RAM cell with column clear

[0002] The present invention relates generally to integrated circuits, and more particularly, to techniques and circuits for storing data in a static random access memory.

[0003] Static random access memory (SRAM) is used in many electronic systems to store digital data without the need for periodic refresh. For example, the cache memory of many computer systems is made up of SRAM. A typical SRAM cell fabricated in a metal-oxide-semiconductor (MOS) process with one read/write port is often made from six transistors—four transistors to hold the data and two transistors per read/write port used as access devices. The access devices are typically controlled by “word lines” running in a first direction (for example, across a row or the horizontal direction when viewed oriented a certain way) and would dump/take data from “bit lines” running in another direction (for example, along a column or the vertical direction when viewed as described above). Therefore, a typical SRAM cell requires one word line running in the horizontal direction per row of SRAM cells and two bit lines running in the vertical direction per column of SRAM cells.

[0004] Some specialized applications, however, require an SRAM cell to have functionality in addition to a single read/write port. One such specialized application involves the storage of “physical hit vectors” as described in U.S. Pat. No. 6,014,732 granted to Naffziger which is hereby incorporated herein by reference. This application needs two read/write ports and the ability to clear an entire column of SRAM cells.

[0005] The addition of another read/write port and the ability to clear an entire column increases the number of word lines running horizontally and the number of bit lines running vertically. In particular, two additional bit lines running vertically for the second read/write port and a “clear” line also running vertically would be needed for each column of SRAM cells. Likewise, an additional word line would have to run horizontally across each row of SRAM cells. This brings the total number of lines running vertically over a given SRAM cell to five and the total number of lines running horizontally to two for a total of seven lines running across each SRAM cell.

[0006] This number of lines, especially lines running in the vertical direction, tends to grow the size of the SRAM cell significantly and reduce its frequency of operation. Accordingly, there is a need in the art for a two-ported SRAM cell with a column clear function that has fewer that five vertical lines and fewer that seven total lines across the cell.

[0007] The present invention provides an SRAM cell and array that has a column clear function with only three vertical lines and six total lines across a cell and a method of operating that cell and array of those cells. Instead of two bit lines per port and two access devices per port as in a traditional SRAM cell, one bit line and one access device per port is used. In addition, one additional bit line, one additional word line, and two devices in series are used to perform the column clear operation and complete a write operation.

[0008] The cell is operated by performing write operations using a two step process. To perform a write, each cell in a row to be written is preset during a first step. Then, each cell that is to have a zero written to it is cleared using the additional bit line and additional word line to address the cells to be cleared. A column of cells may be cleared by enabling all the rows for clearing, then asserting column clear control signals for each of the columns in the array to be cleared.

[0009] Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

[0010] FIG. 1 is a schematic illustration of an SRAM with column clear functionality having six total lines across it.

[0011] FIG. 2 is a flowchart illustrating a method of writing data into a row of the SRAM cells shown in FIG. 1.

[0012] FIG. 3 is a flowchart illustrating a method of clearing at least one column of SRAM cells shown in FIG. 1.

[0013] FIG. 1 is a schematic illustration of an SRAM with column clear functionality having six total lines across it. In FIG. 1, a first, second, and third word lines WL1102, WL2104, WL3106, respectively, run across the cell in a first direction. For illustration purposes, this first direction will be hereinafter referred to as the horizontal direction. A first, second, and third bit lines BL1112, BL2114, BL3116, respectively, run across the cell in a second direction. For illustration purposes will be hereinafter referred to as the vertical direction. Also, when the SRAM cells of FIG. 1 are placed in a two dimensional array, the cells adjacent to each other in the horizontal direction will be referred to as being in a row and the cells adjacent to each other in the vertical direction will be referred to as being in a column. Finally, note that the total number of lines crossing the cell is six and the total number in the vertical direction is three. This compares favorably with the seven total and five vertical a traditional two-port cell with column clear needs.

[0014] P-channel field effect transistors (PFETs) 120 and 122 and N-channel field effect transistors (NFETs) 124 and 126 form a pair of cross-coupled CMOS inverters that act as a memory element of the SRAM cell shown in FIG. 1. This is for illustrative purposes only. Other type of memory elements could be used as well. For example, the invention could be used with cross-coupled NMOS inverters, a flip-flop formed from NAND or NOR gates, or other types of memory elements using different technologies such as magnetic random access memory cells (MRAM cells).

[0015] A first of the cross-coupled inverters is formed by PFET 120 and NFET 124. The input to this inverter is node BL and the output node BH. The source of PFET 120 is connected to the positive supply voltage. The drain of PFET 120 is connected to node BH which is connected to the gate of PFET 122, the gate of NFET 126, the drain of NFET 124, and the drain of NFET 140. The source of NFET 124 is connected to the negative supply voltage. The gates of both PFET 120 and NFET 124 are both connected to the drains of PFET 122 and NFET 126.

[0016] A second of the cross-coupled inverters is formed by PFET 122 and NFET 126. The input to this inverter is node BH and the output node BL. The source of PFET 122 is connected to the positive supply voltage. The drain of PFET 122 is connected to node BL which is connected to the gate of PFET 120, the gate of NFET 124, the drain of NFET 126, and the drains of NFETs 130 and 132. The source of NFET 126 is connected to the negative supply voltage. The gates of both PFET 122 and NFET 126 are both connected to the drains of PFET 120 and NFET 124.

[0017] NFET 130 is a first access device. The gate of NFET 130 is connected to word line WL1102. The drain of NFET 130 is connected to bit line BL1112. NFET 132 is a second access device. The gate of NFET 132 is connected to word line 104. The drain of NFET 132 is connected to bit line 114.

[0018] NFETs 140 and 142 form column clear logic that provides the capability to clear the cell when properly addressed by row and column. The gate of NFET 140 is connected to bit line BL3116. The source of NFET 140 is connected to the drain of NFET 142. The gate of NFET 142 is connected to word line WL3106. The source of NFET 142 is connected to the negative supply voltage.

[0019] To read the cell shown in FIG. 1, either, or both bit lines 112 and 114 are precharged and the precharging device turned off so that bit lines 112 or 114 are floating at a precharge level. Then, when word line 102 and/or word line 104 are pulled high, one or both of access devices 130 and 132 are turned on, respectively. This allows the value stored by the memory element as output on node BL to charge or discharge bit lines 112 or 114. Bit lines 112 and 114 are charged or discharged depending upon the precharge level and the value on node BL. For example, if bit line 112 was precharged to the positive supply voltage and the memory element was driving BL low (i.e. at or near the negative supply voltage level), then bit line 112 would be discharged through access device 132 and the memory element. The lowered voltage level of bit line 112 could then be read by a sense amplifier on bit line 112. Another example is if bit line 112 was precharged to an intermediate value (i.e. not near the positive or the negative supply voltage levels) and the memory element was driving BL to a high (i.e. at or near the positive supply voltage level). In this case, bit line 112 would be charged through access device 132 and the memory element. This raised voltage level on bit line 112 could then be read by a sense amplifier on bit line 112. Note that since there are two access devices 130, 132 and two bit lines 112, 114 two reads can be accomplished simultaneously either from the same or different rows.

[0020] A process for writing to a row of SRAM cells shown in FIG. 1 is illustrated by a flowchart shown in FIG. 2. In a step 202 logical 1's are written into the entire row of cells that are being written. This is done by driving all the bit lines BL1112 for the row low while the word line WL1102 is active. This causes the memory element to hold a logical 0 on BL and a logical 1 on BH. Note that for the cell shown in FIG. 1, word lines WL1102, WL2104, WL3106, and bit line BL3116 are active when they are driven high (i.e. driven towards the positive supply voltage).

[0021] In a step 204, the row is enabled for clearing. This is done by activating WL3106. To prevent a drive fight, when WL3106 is activated, WL1102 and WL2104 should be deactivated.

[0022] In a step 206, the column clear signal for all cells in the row that are to store logical 0's are activated. This is done by activating BL3116 for the columns corresponding to each cell in the row that is to store a logical 0. Accordingly, this causes the memory element for those cells to hold a logical 1 on BL and a logical 0 on BH.

[0023] A process for clearing a column of SRAM cells shown in FIG. 1 is illustrated by a flowchart shown in FIG. 3. In a step 302, all of the rows in the SRAM to be cleared are enabled for clearing. This is done by activating WL3 for all of the rows to be cleared. To clear an entire column, WL3 for all of the rows would be activated. To clear columns in only a subset of rows, then a subset of WL3 lines in the array would be activated.

[0024] In a step 304, the column clear signal is activated for the columns that are to be cleared. This is done by activating BL3116 for the one or more columns to be cleared.

[0025] Although a specific embodiment of the invention has been described and illustrated, the invention is not to be limited to the specific form or arrangement of the parts so described and illustrated. The invention is limited only by the claims.