Patent Abstract:
A SRAM system which provides for reduced power consumption. The SRAM system utilizes an array of bit cells. Columns of bit cells in the array are partitioned into sections. Each section of bit cells shares a local bit line. A sector select circuit provides for precharging the local bit lines. The sector select circuit also includes a mux for connecting a local bit line to a global bit line. The sector select circuit includes a device for detecting when a sector select signal and a column select signal are present. When both of these signals are present the sector select circuit couples the local bit line with the global bit line, and disengages the precharging of the local bit line.

Full Description:
TECHNICAL FIELD 
     The present invention relates to the field of static random access memory (SRAM) arrays, and to a SRAM memory architecture providing for bit line partitioning. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 shows a SRAM memory array architecture of the prior art. This architecture utilizes a six transistor memory cell  200  as shown in FIG.  2 . The six transistor SRAM bit cell  200  shown in FIG. 2 utilizes a first supply voltage, VDD  217 , and a ground connection  218 . The cell also includes a word line WLC. Bit lines BTC and BBC provide a connection to read and write data to the cell. The bit cell also includes a storage cell which includes four transistors,  206 ,  208 ,  210 ,  212 , configured to store data. As is known in the art, transistors  206  and  208  act as load transistors and transistors  210  and  212  act as cross coupled storage transistors. As shown in the bit cell  200 , the load transistors  206  and  208  are PMOS transistors, and storage transistors  210  and  212  are NMOS transistors. NMOS Transistors  214  and  216  arc word line, or row select, pass transistors. 
     In a static mode, when the cells in the memory array are not in write or read mode, bit lines BTC and BBC, shown in FIG. 2, are precharged to a VDD level, and the word line shown in FIG. 2 as WLC is at logic zero. In this static state, a programmed cell can maintain the information equivalent to logic 0 or logic 1, since n-channel devices  214  and  216  are off, which isolates the storage cell that includes devices  206 ,  208 ,  210  and  212 . 
     In a write mode, the WLC line (e.g. WL 0 , WL 1  . . . WLN) which is coupled to a row of cells (e.g. N 00 , N 01  . . . N 0 M), as shown in FIG. 1, which contains the cell being written to, is driven to logic 1 or VDD to turn on (open) the pass transistors, thereby providing access to the storage cell. To write to the cell to be programmed to store a binary  1 , the bit line BTC for the cell being written to is driven to logic 1, and the bit line BBC is driven to logic 0. This results in the cell being programmed to logic 1, where the voltage at node  202  will be set at logic 1 and the voltage at node  204  will be set at logic 0 as is known in the art. To program the cell to logic 0 the bit line BTC is driven to logic 0 and the bit line BBC is driven to logic 1, such that  202  will be set at logic 0 and  204  will be set at logic 1 as is known in the art. 
     In the static mode, in between read and write operations, the bit lines BTC and BBC are held at a precharge voltage VDD using the PMOS transistors  102  of the precharge circuit  106  shown in FIG.  1 . In the static mode the word line (WL 0 , WL 1 , . . . WLN) pass transistors  214  and  216  shown in FIG. 2 are held closed as the WLC voltage is at logic zero. 
     To read the data from the cell the WLC voltage is changed to logic 1. The signal of voltage logic 1 on WLC is applied to the gates of the word line pass transistors  214  and  216 , which opens the word line pass transistors  214  and  216 , so that current can flow through the transistors. In addition to the WLC voltage being set to logic 1, the precharge circuit  106  is closed so that the bit lines BTC and BBC are allowed to float. With the word line pass gate transistors open, one of the bit lines BTC and BBC will discharge depending on which node  202  or  204  is at zero. For example, if the cell is programmed at logic 0 then the BTC bit line will discharge through the NMOS transistor  214  and the cross coupled storage transistor  210 , and BBC would remain floating at the VDD level. If the cell was programmed at logic 1 then BBC would discharge through  216  and  212 , and BTC would remain at VDD. The switch (SW 0 , SW 1  . . . SWM) connected to the cell which is being read will be closed (conductive) and the sense circuitry  104  will read the difference in voltage in the bit lines BTC and BBC to determine whether the data is  1  (one) or  0  (zero). 
     In the prior art Static Random Access (SRAM) memory architecture  100  as shown in FIG. 1, there are three stages of operation. At stage  1  memory read/write operations require that all bit lines (BT 0 , BB 0 , BT 1 , BB 1 , . . . BTM, BBM) be precharged to logic 1 by the precharge circuitry  106 , the precharge circuitry provides PMOS transistors  102 , which in the static mode are opened by a PRCHG voltage signal  108  being at logic 0, which is applied to the gates of the PMOS transistors  102 . Also all word lines (WL 0 , WL 1  . . . WLN) are set to logic 0 before read read/write operation for any cell occurs. 
     At stage  2  of the memory read/write mode all are of the PMOS transistors  102  are closed (PRCHG voltage  108  is set to logic 1), so that the voltage on the bit lines is allowed to float, instead of being held at VDD. One of the word lines (e.g. WL 0 ) is driven to logic 1 All the 6T (6-transistor)core memory cells (e.g. bit cells N 00 , N 01  . . . N 0 M) coupled to this word line begin to discharge the bit lines (e.g. BT 0 , BB, BT 1 , BB 1  . . . BTM, BBM). The discharge of the bit lines at this stage causes a large active AC power dissipation. 
     Stage  3  of the memory bit cell read/write operation is selecting one of the switches (SW 0 , SW 1  . . . SWM) in the MUX block  103  by setting Y 0 , Y 1  . . . or YM to logic 1. As shown in FIG. 1, Y 0  is selecting column  1 . To write data to a bit cell at this stage requires using a write circuit  104  to program the selected individual bit cell, by applying a voltage differential to bit lines BT 0  and BB 0 . (The write circuitry and sense circuitry is known to one of skill in the art, and shown as block  104  in FIG. 1.) To read data from the bit cell requires amplifying the differential signal between the bit lines BTC and BBC using a sense amplifier and then routing this to an output circuit. 
     Regardless of which mode is used, whether read or write, a bit line for each column of SRAM memory bit cells of the complete array will be discharged during every read/write operation, and before a new read/write cycle can begin, and the array has to precharged again. This is because the same PRCHG signal is applied to the gates of all of the PMOS transistors  102  of the precharge circuit  106 , and all of the bit cells coupled to word line with logic 1 have word line pass transistors (e.g.,  214  and  216 ) which are opened as a result of the word line generating a logic 1 signal. Stated another way, all the bit lines have to precharged again because all have been discharged during the read/write operation. 
     One problem with this prior approach is that, for each read/write cycle, enough power to precharge and discharge all of the bit line pairs in the array is consumed, while all that is really needed is to program or read information for one bit line pair (e.g. BTC and BBC) during each read or write cycle. 
     As disclosed in the patent application filed on Apr. 9, 2002, entitled LOW POWER STATIC RAM ARCHITECTURE (U.S. application Ser. No. 10/119,191) which has common inventors to the present application, and is assigned to the National Semiconductor Corporation, the assignee of the present application, one approach to reduce the power consumed during each read write cycle is to implement an 8 bit memory cell where a column select signal can be used in conjunction with the a word select signal to limit the power discharge during each cycle to a particular column. The U.S. application Ser. No. 10/119,191 referred to above is hereby incorporated by reference in its entirety. As further discussed in the pending patent application Ser. No. 10/215,678 filed on Aug. 10, 2002, entitled LOW AC POWER STATIC RAM ARCHITECTURE, and in the pending patent application Ser. No. 10/215,676 filed on Aug. 10, 2002, entitled BIT LINE SHARING AND WORLD LINE LOAD REDUCTION FOR LOW AC POWER SRAM ARCHITECTURE the SRAM architecture can be further modified to decrease power consumption by providing for word line and bit line sharing and by providing for sector selection where sections of the columns of memory cells can be selected for reading and writing. Both the LOW AC POWER STATIC RAM ARCHITECTURE application and the BIT LINE SHARING AND WORLD LINE LOAD REDUCTION FOR LOW AC POWER SRAM ARCHITECTURE application referred to above are hereby incorporated by reference in their entirety. 
     The invention herein provides further designs which further decrease the power consumption of the SRAM. 
     SUMMARY 
     The present invention is directed to a static RAM system which allows for significant reduction in power consumption over prior systems by providing for partitioning columns of bit cells. One embodiment includes a plurality of columns of bit cells, wherein the columns of bit cells are partitioned into a plurality of sectors of bit cells. This embodiment provides a number of sector select circuits, where local bit lines couple sectors of bit cells with sector select circuits. The embodiment also provides a number of global bit lines which are coupled to the sector select circuits. The sector select circuits include a switch which couples the local bit line with the global bit line so that a selected bit cell in the sector of bit cells can be read from, or written to. 
     Another embodiment provides a sector select circuit for use in a SRAM system having a plurality of local bit lines and a plurality of global bit lines. The sector select circuit includes a mux circuit for coupling a local bit line of the SRAM with a global bit line of the SRAM system, and a first input for receiving a column select signal form the SRAM system. The sector select circuit also includes a second input for receiving a sector select signal from the SRAM system, and a local column select signal generation circuit which generates a local column select signal in response to receiving a sector select signal and column select signal. The mux circuit is coupled to the local column select signal circuit, and in response to a local column select signal, the mux couples the local bit with the with the global bit line. 
     Another embodiment provides a SRAM system having an array of bit cells including columns and rows of bit cells, wherein the columns of bit cells are partitioned into sectors of bit cells, and form an array of columns and rows of sectors of bits cells. Bit cells of the sectors of bit cells are coupled to a local bit line. An array of sector selection circuits including columns and rows of sector selection circuits are coupled to the sectors of bit cells, wherein a sector selection circuit is coupled to the local bit line coupled to bit cells of the sector of bit cells. The system also provides for column select lines which are coupled to a column of sector selection circuits, and sector selection lines, which are coupled rows of sector selection circuits. The system also provides global bit lines coupled to the column of sector selection circuits. The sector selection circuits include a first circuit and a switch, wherein the first circuit detects when a sector selection signal is present on the sector selection line and when a column selection signal is present on the column selection line and when both signals are present closes the switch whereby a local bit line to be coupled with a global bit line. 
     The features and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description of the invention and the accompanying drawings, which set forth an illustrative embodiment in which the principles of the invention are utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view of an SRAM system of the prior art. 
     FIG. 2 is a view of an SRAM bit cell of the prior art. 
     FIG. 3 is a view of the SRAM memory system of the present invention. 
     FIGS. 4 a-d  are illustrations describing SRAM bit cells used in an embodiment of the present invention. 
     FIGS. 5 a-c  are views of embodiments of sector select circuits of the present invention. 
     FIG. 6 is a view of an embodiment of an SRAM memory system of the present invention. 
     FIGS. 7 a-c  are views of embodiments of sector select circuits of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 shows an embodiment of the present invention. The SRAM architecture shown in FIG. 3 includes word lines (WL 0 , WL 1  . . . WLN- 1 , and WLN) and an array, including columns and rows, of bit cells  410 . As shown in FIG. 3, and as discussed in more detail below, the 6T bit cells  410  share local bit lines and share word line pass gate transistors  304 . 
     FIGS. 4 a - 4   d  show the relationship between an 8 transistor bit memory cell  400  (as described in the above referenced U.S. application Ser. No. 10/119,191) and the 6T bit cell  410  with bit line sharing and word line sharing as shown in FIG.  3 . Specifically, FIG. 4 a  shows a four transistor configuration  401  used for storing a bit of information. To perform a write or read operation to, or from, the four transistor configuration  401  the column select transistors  402  must be opened by providing a column select signal on line YSC, and the word line pass gate transistors  404  must also be opened by providing a signal on the word line WLC. Once the word line pass gate transistors  404  and column select transistors  402  are opened the four transistor configuration  401  can be read from, or written to, by either sensing or applying a voltage across the bit lines BTC and BBC. FIG. 4 b  shows a configuration where adjacent 8 transistor bit memory cells  400  share bit lines. Specifically area  412  shows the word line pass gate transistors  404  as both being coupled to the bit line BB 01  which is shared by the eight transistor bit cells  414  and  416 . 
     FIG. 4 c  shows a cell where the column select pass gate transistors  402  are coupled to a shared word line pass gate transistor  418 , which is in turn coupled to the shared bit line BB 01 . This configuration allows the overall number of word line pass gate transistors to be reduced by 50%. FIG. 4 d  represents the configuration of the four transistors  401  and the column select transistors  402 , as shown in FIG. 4 c , by showing a box corresponding to the “6T bit cell”  410 . FIG. 4 d  also shows the shared bit lines are BT 00 , BB 01  and BT 12  and the shared word line gate pass transistors  418 . 
     Turning now back to FIG. 3, the SRAM architecture  300 , shows an embodiment of the present invention with shared word line pass gate transistors  304 , and with shared local bit lines (e.g.  0 LBB 01 , KLBB 01  . . . KLBTMM) between adjacent 6T bit cells  410 , combined with bit line partitioning to further reduce power consumption and total capacitive load associated with each read/write operation. The overall power saving ratio depends on the sector size relative to the total memory size. 
     In the SRAM architecture  300 , the column select signals YS are sectorized, or partitioned, into local column select signals LYSA. This partitioning of the column select line YS allows for further load reduction, where the load reduction depends on the ratio of the sector size over the total memory size. 
     As shown in FIG. 3 the bit lines are partitioned into sections of local bit lines which correspond to the partitioning of the column select lines. Specifically, each bit line is partitioned into K sectors. Thus, instead of having bit lines BB 00  and BT 01  for column  0 , as shown in FIG. 1, there are local bit lines  0 LBT 00  and  0 LBB 01  for section  1 ,  1 LBT 00  and  1 LBT 01  for section  2  and so forth, where the local bit lines are partitioned as  01 BT 00 ,  0 LBB 01 ,  1 LBT 00 ,  1 LBB 01 , KLBBMM, KLBTMM for K sectors and M columns. During each read/write operation, only one sector is activated, and the local column select signal is present on only one LYSA line, where the LYSA line with the local column select signal is coupled to the 6T bit cells in the sector which contains the 6t bit cell which is being written to, or read from. 
     Each sector of the memory array of bit cells includes a sector selection circuit  302 . As discussed in more detail below, the operation and principles of different sector selection circuits, in the SRAM architecture is essentially identical, but some modifications are necessary to account for the sector selection circuits position in the overall array of 6T bit cells  410 . 
     As shown in FIGS. 5 a - 5   c , the sector selection circuit includes a column mux circuit  502 , and a precharge circuit  504 . The sector selection circuit also includes an AND gate  508 , which operates as a circuit for receiving a sector select signal (SS) and for receiving a column select signal YS. The sector select signal is received at input port  510 , and the column select signal is received at input port  512 . The precharge circuit  504  of the sector select circuit  302  serves to precharge local bit lines LBB and LBT to the voltage VDD. The mux  502  serves to couple the selected local bit lines LBT and LBB with the corresponding global bit lines GBT and GBB. 
     Because the local bit lines LBT and LBB are shared between adjacent 6T bit cells  410 , the precharge circuit  504  is shared between adjacent sectors of adjacent columns of bit cells  410 . One exception to this is noted in connection with the bit line  0 LBT 00  of column  0  at the left hand side of the SRAM architecture shown in FIG.  3 . For the sector select circuit in the far left hand column, the “sector corner” circuit, one additional precharge  504  and mux  502  circuit is provided (detail for the “sector corner” circuit is shown in FIG. 5 a ). The shared global bit lines are GBT 00  and GBB 01  for column  0 , GBB 01  and GBT 12  for column  1  and so forth. The bit lines BT and BB alternate every other column as shown in FIG.  3 . Hence the “sector A” selection circuitry, shown in FIG. 5 b , and the “sector B” selection circuitry, shown in FIG. 5 c  alternate, every other column like the bit lines. 
     The local bit lines LBT and LBB can be accessed for reading or writing to a selected 6T bit cell  410  through the global bit lines GBT and GBB, where the selected mux switches  502  are opened to connect a specific global bit line with a selected pair of local bit lines, and the word lines (e.g. WL 0 , WL 1  . . . ) and the local column select signals, LYSA, are utilized to select a specific 6T bit cell for reading or writing. In one embodiment the transmission gates  502  are implemented using NMOS transistors. The switches  502  are controlled by the local column select signals LYSA. 
     Further details of the SRAM architecture can be illustrated by example. Consider for instance, the situation where 6T bit cell N01 is being read from. In this situation a column select signal voltage is generated on column select line Y 1 . This voltage on line Y 1  is received by the NOR gate  306 , of the global bit line precharge circuit  308 . The NOR gate  306  then outputs a voltage which causes the PMOS transistor  308 , which is coupled to the global bit line GBB 01 , to close, and thus GBB 01  is allowed to float. Further, the NOR gate  310  receives the voltage on Y 1  and in response outputs a voltage which closes the PMOS transistor  312  which allows the global bit line GBT 12  to float. The column select signal Y 1  is also transmitted through the buffer  318  as signal YS to the sector select circuits in column  1 . (As shown these are “sector A” circuits.) The sector select circuit in column  1  corresponds to the sector select circuit  506  shown in FIG. 5 b . The column signal YS is received by the AND gate  508 . A sector select signal is also generated sector select line SS 0 . The sector select signal SS 0  is also received by the AND gate  508 . In response to receiving the signals on sector select line and the column select line the AND gate  508  outputs a local column select signal LYSA, on the local column select line LYSA. This local column select signal closes (makes conductive) a switch of the mux  502  thereby coupling the local bit line LBT with the global bit line GBT. Additionally, the voltage output by the AND gate, LYSA, is applied to the gate of the one of the PMOS transistors of the precharge circuit  504  which causes one of the transistors to close and thus the local bit line LBT of sector  1  of column  1  is allowed to float. Note that the “sector A” circuit  520  is the only sector select circuit of the SRAM architecture  300  which receives a sector select signal and a column select signal, and hence it is the only sector select circuit which outputs a local column select signal LYSA. 
     The signal LYSA output by the AND gate  508  is also output to the 6T bit cells  410  of the first sector of column  1  on a local column select line LYSA. This LYSA signal is received by the column select pass gate transistors  402  (see FIG. 4 c ) of the 6T bit cells  410 , and in response the column select pass gate transistor open. A signal is also generated on the word line WL 0  and in response to this signal the word line pass gate transistors  304  (as shown in FIG. 3) coupled to the WL 0  word line open. 
     In addition to outputting a signal LYSA in response to the sector select signal SS 0  and the column select YS signal, the sector select circuit also outputs a line select signal LYSB. The LYSB signal is input to the line select signal input port LYSB for sector select circuit shown as “Sector Corner”  302  in FIG.  3 . This sector select circuit is shown in detail in FIG. 5 a . The line select signal LYSB causes a PMOS transistor of the precharge circuit  504  of the sector selection circuit which receives it to close which allows the local bit line LBB 01  to float. The signal LYSB also closes a switch in the mux  502  of the Sector Corner  302  circuit, which couples the local bit line LBB 01  with the global bit line GBB 01 . 
     Reference is now made to the SRAM mux circuit  314 , where in response to the signal on Y 1  switches S 1  and S 2  of the mux  314  are closed, and thereby couples the global bit lines GBB 01  and GBT 12  with the sense amplifier write circuit  316 . The sense amplifier write circuit operates to sense a voltage differential between the global bit lines GBB 01  and GBT 12  thereby reading data stored in the bit cell  410  shown as N 01 . To write data to the N 01  the sense amplifier write circuit applies a voltage differential between the bit cells GBB 01  and GBT 12 . 
     In the manner described above data can be written to read from any of the bit cells (N 00 -NNM). By way of a hi-level summary example to read information at bit cell N 00 , the YS 0  and SS 0  are turned on. The corresponding precharge circuits are turned off and the switches are closed to couple the local bit lines with the global bit lines. The differential signal at  0 LBB 01  and  0 LBT 00  will be passed to the global bit line GBB 01  and GBT 00  respectively. 
     The load reduction ratio depends on the number of sectors in memory. For example, in a 256×256 memory with 16 sectors, each sector will contain 16 bit cells vertically. The load for the local bit line will be the  16  bit cells in the sector. Transistors in the sector will be the total load for the global bit line. If the load of the sector mux is similar to the load for a bit cell, then the load for the global bit line equals the load of the 16 bit cells. Therefore, the load can be reduced from 256 to 32. In addition sector select circuitry which generates the sector select signal may also create an additional load. In one embodiment the load of the sector select circuit is equivalent to 16 bit cells, so the total load reduction is from 256 to 48. 
     It should be noted that the invention has been described above in connection with a synchronous SRAM where the precharge circuit is turned on and off in connection.,with reading and writing information to the bit cell. The invention herein can also be applied to an asynchronous SRAM where the precharge circuits are not clocked on and off, and are instead always held in an on state. FIG. 6 shows an asynchronous SRAM system  600 . In this asynchronous system  600 , the gates of the PMOS transistors  602  of the precharge circuits  608  sector select circuits  604 ,  606  and  608  are connected to ground. Further, the gates of the PMOS transistors  610  of the global bit line precharge circuit  612  are connected to ground. This means the local bit lines ( 0 LBT 00 -KLBTMM) and the global bit lines (GBT 00 -GBTMM) are in a state of constant precharge. In prior asynchronous SRAM (similar to the system shown in FIG. 1, but with the PMOS transistors  102  held in an open condition) large power consumption occurred because the there is a short circuit path between Vdd and ground through the PMOS precharge transistors  602  and through the bit cells for both the selected columns and the non-selected columns. As discussed in more detail in patent application Ser. No. 10/119,191 this large power consumption has limited the actual use of asynchronous SRAM systems. By employing the asynchronous system shown in FIG. 6, there is a significant reduction in power consumption. This reduction in AC power dissipation for asynchronous memories is realized by partitioning columns of bit cells into sectors as described above. Reduced power consumption within the memory is achieved due to the proportional reduction of the short circuit current between Vdd and Gnd as only the one selected sector of bit cells consumes power, while the rest of the unselected sectors will be inactive and remain in the precharged state. This technique also reduces the peak AC current by the same argument. 
     For example, to read information at bit cell N 00 , the YS 0  and SS 0  are turned on, and the switches of the mux of corresponding sector selection circuit are closed to couple the local bit lines with the global bit lines. The corresponding word line WL 0  is also turned on, and the differential signal at  0 LBB 01  and  0 LBT 00  will be passed to the global bit line GBB 01  and GBT 00  respectively. Thus, the present a SRAM asynchronous system as shown in FIG. 6 reduces the amount of power consumed during read and write operations, as only the local bit lines discharged are  0 LBB 01  and  0 LBT 00 . Additionally aspects of asynchronous SRAM systems are discussed in patent application Ser. No. 10/119,191. 
     Although specific embodiments and methods of the present invention are shown and described herein, this invention is not to be limited by these methods and embodiments. Rather, the scope of the invention is to be defined by the following claims and their equivalents.

Technology Classification (CPC): 6