Multi-port static random access memory with fast write-thru scheme

Abstract: A fast write-thru scheme is proposed for use in a multi-port static random access memory. This is achieved by operating the read and write ports of the SRAM circuitry in two separate but interleaved stages. In a first stage, a write path is set up comprising a write address decoder (244), an AND gate (245) connected to a clock signal (CE) and the write port (260) and latch (290) of a memory cell (200). In the second stage, a read path is set up comprising a read address decoder (254), a latch (290), read port (270-274) and a data out buffer (225). To minimise the write-thru access time, the asynchronous read path controlled by the read address is interleaved with the write path triggered by a write clock (CE), so that the read address is delayed with respect to the clock and the write addresses. Thus the write-thru access time becomes independent from the write time needed for overwriting the multi-port SRAM cell and equal to the read address access time (Taa) achieved in a fully static or asynchronous read operation. The proposed timing scheme is particularly suited for structured and expandable multi-port SRAMs embedded on CMOS VLSI logic chips.

Description: Field of the Invention The invention concerns a multi-port memory with memory cells, bit lines, word lines, read ports and write ports, whereby the word line used for reading is connected to a read address decoder, the word line used for writing is connected to a write address decoder and a clock signal is provided. The invention further comprises a method for writing data through the multi-port memory cell. Description of Prior Art High-speed general purpose registers used in a central processing unit (CPU) are built up from multi-port static random access memories (SRAM). The multi-port SRAMs have n data input and data output ports and n separate write and read address ports. They are thus able to perform n separate data transfers during a single machine cycle. Such registers are normally required to possess so-called write-thru capability, i.e. the data which is written into one of the storage cells of the memory via a write port is required to be read out through one or more read ports during the same cycle. This procedure is described in a number of prior art documents, including German Patent DE-C-28 17 431 (IBM). An example of the realisation of one such register 10 is known from European Patent Application EP-A-0 434 852 (IBM) and is shown in Fig. 1. The example depicted has only one write port 30 which is connected to a data input line 20. Three read ports 50a, 50b and 50c are shown in the figure. They are each connected to separate data output lines 60a, 60b and 60c respectively. A decoder 70 decodes the read AR1, AR2, AR3 and write AW address signals appearing on lines 100a-c and 80 respectively and selects the indicated word lines within a cell array 40. The cell array 40 comprises a number of static latches as is well known in the prior art. A clock signal (CE) is provided on line 90 to the cell decoder 70 and to the cell array 40. Fig. 2 shows a timing diagram for a prior art write-thru procedure. The write-thru procedure is clocked from a clock signal (CE) and edge triggered. The clock signal (CE) is used to strobe valid input data (DI) appearing on data input line 20 through the write port 30 into the cell array 40. Before the clock signal (CE) rises, the data input (DI) signal on data input line 20 and address signals (AW, AR) must be valid. The address signals comprise the write address signal (AW) indicating the address at which the input data (DI) on data input line 20 is to be written and the read address signals (AR; AR1, AR2, AR3) indicating the address from which output data (DO; DO1, DO2, DO3) is to be output on data out lines 60a-c through read ports 50a-c. The minimum required set up time for the address signals (AW, AR) is shown on Fig. 2 as Ts. The register 10 starts the write-thru cycle as soon as the clock signal (CE) goes positive. The clock signal (CE) must be held high for a minimum time Tce until valid output data (DO) appears on the data output lines 60a-c after access time Tac. The data input (DI) and address signals (AR, AW) must stay valid for longer than time Tce. The extra time these signals remain valid is denoted Th and is referenced to the negative clock edge. Before the next write cycle begins, the clock signal (CE) goes low. The problem with this prior art solution is that the write-thru access time Tac is large since it comprises the time needed to overwrite the latch of the SRAM cell, as well as the time needed to read the data out of the cell. EP-A-0 440 191 discloses a multiport memory device in which both the write and the read operations are synchronized to a clock signal, respectively. In EP-A-0 178 163 there is disclosed a multiport memory device in which the write and the read operations overlap each other. The read and write operations are synchronized by clock signals. Summary of the Invention The object of the invention is therefore to produce a memory register and a method for operating the memory register in which the write-thru access time is reduced. This object is achieved by providing an asynchronously controlled static read word line to the memory cell and by clocking a write word line to the memory cell using a clock signal as specified in claims 1 and 13. The clocked write word line is constructed by connecting the output of the write address decoder of the memory cell to an AND gate, the other input of which is connected to the clock signal. The memory cell of the memory register comprises a buffered latch, one node of which is connected to a write bit line and the other node of which is connected to a read bit line through a transfer device. As the read address decoder is not clocked, the timing of the read address is no longer critical and thus it can be set up at a later time than the write address. The memory cell is operated in a pipelined mode in which the write and read phases are interleaved. The method for writing data through the cell comprises the following steps: placing data to be written onto the write bit line connected to the memory cell; strobing the write address from a write address port to a clocked write word line; overwriting the contents of a latch in the memory cell; setting up a read address to select a static read word line and reading data out of the latch in the memory cell onto a read bit line. Description of the Drawings Fig. 1 shows a prior art implementation of the register. Fig. 2 shows a timing diagram of a write-thru procedure using the circuit of Fig.1. Fig. 3 shows a four-port SRAM employing the proposed invention. Fig. 4 shows a timing diagram of a write-thru procedure using the circuit of Fig. 3. Fig. 5 shows an interleaved timing diagram used for a simulation of the invention. Fig. 6 shows the timing signals and waveforms generated by the simulation of the invention. Detailed Description of the Invention Fig. 3 illustrates an embodiment of the current invention. It comprises a four-port memory cell 200 with one write port 210 and three read ports 220. The write port 210 is connected to a data in buffer 215 and each of the read ports 220 to data out buffers 225. In the embodiment the four-port memory cell 200 is accessed by a clocked write word line 246 and three separate static read word lines 256. The read word lines 256 are, unlike the write word line 246, not clocked by a clock signal (CE) supplied to the memory register. In Fig. 3 only one of the read word lines 256 is shown for simplicity. Write address signals (AW) appear at write address port 240 and are buffered by write address buffer 242. Write address word decoder 244 decodes the write address signals (AW) and outputs a signal onto an output line 241 corresponding to the cell into which data is to be written. This output line 241 is connected to an AND gate 245 whose other input is the clock signal (CE) on line 230. From the AND gate 245, the clocked signal is passed to the write word line 246 of the cell 200 into which data is to be written. Read address signals (AR) appear at read address ports 250 and are buffered by read address buffers 252. Read address word decoders 254 decode the read address signals (AR) and output a signal onto the read word line 256 corresponding to the cell 200 from which data is to be read. The memory cell 200 itself comprises a static CMOS latch 290 connected to one write port and three read ports, each arranged in a single ended configuration. The write port is realised by an NFET transfer device 260 connected between a first latch node 282 of the latch 290 and a write bit line 217 which is in turn connected to the data in buffer 215. The NFET transfer device 260 is gated by the clocked write word line 246. Each of the read ports are realised by an NFET transfer device 270, 272, 274 connected between a second latch node 284 and the appropriate one of read bit lines 227 which are in turn connected to the data out buffers 225. The gates of the NFET transfer devices 270, 272, 274 are connected to the appropriate one of the static read word lines 256. The latch 290 is an inverting latch and an inverter buffer 280 is connected between the output of the latch 290 and the second latch node 284 in order to provide sufficient driving capability for the connected read bit lines. Referring now to Fig. 4, the write-thru operation of the circuit shown in Fig. 3 can be described. For the purpose of illustration, it is assumed that a logical '0' is initially stored in the latch 290 of the memory cell 200 and that the write bit line 217 is at an up-level as data is to be written into the latch 290 of the memory cell 200. The first phase of the write-thru operation comprises a write pipeline operation which is initiated by the clock signal (CE) going high at time To as long as the write address (AW) has been previously set up at time Ts prior to To, as shown on Fig. 4. In the first stage of the write pipeline operation, the clock signal (CE) on the line 230 strobes, using the AND gate 245, the output of the fully static write address word decoder 244 along the clocked write word line 246 to the write port of the selected memory cell. In the second stage of the write pipeline operation, as the write word line 246 rises, the write port NFET 260 is turned on and pulls up the first latch node 282, thus overwriting the stored '0' in the latch 290. In the embodiment of the invention illustrated, the data on the write bit line 217 may become valid at a time Tdi after To as shown on Fig. 4. The up-level obtained at the first latch node 282 is inverted by the latch 290 and then reinverted by the attached inverter buffer 280 and thus also appears as an up-level at second latch node 284. After the value in the latch 290 has been overwritten, the write operation is completed and the latch 290 is ready for selection of the read port. The write word line 217 may be turned off at this stage in order to ensure that invalid data at the data in port 210 does not switch the state of the latch 290. After the clock signal (CE) has gone negative at time Tce after To, the write address signal (AW) must remain valid for a further period, hold time Th, after which it and the data in signal (DI) may become invalid. The second phase of the write-thru operation comprises a read pipeline. This can already be started whilst the writing of the latch 290 of the memory cell 200 is being executed and is initiated as soon as the read address signal (AR) becomes valid. In stage 1 of the read pipeline operation, the read address signal (AR) is decoded in the read address decoder 254, resulting in a select signal on one of the read word lines 256. The read address signal (AR) becomes valid at time Tw after To as shown on Fig. 4. In stage 2, as the read word line 256 rises, the selected one of the read NFET transfer devices 270-274 is turned on. The inverter buffer 280 charges up the selected one of the read bit lines 227 and, after amplification by the output buffer 225, the overwritten cell data appears at the selected output ports 220 at a time Tv after To as shown on Fig. 4. The difference between the times Tv and Tw is denoted Taa and is the address access time for the data in the cell. It is shorter than the access time Tac shown on Fig. 2 for the prior art memory register shown in Fig. 1. Should the latch 290 originally be charged to '1' and need to be overwritten with a '0', this is carried out in a similar manner. The first latch node 282 is discharged from supply voltage to ground through the write port NFET 260 and this down-level signal propagates through the subsequent inverting stages and can be read out through the selected ones of the read port transfer NFETs 270-274 to the respective output port 220. A basic timing diagram of the external and internal signals of the invention is shown in Fig. 5 to illustrate the advantages of the invention over prior art solutions. The reference letters shown on the figure are the same as those used previously. The reference letters WLW indicate the signal developed on the write word line 246, BLW the signal developed on write bit line 217, WLR the signal developed on one of the read word lines 256, BLR the signal developed on one of the read bit lines 227 and NT the signal developed at the first latch node 282 of the memory cell 200. The first phase of the write-thru operation is shown in the top half of Fig. 5 and comprises the write operation. As noted above and indicated on Fig. 5 it can be divided into two stages, the first stage consists of decoding a write address (indicated as W-dec on Fig. 5) and the second stage consists of writing data into cell 200 (indicated as W-cell on Fig. 5). Similarly the read operation shown in the bottom half of Fig. 5 is also divided into two stages, the first stage consist of decoding a read address (indicated as R-dec on Fig. 5) and the second stage consist of reading data out of cell 200 (indicated as R-cell on Fig. 5). As shown in Fig. 5, the write address signal (AW) is set up at a time Ts before the write clock signal (CE) goes positive at time To and has to remain valid until after the write clock signal (CE) falls at time Tce after To. These two signals are combined at the AND gate 245 and cause the signal on the write word line 246 to rise. At a time Tdi after To the data in signal (DI) appears and, if it is a '1' as shown in the example, it causes a write bit line signal (BLW) to appear on the write bit line 217. The effect of the write word line (WLW) and write bit line (BLW) signals is to cause the potential of the first latch node 282 to rise in the cell 200 as explained above (signal NT). Unlike in the prior art apparatus, the read address signal (AR) does not have to be valid before the write clock signal (CE) rises. It becomes valid after time Tw after To. After a delay, the read word line signal (WLR) rises on the read word line 256. The effect of the rising of this signal is to couple the value at the first latch node 282 of the memory cell 200 to the read bit line 227 as explained above and a rising read bit line signal (BLR) appears on the read bit line 227. After a delay, a data out signal (DO) appears at output 220. The read address access time Taa is shown on Fig. 5 as the length of the read cycle starting at time Tw after To. Fig. 6 shows the results of the simulation that can be achieved using the external signals shown in Fig. 5. The vertical axis of Fig. 6 shows the value of the voltages of the signals in volts. The horizontal axis is a time scale marked in nanosecond intervals. Fig. 6 shows two complete write and read cycles. The two write cycles are denoted W1-CYCLE and W0-CYCLE and are marked on the top of the Fig. The two write cycles are divided into decoding and writing stages and, in correspondence with Fig. 5, these stages are marked W-DEC and W-CELL respectively. The two read cycles are denoted R1-CYCLE and R0-CYCLE and are marked on the bottom of the Fig. The two read cycles are divided into two decoding and reading stages and, in correspondence with Fig. 5, these stages are denoted R-DEC and R-CELL respectively. On Fig. 6 are shown the signal values against time for the following signals: address write signal (AW), address read signal (AR), clock signal (CE), signal on write word line 246 (WLW), signal on read word line 256 (WLR), data in signal (DI), data out signal (DO) and signal at the second latch node 284 of the latch 290 (NB). Each of these signals is shown using a different line code on the Figure. In the second read cycle (RO-CYCLE) and second write cycle (W0-CYCLE), the write (WLW) and read (WLR) word line signals, produced by the falling edges of write address (AW at time 2ns) and read address (AR at time 4ns) are shown for another selected memory cell attached to the same write (BLW) and read (BLR) bit lines as the memory cell considered in the first cycle (R1 & W1-CYCLES). The first write cycle on the Fig. is the W1-CYCLE. In this cycle a value of 1 is to be placed into the latch 290 of the memory cell 200. The clock signal (CE) reaches the threshold value above which it is deemed to be "1" (active) at a time of -2ns. At this time, a valid address write signal (AW) is also present. At a time of -1ns, the data in signal (DI) passes the threshold value above which it is deemed to be a logical "1". This value is then written into the latch 290 of the memory cell 200 as described above. The address read signal (AR) becomes valid at time of 0ns. It is at this point that the first read cycle, R1-CYCLE, begins. At a time of about 3.5ns, the data out signal (DO) rises above its threshold value and valid data is obtained at one of the read ports 220. The time from setting up the read address (AR) to receiving a valid data out signal (DO) is approximately 3.5ns and is shown on Fig. 6 as Taal. In the prior art solution, the read address signal (AR) has to be valid at the time at which the clock signal (CE) crosses its threshold value. This is 2ns earlier than in the described simulation and thus, in the prior art solution, the value of Taal would be approximately 5.5ns. The second write cycle is the W0-CYCLE. In this cycle a value of 0 is to be placed into the latch 290 of the memory cell 200. The clock signal (CE) reaches the threshold value above which it is deemed to be "1" (active) at a time of 2ns. At this time, a valid address write signal (AW) is also present. At a time of 3ns, the data in signal (DI) passes the threshold value below which it is deemed to be a logical "0". This value is then written into the memory cell 200 as described above. At a time of 4ns, the address read signal (AR) for the second read cycle, R0-CYCLE, becomes valid as it falls below the threshold value. It is at this point that the second read cycle, RO-CYCLE, begins. Another read word line signal (WLR), is produced which directs the written-thru data from another memory cell to the read ports 220. At a time of about 7.5ns, the data out signal (DO) falls below its threshold value and a valid reading is obtained at the read ports 220. The time from setting up the second read address signal (AR) to receiving a valid data out signal (DO) is again approximately 3.5ns and is marked on the Figure as Taa0. As described above this is 2ns earlier than in the prior art solution.

Claims: A multi-port memory, comprising: an array of memory cells (200), each memory cell being coupled to at least one write port (210) for supplying write data to said memory cell and at least one read port (220) for reading data from said memory cell, said write port and said read port selected by activation of first and second control lines (246, 256), respectively; a first plurality of address inputs (240) for receiving a first plurality of address signals (AW) for selecting said write port coupled to one of said memory cells; a second plurality of address inputs (250) for receiving a second plurality of address signals (AR1) for selecting said read port coupled to one of said memory cells; a first circuit (244) coupled to said first plurality of address inputs for decoding said first plurality of address signals and activating said first control line (246); a second circuit (254) coupled to said second plurality of address inputs for decoding said second plurality of address signals and activating said second control line (WLR1); said first circuit activating said first control line before said second circuit activates said second control line, characterized in that said second circuit is not synchronized by a clock, and wherein said at least one read port is activated during the time when data is written to said memory cells. The memory of claim 1, wherein said memory is accessed in a write/read access cycle, wherein a write portion of said access cycle comprises a first portion in which one of said write ports is selected and a second portion in which write data is written to a selected memory cell through said selected write port, and wherein a read portion of said access cycle comprises a first portion in which one of said read ports is selected and a second portion in which data is read from a selected memory cell through said selected read port. The memory of claim 2, wherein said second portion of said write portion of said access cycle occurs at the same time as said first portion of said read portion of said access cycle. The memory as recited in claim 3, further comprising logic means (245) coupled between said first circuit and said first control line. The memory as recited in claim 4, wherein said logic means is controlled by a clock signal (CE). The memory as recited in claim 1, wherein each of said memory cells comprise first and second cross-coupled inverters (220), and an output buffer (280) for driving data from said memory cells to said read port. The memory as recited in claim 6, wherein said write port comprises a transistor (260) having a control electrode coupled to said first control line, a first controlled electrode receiving a data input, and a second controlled electrode coupled to an input of said first and second cross-coupled inverters. The memory as recited in claim 7, wherein said read port comprises a transistor (274) having a control electrode coupled to said second control line, a first controlled electrode coupled to an output of said first and second cross-coupled inverters, and a second controlled electrode coupled to a read bit line. The memory as recited in claim 1, wherein said first plurality of address signals are valid before said second plurality of address signals. A multi-port memory as recited in claim 1, further comprising: first synchronous means (244, 245) for activating said write port in response to said first address upon receipt of a clock signal (CE); and second asynchronous means (245) for activating said read port in response to said second address without receipt of a clock signal, wherein said read port is activated at substantially the same time as data is written to said memory cell storage means. The memory as recited in claim 10, wherein said first synchronous means further comprises: a first decoder (244) coupled to said first plurality of address inputs and producing a decoded output; and a logic gate (245) having a first input coupled to said decoded output of said first decoder, a second input coupled to said clock signal, and an output coupled to said write port. The memory of claim 11, wherein said second asynchronous means further comprises a second decoder (254) coupled to said second plurality of address inputs and producing an output coupled to said read port. A method for writing data through a multi-port memory cell having a latch (250), write and read address ports (260, 274) coupled to the latch, a clocked write word line (246) coupled to said write address port, a write bit line (217) coupled to said latch and having data thereon, a static read word line (256) coupled to said read address port, and a read bit line (227) coupled to the latch for reading data from said multi-port memory cell, comprising the steps of: a) strobing a decoded write address enable signal (WLW) from said write address port onto said clocked write word line; b) writing data from said write bit line into said latch; c) producing a decoded read address enable signal (WLR1) from said read address port onto said static read word line; and d) reading data out of the latch onto the read bit line; characterised in that said step (c) is not synchronized by a clock, said steps (b) and (c) being carried out substantially simultaneously. The method for writing data through a multi-port memory cell as recited in claim 13 wherein said step (a) additionally comprises the steps of: decoding the write address in a write address decoder (244) and passing it to an AND gate (245), the other input of which is connected to a clock signal (CE) and the output of which is connected to the write word line.

Inventors: CLEMEN RAINER DIPL-ING, GETZLAFF KLAUS JOERG ING GRAD, CLEMEN, RAINER, DIPL.-ING., GETZLAFF, KLAUS JOERG ING. GRAD., Getzlaff, Klaus Jörg Ing. grad.
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1992
Main classification: G11C8
IPC codes: G11C11, G11C8