., a 6 transistor
SRAM cell), although the particular implementation of memory cells 104-105 is a matter of
design choice.
[0017] In this embodiment, read circuit 116 does not include level shifters in the read
data path between memory cells 104-105 and OUTPUT 122 of read circuit 116, although read
circuit 116 is capable of translating bit data stored by memory cells 104-105 from the cell
voltages of the memory cells 104-105 to the peripheral voltage of OUTPUT 122. This reduces
the delay along the read data path between memory cells 104-105 and OUTPUT 122, and
improves the read performance of architecture 100. The specifics of how read circuit 116 may
operate will become more readily apparent in discussing FIGS. 2-7
[0018] FIG. 2 is a block diagram of a single-ended SRAM read circuit for translating
data stored in a memory cell from a cell voltage of the memory cell to a peripheral voltage in an
exemplary embodiment. In this embodiment, read circuit 116 includes a data path circuit 202, a
level shifter circuit 204, and an output circuit 206. Generally, read circuit 116 translates memory
cell data read by bit line 106 from cell voltage 208 to peripheral voltage 210. Data path circuit
202 is coupled to cell voltage 208, peripheral voltage 210, and a common ground 212, and
toggles an MX signal 214 that is used by output circuit 206 to generate OUTPUT 122. In
particular, data path circuit 202 includes any component or device that is able to toggle MX 214
between ground 212 (a logical zero or low) and peripheral voltage 210 (a logical one or high)
based on a read signal on bit line 106 (e.g., data read from a memory cell, which may be a logical

zero at approximately ground 212 or a logical one at approximately cell voltage 208) and an
RSDEC signal 216. Data path circuit 202 also toggles an LS signal 218 that is used by level
shifter circuit 204 to generate RSDEC 216. In particular, data path circuit 202 includes any
component or device that is able to toggle LS 218 between ground 212 (a logical zero or low)
and peripheral voltage 210 (a logical one or high) based on an ENABLE signal 120 and read data
on bit line 106. Both ENABLE 120 and the signals on bit line 106 toggle between a logical zero
at approximately ground 212 and a logic one at approximately cell voltage 208.
[0019] Level shifter circuit 204 is coupled to cell voltage 208, peripheral voltage 210,
and ground 212, and toggles RSDEC 216 that is used by data path circuit 202 to generate MX
214. In particular, level shifter circuit 204 includes any component or device that is able to
toggle RSDEC 216 between ground 212 (a logical zero or low) and peripheral voltage 210 (a
logical one or high) based on LS 218 and a RESET signal 118. RESET 118 toggles between a
logical zero at approximately ground 212 and a logical one at approximately peripheral voltage
210.
[0020] FIG. 3 is a schematic diagram of read circuit 116 of FIG. 2 in an exemplary
embodiment. In this embodiment, a number of Field-Effect Transistors (FETs) 302-315,
inverters 316-317, and a NAND gate 318 are illustrated in a particular configuration for
implementing the functionality previously described for read circuit 116 of FIG. 2 Generally,
FETs 302-315 may include any type of field-effect transistor as a matter of design choice. One
example of a FET is a Metal Oxide Semiconductor Field Effect Transistor (MOSFET).
Although FIG. 3 illustrates one configuration of a number of FETs, inverters, and aNAND gate,
one skilled in the art will understand other configurations may be implemented as a matter of
design choice. Thus, it is not intended that read circuit 116 be limited to only the configuration
and types of components illustrated in FIG. 3.
[0021] When ENABLE 120 is high and the read data on bit line 106 is low, NAND gate
318 sets LS 218 low. All other logic combinations of ENABLE 120 and the read data on bit line
106 set LS 218 high. With LS 218 low and RESET 118 high, FET 315 is on, and FET 314 is on.
This discharged RSDEC 216 to ground 212. FET 306 and FET 305 are on, which turns off FET
308. FET 307 may be partially on due to the voltage differential between cell voltage 208 output

by inverter 317 and peripheral voltage 210, but the gate on FET 308 is charged to peripheral
voltage 210 so there is no leakage path between peripheral voltage 210 and ground 212. A low
RSDEC 216 turns on FET 303 of data path circuit 202. With both FET 302 and FET 303 on,
MX 214 charges to peripheral voltage 210. FET 304 and FET 311 of output circuit 206 invert
MX 214 to set OUTPUT 122 low.
[0022] With ENABLE 120 high and the read data on bit line 106 high, NAND gate 318
sets LS 218 high. With LS 218 high, FET 312 is on, which turns on FET 308. FET 307 is on,
which charges RSDEC 216 to peripheral voltage 210. FET 306 may be partially on due to the
voltage differential between cell voltage 208 output by NAND gate 318 and peripheral voltage
210, but the gate on FET 305 is charged to peripheral voltage 210, so there is no leakage path
from peripheral voltage 210 to ground 212. With RSDEC 216 at peripheral voltage 210 FET
303 in data path circuit 202 is off.
[0023] With FET 303 off and the read data on bit line 106 high, FET 310 is on, which
pulls MX 214 low. Output circuit 206 inverts MX 214 and sets OUTPUT 122 high. For read
circuit 116, accessing data stored by memory cells 104-105 occurs faster because there is not a
voltage translation circuit in the data path between memory cells 104-105 and OUTPUT 122.
This allows for reading data from memory cells 104-105 faster, which improves the performance
of architecture 100.
[0024] In some embodiments, a latch circuit (not shown) may be coupled with MX 214
to latch MX 214 at the end of the read cycle for the memory cell. In those embodiments, the
latch circuit may latch MX 214 when RESET 118 toggles from high to low.
[0025] FIG. 4 is a schematic diagram of data path circuit 202 of FIG. 3 in another
exemplary embodiment. In this embodiment, a decoder 400 is used to generate ENABLE 120,
and a N-channel FET 412 has been added to data path circuit 202 of FIG. 3. FET 412 is utilized
to ensure that bit line 106 is pulled low whenever ENABLE 120 is low. In this embodiment,
decoder 400 generates ENABLE 120 based on a PREDEC signal 402 and a GMXRSP signal
403. PREDEC 402 may be a column select signal used to select which column to connect to bit
line 106. GMXRSP signal 403 may be a tracking signal which tracks the reading and transfers
the data onto bit line 106.

[0026] FIG. 5 is a timing diagram for the circuits of FIGS. 3-4 in an exemplary
embodiment. FIG. 5 illustrates the various relationships between OUTPUT 122, MX 214,
RESET 118, and RSDEC 216 based on the logical states of PREDEC 402, GMXRSP 404, and
the read data applied to bit line 106.
[0027] FIG. 6 is a block diagram of a single-ended SRAM read circuit for translating
data stored in a memory cell from a cell voltage of the memory cell to a peripheral voltage in
another exemplary embodiment. In this embodiment, read circuit 116 includes a data path circuit
602, a level shifter circuit 604, and an output circuit 206. Generally, read circuit 116 translates
memory cell data read by bit line 106 from cell voltage 208 to peripheral voltage 210. Data path
circuit 602 is coupled to cell voltage 208, peripheral voltage 210, and a common ground 212, and
toggles MX 214 that is used by output circuit 206 to generate OUTPUT 122. In particular, data
path circuit 602 includes any component or device that is able to toggle MX 214 between ground
212 (a logical zero or low) and peripheral voltage 210 (a logical one or high) based on read data
on bit line 106 (e.g., data read from a memory cell, which may be a logical zero at approximately
ground 212 or a logical one at approximately cell voltage 208), RSDEC 216, and a compliment
of ENABLE 120, illustrated as ENABLE 414. Both ENABLE 414 and the read data on bit line
106 toggle between a logical zero at approximately ground 212 and a logic one at approximately
cell voltage 208.
[0028] Level shifter circuit 604 is coupled to cell voltage 208, peripheral voltage 210,
and ground 212, and toggles RSDEC 216 that is used by data path circuit 602 to generate MX
214. In particular, level shifter circuit 604 includes any component or device that is able to
toggle RSDEC 216 between ground 212 (a logical zero or low) and peripheral voltage 210 (a
logical one or high) based on a RESET 118 and ENABLE 120.
[0029] FIG. 7 is a schematic diagram of read circuit 116 of FIG. 6 in an exemplary
embodiment. In this embodiment, a number of FETs 702-711 and an inverter 712 are illustrated
in a particular configuration for implementing the functionality previously described for read
circuit 116 of FIG. 6. Generally, FETs 702-711 may include any type of field-effect transistor as
a matter of design choice (e.g., MOSFET). Although FIG. 7 illustrates one configuration of a
number of FETs and an inverter, one skilled in the art will understand other configurations may

be implemented as a matter of design choice. Thus, it is not intended that read circuit 116 be
limited to only the configuration and types of components illustrated in FIG. 7.
[0030] In this embodiment, when RESET 118 is high and ENABLE 120 is high, FETs
705-706 of level shifter circuit 604 are on. FET 704 is off. This pulls RSDEC 216 low. With
ENABLE high and RSDEC 216 low, FETs 710-711 are off and FET 708 of data path circuit 602
is on. FETs 702-703 are also off. When the read data signal on bit line 106 is low, FET 707 is
on and FET 709 is off, which charges MX 214 to peripheral voltage 210. FET 304 and FET 311
of output circuit 206 (see FIG. 2) invert MX 214 to set OUTPUT 122 low. When the read signal
on bit line 106 is high, FET 707 is off or partially off. FET 709 is on, which discharges MX 214
to ground 212. FET 304 and FET 311 of output circuit 206 invert MX 214 to set OUTPUT 122
high.

Claims:
What is claimed is:
1. A Static Random Access Memory (SRAM) device, comprising:
a memory cell array;
a bit line traversing the memory cell array for reading data from memory cells of the
memory cell array;
a read circuit coupled to the bit line for translating data stored in a memory cell from a
cell voltage of the memory cells to a peripheral voltage of an output of the SRAM device,
wherein the cell voltage and the peripheral voltage share a common ground, the read circuit
comprising:
a data path circuit configured to toggle a first internal signal between a logical
zero of the peripheral voltage and a logical one of the peripheral voltage based on read
data on the bit line and a second internal signal, and to toggle a third internal signal
between a logical zero of the cell voltage and a logical one of the cell voltage based on an
enable signal and the read data on the bit line;
a level shifter circuit configured to toggle the second internal signal between a
logical zero of the peripheral voltage and a logical one of the peripheral voltage based on
the third internal signal and a reset signal; and
an output driver circuit configured to toggle the output of the SRAM device
between a logical zero of the peripheral voltage and a logical one of the peripheral
voltage based on the first internal signal.

2. The SRAM device of claim 1 wherein the data path circuit comprises:
a first P-channel Field Effect Transistor (FET) having a source terminal coupled to the
peripheral voltage, a gate terminal coupled to the bit line, and a drain terminal;
a second P-channel FET having a source terminal coupled to the drain terminal of the
first FET, a gate terminal coupled to the second internal signal, and a drain terminal coupled to
the first internal signal; and
a third N-channel FET having a drain terminal coupled to the first internal signal, a gate
terminal coupled to the bit line, and a source terminal coupled to the common ground.
3. The SRAM device of claim 2 wherein the data path circuit further comprises:
an inverter having a supply terminal coupled to the cell voltage, a ground terminal
coupled to the common ground, an input coupled to the bit line, and an output terminal; and
a NAND gate having a supply terminal coupled to the cell voltage, a ground terminal
coupled to the common ground, a first input terminal coupled to the output terminal of the
inverter, a second input terminal coupled to the enable signal, and an output terminal coupled to
the third internal signal.
4. The SRAM device of claim 3 wherein the data path circuit further comprises:
a fourth N-channel FET having a drain terminal coupled to the bit line, a gate terminal
coupled to a compliment of the enable signal, and a source terminal coupled to the common
ground.
5. The SRAM device of claim 1 wherein the output circuit comprises:
a first P-channel Field Effect Transistor (FET) having a source terminal coupled to the
peripheral voltage, a gate terminal coupled to the first internal signal, and a drain terminal
coupled to the output of the SRAM device; and
a second N-channel FET having a drain terminal coupled to the output of the SRAM
device, a gate terminal coupled to the first internal signal, and a source terminal coupled to the
common ground.

6. The SRAM device of claim 1 wherein the level shifter circuit comprises:
a first P-channel Field Effect Transistor (FET) having a source terminal coupled to the
peripheral voltage, a gate terminal coupled to the second internal signal, and a drain terminal;
a second P-channel FET having a source terminal coupled to the drain terminal of the
first FET, a gate terminal coupled to the third internal signal, and a drain terminal;
a third N-channel FET having a drain terminal coupled to the drain terminal of the second
FET, a gate terminal coupled to the third internal signal, and a source terminal coupled to the
common ground;
a fourth N-channel FET having a drain terminal coupled to the drain terminal of the third
FET, a gate terminal coupled to the second internal signal, and a source terminal coupled to the
common ground;
a fifth P-channel FET having a source terminal coupled to the peripheral voltage, a gate
terminal, and a drain terminal;
a sixth P-channel FET having a source terminal coupled to the drain terminal of the fifth
FET, a gate terminal coupled to the drain terminal of the fourth FET, and a drain terminal
coupled to the second internal signal;
a seventh N-channel FET having a drain terminal coupled to the second internal signal, a
gate terminal coupled to the reset signal, and a source terminal;
an eighth N-channel FET having a drain terminal coupled to the source terminal of the
seventh FET, a gate terminal coupled to the gate terminal of the fifth FET, and a source terminal
coupled to the common ground;
a ninth P-channel FET having a source terminal coupled to the peripheral voltage, a gate
terminal coupled to the reset signal, and a drain terminal coupled to the second internal signal;
and
an inverter having a supply terminal coupled to the cell voltage, a ground terminal
coupled to the common ground, an input terminal coupled to the third internal signal, and an
output terminal coupled to the gate terminal of the eighth FET.

7. A Static Random Access Memory (SRAM) device, comprising:
a memory cell array;
a bit line traversing the memory cell array for reading data from memory cells of the
memory cell array;
a read circuit coupled to the bit line for translating data stored in a memory cell from a
cell voltage of the memory cells to a peripheral voltage of an output of the SRAM device,
wherein the cell voltage and the peripheral voltage share a common ground, the read circuit
comprising:
a data path circuit configured to toggle a first internal signal between a logical
zero of the peripheral voltage and a logical one of the peripheral voltage based on read
data on the bit line and a second internal signal, and to set the bit line to a logical zero
based on the second internal signal and a compliment of an enable signal;
a level shifter circuit configured to toggle the second internal signal between a
logical zero of the peripheral voltage and a logical one of the peripheral voltage based on
the enable signal and a reset signal; and
an output driver circuit configured to toggle the output of the SRAM device
between a logical zero of the peripheral voltage and a logical one of the peripheral voltage based
on the first internal signal.
8. The SRAM device of claim 7 wherein the data path circuit comprises:
a first P-channel Field Effect Transistor (FET) having a source terminal coupled to the
peripheral voltage, a gate terminal coupled to the bit line, and a drain terminal;
a second P-channel FET having a source terminal coupled to the drain terminal of the
first FET, a gate terminal coupled to the second internal signal, and a drain terminal coupled to
the first internal signal; and
a third N-channel FET having a drain terminal coupled to the first internal signal, a gate
terminal coupled to the bit line, and a source terminal coupled to the common ground.

9. The SRAM device of claim 8 wherein the data path circuit further comprises:
a third N-channel FET having a drain terminal coupled to the bit line, a gate terminal
coupled to the second internal signal, and a source terminal; and
a fourth N-channel FET having a drain terminal coupled to the source terminal of the
third FET, a gate terminal coupled the compliment of the enable signal, and a source terminal
coupled to the common ground.
10. The SRAM device of claim 7 wherein the level shifter circuit comprises:
a first P-channel Field Effect Transistor (FET) having a source terminal coupled to the
peripheral voltage, a gate terminal coupled to the reset signal, and a drain terminal coupled to the
second internal signal;
a second N-channel FET having a drain terminal coupled to the second internal signal, a
gate terminal coupled to the reset signal, and a source terminal;
a third N-channel FET having a drain terminal coupled to the source terminal of the
second FET, a gate terminal coupled to the enable signal, and a source terminal coupled to the
common ground;
a fourth P-channel FET having a source terminal coupled to the peripheral voltage, a gate
terminal coupled to the enable signal, and a drain terminal;
a fifth P-channel FET having a source terminal coupled to the drain terminal of the fourth
FET, a gate terminal, and a drain terminal coupled to the second internal signal; and
an inverter having a supply terminal coupled to the peripheral voltage, a ground terminal
coupled to the common ground, an input terminal coupled to the second internal signal, and an
output terminal coupled to the gate terminal of the fifth FET.
11. The SRAM device of claim 7 wherein the output circuit comprises:
a first P-channel Field Effect Transistor (FET) having a source terminal coupled to the
peripheral voltage, a gate terminal coupled to the first internal signal, and a drain terminal
coupled to the output of the SRAM device; and
a second N-channel FET having a drain terminal coupled to the output of the SRAM
device, a gate terminal coupled to the first internal signal, and a source terminal coupled to the
common ground.

ABSTRACT

Single-ended read circuits for SRAM devices are disclosed for high performance sub-micron
designs. One embodiment is an SRAM device that includes a memory cell array and a bit line
traversing the memory cell array for reading data from memory cells of the memory cell array.
A read circuit coupled to the bit line translates data stored in a memory cell from a cell voltage of
the memory cells to a peripheral voltage of an output of the SRAM device while bypassing a
level shifter in the read data path.