Abstract:
A method for sensing the contents of a memory cell within a static random access memory (SRAM) includes holding a bit line associated with the memory cell at a zero voltage potential when the memory cell is not being accessed; energizing the bit line to a first voltage potential different than the zero voltage potential during an access of the memory cell; and sensing the memory cell contents when the associated bit line has reached the first voltage potential.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to memory devices and, more particularly, to an apparatus and method for low power sensing in a multi-port SRAM using pre-discharged bit lines. 
         [0002]    A typical static random access memory (SRAM) cell includes an array (rows, columns) of individual SRAM cells. Each SRAM cell is capable of storing a voltage value therein, which voltage value represents a corresponding binary logical data bit value (e.g., a “low” or “0” value, and a “high” or “1” value). One existing configuration for an SRAM cell includes a pair of cross-coupled devices such as inverters. Using CMOS (complementary metal oxide semiconductor) technology, each inverter comprises a pull-up PFET (p-channel) transistor connected to a complementary pull-down NFET (n-channel) transistor, with the two transistors in each inverter typically connected in series between a positive voltage potential and ground. The inverters, further connected in a cross-coupled configuration, act as a bistable latch that stores the data bit therein so long as power is supplied to the memory array. 
         [0003]    The transistors within the typical SRAM cell exhibit relatively significant current leakage, particularly at the word-line transistor gates and the bit-line transistor gates. Since known SRAM cell designs require a constant power level both to maintain the data bit stored in the SRAM latch and to allow the reading from and the writing to of data, the current leakage increases the power used by the array of SRAM cells. For example, one common technique is to continuously pre-charge all of the read bit lines within the SRAM to a logical high level; that is, to a positive voltage of, e.g., +1 volts. This is done when the bit lines are not being accessed. After a read cycle involving certain read bit lines, those bit lines are returned to their pre-charge state. The resulting undesirable use of power in these prior art designs increases with the increase in SRAM cell density and the overall number of cells on an integrated circuit (IC), such as a stand-alone memory device, or as part of a processor or application-specific integrated circuit (ASIC). 
         [0004]    Various techniques to reduce the leakage current have been proposed, such as increasing the size of the cell by making the devices longer, increasing the threshold voltages of the cell, adding additional transistors to the cell, or lowering the voltage to the array when the cell is not being accessed. However, all of these techniques can increase the area of the array, or significantly reduce the performance of the array. 
         [0005]    What is needed is an apparatus and method to reduce the DC power consumption in a multi-port SRAM cell due to relatively large cell current leakage as well as to reduce the AC power consumption in the multi-port SRAM cell due to relatively large read bit line voltage swings. 
       SUMMARY 
       [0006]    The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated, in an exemplary embodiment, by an apparatus for low power sensing in a multi-port SRAM using pre-discharged bit lines. In an exemplary embodiment, the apparatus includes a first switch that holds a bit line associated with the memory cell at a zero volt potential when the memory cell is not being accessed; a second switch that holds a sense line at a first voltage potential for a period of time after access to the memory cell has been allowed, wherein the sense line is connected to the bit line when the memory cell is being accessed, and wherein the bit line is energized to a second voltage potential different than the first voltage potential when the memory cell is being accessed; and a sense amplifier that senses the second voltage potential on the bit line when the memory cell is being accessed. 
         [0007]    In another exemplary embodiment, a method for low power sensing in a multi-port SRAM using pre-discharged bit lines includes holding a bit line associated with the memory cell at a zero voltage potential when the memory cell is not being accessed; energizing the bit line to a first voltage potential different than the zero voltage potential during an access of the memory cell; and sensing the memory cell contents when the associated bit line has reached the first voltage potential. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
           [0009]      FIG. 1  is a schematic diagram of a multi-port SRAM cell; 
           [0010]      FIG. 2  is a schematic diagram of circuitry that includes the SRAM cell of  FIG. 1  and illustrates an exemplary embodiment of the present invention; and 
           [0011]      FIG. 3  shows several signal timing diagrams within the circuitry of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Disclosed herein is an apparatus and method for low power sensing in a multi-port SRAM using pre-discharged bit lines. Briefly stated, the apparatus and method pre-charges the SRAM read port bit lines to a logic low level of zero volts (i.e., “pre-discharges” the bit lines). As a result, the read port bit lines of the multi-port SRAM do not leak DC current when pre-discharged as such. The apparatus and method holds the SRAM read port bit lines that are not being read at any particular point in time at ground (zero voltage) potential, and energizes selected read port bit lines (i.e., applies a potential thereto) only when the selected read port bit lines are accessed to read or sense the stored information within the selected memory cell. In addition, the potential applied to the selectively energized read bit lines is lower in value than the full rail voltage potential (typically +1 volts or Vdd); that is, the applied potential is at an intermediate value between Vdd and ground, thereby saving AC power due to relatively lower voltage swings on these lines. 
         [0013]    Referring to  FIG. 1 , there is shown a typical multi-port (e.g., two port) SRAM memory cell  100 . The cell  100  includes a base cell  102  that comprises six CMOS transistors  104 - 114 , wherein the base cell  102  constitutes both the write port of the memory cell  100  and the basic storage element of the memory cell  100 .  FIG. 1  also shows a read port  116  in which both the true and complement read bit lines, rblt  118 , rblc  120 , are connected to a sense amp  122  (SA), shown in  FIG. 2 , for sensing the logic value stored in the cell  100 . A plurality of the read ports  116  may be used as part of a single memory cell  100 , if desired. 
         [0014]    The base cell  102  of  FIG. 1  includes a bistable latch  124  comprising a first pair of PMOS (e.g., PFET) and NMOS (e.g., NFET) transistors  104 ,  106  connected in series as an inverter between a positive power supply potential Vdd (e.g., +1 volts) and a ground potential (e.g., 0 volts). The latch  124  further comprises a second pair of PMOS and NMOS transistors  108 ,  110 , also connected in series as an inverter between the power supply potential Vdd and ground. The transistors  104 ,  106  have their respective gate terminals connected together at a storage node  126 , which is also connected to the drain terminals of both transistors  108 ,  110 , which drain terminals are connected together. This storage node  126  is referred to as the “complement” node. Similarly, the transistors  108 ,  110  have their respective gate terminals connected together at a storage node  128 , which is also connected to the drain terminals of both transistors  104 ,  106 , which drain terminals are connected together. This storage node  128  is referred to as the “true” node. In normal operation of the base cell  102 , the true storage node  128  and the complement storage node  126  typically store complementary logic levels (i.e., one node stores a binary “1” while the other node stores a binary “0”, or vice versa). Thus, the PMOS transistors  104 ,  108  operate as load transistors and the NMOS transistors  106 ,  110  operate as drive transistors within the base cell  102 . 
         [0015]    The base cell  102  also includes two NMOS transistors  112 ,  114 . A first transistor  112  is connected between a true write bit line, wblt  130 , and the storage node  128 . A second transistor  114  is connected between a complement write bit line, wblc  132 , and the storage node  126 . Gate terminals of these transistors  112 ,  114  are connected to a common write word line, wwl  134 . As such, the transistors  112 ,  114  each have their respective gate potentials controlled by the write word line, wwl  134 . 
         [0016]    The read port  116  further includes four NMOS transistors  136 - 142 . Two of the transistors  136 ,  138  are connected in series between the true read bit line, rblt  118 , and ground. Another two of the transistors  140 ,  142  are connected in series between the complement read bit line, rblc  120 , and ground. Gate terminals of two of the transistors  136 ,  140  are connected to a common read word line, rwl  144 . As such, the transistors  136 ,  140  have their respective gate potentials controlled by the read word line, rwl  144 . The gate of transistor  138  is connected to the complement storage node  126  in the base cell  102 , while the gate of transistor  142  is connected to the true storage node  128  in the base cell  102 . In general, the transistors  136 - 142  within the read port  116  do not necessarily need to be long channel or SRAM-type high voltage threshold devices. It suffices that these transistors  136 - 142  are such that any current leakage therethrough does not degrade the signal to a large enough extent to cause any read errors. 
         [0017]    In operation of the base cell  102  and the read port  116 , when the common write word line, wwl  134 , is active, access to the cell for write or read operations is enabled. Thus, when wwl  134  is active, data may be written to the storage nodes  126 ,  128  via the two complementary write bit lines, wblt  130 , wblc  132 , respectively. When the common write word line, wwl  134 , is inactive, the data previously written to the storage nodes  126 ,  128  is held steady by the latch  124  comprised of the transistors  104 - 110 . When the common read word line, rwl  144 , is active, data is read from the storage nodes  126 ,  128  via the two complementary read bit lines, rblt  118 , rblc  120 . In a typical SRAM memory cell  100 , it is not necessary to periodically assert the common write word line  134  (i.e., apply a voltage thereto) to refresh the data held in the latch  124 . The data will be held in a steady state in the latch  124  as long as power is continuously applied to the cell  100 . 
         [0018]      FIG. 2  shows an exemplary embodiment of the present invention. In the multi-port SRAM, multiple rows  200 ,  202  of SRAM memory cells  100  (two rows  200 ,  202  are shown, each row having a plurality of cells  100 ) may each be connected to the sense amp  122 . For each row  200 ,  202  of cells  100 , the cells  100  are connected together by the respective read bit lines: rblt 0   204  and rblc 0   206  for row zero  200 ; rblt 1   208  and rblc 1   210  for row one  202 . These read bit lines  204 - 210  are the read bit lines  118 ,  120  originating from the read port  116  in  FIG. 1 . Each read bit line  204 - 210  is connected to the sense amp  122  through a corresponding bit switch circuit, where each bit switch circuit is comprised of an NFET pass gate transistor  212 - 218 , each having a relatively high voltage threshold. The true read bit lines  204 ,  208  of each row  200 ,  202  pass through the corresponding bit switch circuits  212 ,  216  and connect together as a true sense line, slt  220 . Similarly, the complement read bit lines  206 ,  210  of each row  200 ,  202  pass through the corresponding bit switch circuits  214 ,  218  and connect together as a complement sense line, slc  222 . The gate terminal of each bit switch circuit NFET transistor  212 ,  214  for row zero  200  is controlled (i.e., the NFET transistor is turned “on”) by a positive active signal line, bso  224 . Similarly, the gate terminal of each bit switch circuit NFET transistor  216 ,  218  for row one  202  is controlled by a positive active signal line, bs 1   226 . The sense lines  220 ,  222  are provided to the sense amp  122 , which is enabled by a sense signal line, set_en  228 . 
         [0019]      FIG. 2  also illustrates that, in accordance with the present invention, the apparatus further includes an NFET transistor  230 - 236  for a corresponding one of each of the read bit lines  204 - 210 . The drain terminal of each NFET transistor  230 - 236  is connected to the corresponding read bit line  204 - 210 , while the source terminal of each transistor  230 - 236  is connected to ground. The gate terminal of each transistor  230 - 236  is connected to a common positive active control signal line, pdbl  238 . As described in detail hereinafter, when one or more of the transistors  230 - 236  are turned on, the corresponding read bit line  204 - 210  is pulled down to ground potential, thereby “pre-discharging” the corresponding read bit line  204 - 210 , with the result being that no DC current leakage occurs on these lines  204 - 210  when pre-discharged as such. 
         [0020]    In  FIG. 2 , the sense lines, slt  220  and slc  222 , are also connected to a sense line pre-charge control circuit that comprises three PFET transistors  240 - 244 . The gate terminals of all three transistors  240 - 244  are connected to a negative active sense line pre-charge signal, xpusl  246 . When this signal, xpusl  246 , turns on each of the transistors  240 - 244 , the sense lines, slt  220  and slc  22 , are pre-charged to a high logic level of Vdd (e.g., +1 volts). 
         [0021]    In the apparatus of  FIG. 2  in accordance with an exemplary embodiment of the present invention, the read bit lines  204 - 210  are pre-charged to a logic low level of, e.g., ground or zero volts (“pre-discharged”), through the NFET transistors  230 - 236 , rather than to a logic high level of, e.g., Vdd or +1 volts, as in the prior art. Also, as compared to prior art dual-ended sensing apparatus and methods, the bit switch circuit transistors  212 - 218  now comprise relatively high threshold voltage (Vt) NFETS instead of PFETS. In addition, the polarity of the transistor controls signals (i.e., the gate voltage signals) agree with their respective transistor device-types, and the timing of the sense line pre-charge control signal, xpusl  246 , has been changed, as illustrated in  FIG. 3 . These changes do not increase the area occupied by the SRAM memory cell  100 . In fact, in practice it has been discovered that the area is reduced slightly. 
         [0022]    When the read bit lines  204 - 210  are pre-discharged to a logic low level, no DC leakage occurs through the read ports  116  of the SRAM. A slight delay in reading out the stored data occurs because the read bit lines  204 - 210  are energized to an intermediate voltage level between Vdd and ground prior to their sensing or reading out of the stored values therefrom. This is done by keeping the sense line pre-charge control signal, xpusl  246 , active for a short period of time after the word line, wwl  134 , has been activated, as shown in  FIG. 3 . AC power is reduced because only selected ones of the bit lines  204 - 210  that are being read are energized at any particular point in time, and also because, even when energized, the selected bit lines  204 - 210  are not fully charged to Vdd but to a voltage that is intermediate between Vdd and ground. 
         [0023]    Referring also to  FIG. 3 , there illustrated are several signal traces of voltage values versus time at different points in the circuit of  FIG. 2 . The respective bit switch control signals, bs 0   224 , bs 1   226 , are active high as shown in  FIG. 2  and in the top trace  300 . In the example shown, row  1   202  is activated because the pass gate transistor control signal, bs 1   226 , assumes a logic high value shortly after time t=1, thereby turning on NFETS  216 ,  218 , while the pass gate transistor control signal, bs 0   224 , remains at a low logic level, thereby keeping NFETS  212 ,  214  off and not allowing the read bit signal lines, rblt 0   204  and rblc 0   206 , to influence the sense lines, slt  220  and slc  222 . 
         [0024]    As shown in the next trace  302 , the pre-discharge control signal, pdbl  238 , for the NFETS  230 - 236  assumes a low logic level also shortly after time t=1, thereby turning off the NFETS  230 - 236  (i.e., removing the “pre-discharge” or zero volt state of the read bit lines  204 - 210 ). Also, shortly after time t=1, the common read word line, rwl  144 , assumes a high logic level, thereby allowing access to the cell  100 . The sense line pre-charge control signal, xpusl  246 , stays at a logic low until approximately t=2, at which time it changes to a logic high, thereby turning off the PFETS  240 - 244 . This delay between rwl  144  going high and xpusl  246  going high allows the selected read bit lines (here, rblt 1   208  and rblc 1   210 ) to become energized to a voltage value intermediate between Vdd and ground, as described hereinafter. 
         [0025]    The next trace  304  shows the read bit lines for the selected row, row  1   202 , in which the true read bit line, rblt 1   208 , and the complement read bit line, rblc 1   210 , are energized and start to increase in voltage beginning shortly after time t=1. The read lines will achieve a voltage value intermediate between Vdd (e.g., +1 volts and ground). This increase in voltage is due to the aforementioned delay between rwl  144  going high and then xpusl  246  going high. That is, the sense line pre-charge PFETS  240 - 244  remain turned on for a short time after rwl  144  is activated until xpusl  246  also goes high, which allows for charge-sharing from the sense lines, slt  220  and slc  222 , to the bit lines rblt 1   208  and rblt 0   210 , of the selected row  202 . One of the bit lines, rblt 1   208 , will not rise in voltage as quickly as that of the other bit line, rblc 1   210 , due to the “0” state of the selected cell  100  in this exemplary embodiment. 
         [0026]    The next trace  306  shows the sense lines, slt  220  and slc  222 , and the set enable signal, set_en  228 . As compared to the prior art, the set enable signal is delayed slightly in transitioning from logic low to logic high (at approximately time t=3) to account for the time it takes to energize the read bit lines  208 ,  210 . Consequently this delays the sense line resolution, which is the time at which the sense line signal, slt  220 , assumes a logic low (i.e., at approximately time t=4). Thereafter the logic bit values on the selected read bit lines can be sensed or read by the sense amp  122 . 
         [0027]    The apparatus and method of the present invention pre-charges the read port bit lines  204 - 210  to a logic low level when they are not being read or sensed so that the read ports  116  of the multi-port SRAM do not unnecessarily leak DC current. The bit lines  204 - 210  are held at ground and energized only when they are accessed, as shown in  FIG. 3 . Hence the read port DC leakage due to the SRAM cells is significantly reduced, as compared to prior art schemes that pre-charge the read bit lines to Vdd. There is a small amount of current leakage through the bit switch transistors  212 - 218 . However, this amount of leakage is significantly less than that of the cells for all but the smallest SRAM sizes. In addition, the AC power is reduced because the bit lines  204 - 210  are not fully charged to a full voltage rail potential (e.g., of Vdd) when they are energized (as shown in  FIG. 3 ), and also because only the bit lines  204 - 210  that are being read are energized. When the selected read bit lines  204 - 210  are energized, some current leakage occurs. However, because typically only a selected few, and not all, of the bit lines are energized at any one point in time, the overall amount of current leakage caused by the energized bit lines is significantly lower then in the prior art where all of the bit lines are typically pre-charged to Vdd. 
         [0028]    The read performance is delayed slightly to allow the bit lines to energize to some intermediate voltage lower than Vdd prior to the read operation. The delay penalty is small, and depends on the technology voltage and temperature. For example, in a 65 nm CMOS bulk technology at a slow process corner and low voltage, the delay penalty is about 60 ps. At a 1 GHz cycle time, this would represent a 6% decrease in performance. With newer technologies, the write time of the cell limits the performance more than the read time, so delaying the read slightly may not affect overall performance at all. Implementation of the present invention requires no additional area over current multi-port SRAM designs and does not change the design structure of the sense amp  122 . 
         [0029]    The expected power savings brought about by the present invention depends on the memory configuration and the operating voltage. For example, at 1V in a 65 nm CMOS-bulk technology, a two-port array configured as 8 columns with 64 cells per column, a savings of approximately 654 nW per sense amp occurs. At a cycle time of 900 ps, the AC power savings are 2400 nW per sense amp. For an ASIC design employing 60 two-port SRAM macros, each with 2000 sense amps, then approximately 78 mW leakage and 0.29 W AC power per chip may be saved. 
         [0030]    While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.