Cell circuit for multiport memory using 3-way multiplexer

The present invention is related to a U.S. patent application entitled “CELL CIRCUIT FOR MULTIPORT MEMORY USING DECODER,” Ser. No. 10/273,567 filed Oct. 17, 2002, assigned to the same assignee, and incorporated herein by reference.

1. Field of the Invention

The invention relates generally to a multiport memory and, more particularly, to an improved cell circuit for data readout for use in a multiport memory.

2. Description of the Related Art

Current microelectronic circuits will achieve complicated systems with a great number of transistors, and the number will keep increasing in the future. Generally, these systems include a plurality of cooperating subsystems for processing data. One apparent problem with realizing these systems is the storage of the data to be processed, as well as their data processing programs. The most powerful systems will surely be realizable if a memory is available to which the subsystems can gain access chronologically parallel and with a high bandwidth. Such memories, which have multiple ports as external terminals, to which the external component units can gain access chronologically parallel, are generally known as multiport memories.

A prior-art multiport memory typically uses a large multiplexer to select one of a plurality of data store cell outputs, resulting in a relatively large space for a readout cell area for multiple read ports as well as a large number of read wordlines. Therefore, a need exists for a multiport memory with new multiple read ports configuration that takes up less space for a readout cell area by reducing both the readout cell area and the number of read wordlines.

The present invention provides a cell circuit for data readout in a multiport memory storing a plurality of write data signals. The cell circuit includes a multiplexer and a discharge device. The multiplexer receives a subset of the write data signals and a plurality of read wordline signals and selects an output enable signal among the subset of the write data signals based on the read wordline signals. The discharge device is coupled to the multiplexer for receiving the output enable signal to generate a drive signal for driving a bitline of the multiport memory.

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1; depicts a multiport memory array structure in a block diagram;

FIG. 2 is a schematic diagram of a memory block as shown in FIG. 1;

FIG. 3 is a schematic diagram of a data store cell circuit as shown in FIG. 2; and

FIG. 4 is a schematic diagram of a readout cell circuit as shown in FIG. 2 .

In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail.

Referring to FIG. 1 of the drawings, the reference numeral 100 generally designates a block diagram of a memory array having M+1 array units for bits 0 through M, wherein M is an integer larger than 0. In this figure, the memory array 100 is shown as an example to illustrate a 64 entry x M-bit array having two write ports and six read ports.

An array unit 102 for bit M is shown in further detail. The array unit 102 generally comprises a plurality of memory blocks 0 - 7 (hereinafter collectively referred to as “memory blocks 104 ”) coupled to bitlines 106 . In this example, the number of the bitlines 106 is six since there are six read ports in this configuration. Each of the memory blocks are coupled to the bitlines 106 . Preferably, each bitline carries a dynamic ORed signal since a precharge circuit 108 is coupled to a discharge device (not shown) through each bitline. A more detailed illustration of these connections is shown in FIG. 2 . Two write datalines 110 are coupled to each of the memory blocks 104 to provide two write data inputs (not shown) to each of the memory blocks 104 . Each memory block also receives write wordlines (not shown) to select one of the two write data inputs.

Now referring to FIG. 2, a schematic diagram of a memory block 200 is depicted to illustrate any one of the memory blocks 104 of FIG. 1 . The memory block 200 generally comprises a plurality of data store (DS) cells 0 - 7 (hereinafter collectively referred to as “DS cells 202 ”), coupled to a plurality of readout cells 204 A, 204 B, 204 C, 204 D, 204 E, and 204 F (hereinafter collectively referred to as “readout cells 204 ”). For example, DS cells 0 - 2 are coupled to the readout cell 204 A. Similarly, there are also other connections (not shown for the sake of simplicity) between the DS cells 202 and the readout cells 204 B-F. These connections are clearly indicated in each of the readout cells 204 B-F and will be easily understood by a person with ordinary skill in the art in the context of the foregoing and following description.

Specifically, these additional connections are as follows. The DS cells 1 - 3 are coupled to the readout cell 204 B. The DS cells 2 - 4 are coupled to the readout cell 204 C. The DS cells 3 - 5 are coupled to the readout cell 204 D. The DS cells 4 - 6 are coupled to the readout cell 204 E. The DS cells 5 - 7 are coupled to the readout cell 204 F. It is noted that this configuration is merely an example of many different possible configurations embodying the features of the present invention.

Each of the readout cells 204 has a 3:1 multiplexer and a discharge device coupled to the multiplexer. Specifically, the readout cell 204 A comprises a multiplexer 206 A and a discharge device 208 A coupled to the multiplexer 206 A. The readout cell 204 B comprises a multiplexer 206 B and a discharge device 208 B coupled to the multiplexer 206 B. The readout cell 204 C comprises a multiplexer 206 C and a discharge device 208 C coupled to the multiplexer 206 C. The readout cell 204 D comprises a multiplexer 206 D and a discharge device 208 D coupled to the multiplexer 206 D. The readout cell 204 E comprises a multiplexer 206 E and a discharge device 208 E coupled to the multiplexer 206 E. The readout cell 204 F comprises a multiplexer 206 F and a discharge device 208 F coupled to the multiplexer 206 F. The outputs of the readout cells 204 are coupled to the bitlines 106 .

In this particular example, the DS cells 202 have eight entries; therefore, there are sixteen write wordlines 210 (2 ports×8 entries). There are six read ports; therefore, a prior-art configuration with a 8:1 multiplexer (not shown) would result in 48 read wordlines. In the memory block 200 , by using 3:1 multiplexers 206 A-F, the total number of read wordlines can be reduced to from 48 to 18.

The discharge devices 208 A-F are respectively coupled to the precharge circuit 108 via the bitlines 106 . Preferably, the read wordlines 212 are dynamic signals; therefore, output signals of the 3:1 multiplexers 206 become clock signals to enable the discharge device 208 , and the bitlines 106 carry dynamic ORed signals. To improve data accessibility, a shuffle circuit 214 is used at the bottom of the bitlines 106 . By using the shuffle circuit 214 , each data output can be accessible through each of readout ports 216 .

FIG. 3 depicts a schematic diagram of a data store (DS) cell circuit 300 . Preferably, the DS cell circuit 300 represents any of the DS cells 202 of FIG. 2 . The DS cell circuit 300 comprises a write data selector 302 having two write data input ports 304 A and 304 B for receiving first and second write data inputs in accordance with the examples having two write data inputs as shown in FIGS. 1 and 2. The write data selector 302 also includes two write wordline ports 306 A and 306 B for receiving first and second write wordlines, respectively, and selecting one of the two write data inputs or neither of them based on the first and second write wordlines.

The write data selector 302 is coupled to a latch 308 , which outputs a DS cell output signal. The DS cell output signal is either updated with one of the two write data inputs or keeps a previous data (e.g., one of the two write data inputs in a previous clock cycle). The write data selector 304 generally comprises first and second three-state inventors 310 and 312 respectively coupled to the write data input port 304 B and write data input port 304 A.

The first three-state inverter 310 is coupled to a first inverter 314 for receiving as an enable signal an inverted signal of the output of the inverter 314 . The first three-state inverter 310 is also coupled to the write wordline port 306 B to receive as a complementary enable signal the second write wordline. Similarly, the three-state inverter 312 is coupled to a second inverter 316 for receiving as an enable signal an inverted signal of the output of the second inverter 316 . The second three-state inverter 312 is also coupled to the write wordline port 306 A to receive as a complementary enable signal the first write wordline. The write data selector 302 also includes a NOR gate 318 for determining whether the latch 308 should be updated with a new input or keep its current state.

Accordingly, the operation of the DS cell circuit 300 is as follows. When only the first write wordline is asserted, the write data selector 302 outputs only the first write data input through the second three-state inverter 312 . This is because the first three-state inverter 310 is not enabled. In this case, the output of the NOR gate 318 is not asserted, resulting in an update of the DS cell output signal with the first write data input.

Similarly, when only the second write wordline is asserted, the write data selector 302 outputs only the second write data input through the first three-state inverter 310 . This is because the second three-state inverter 312 is not enabled. In this case, the output of the NOR gate 318 is not asserted, resulting in an update of the DS cell output signal with the second write data input.

When both the first and second write wordlines are disabled, the NOR gate 318 disables the latch 308 and the DS cell output signal keeps its previous data.

FIG. 4 shows a schematic diagram of a readout cell circuit 400 . Preferably, the readout cell circuit 400 represents any of the readout cells 204 A-F of FIG. 2 . The readout cell circuit 400 comprises a static 3:1 data multiplexer 402 and a discharge device 404 . The 3:1 multiplexer 402 selects one of the three DS cell outputs as a control signal, which input to the discharge device 404 . Preferably, the discharge device 404 comprises a metal-oxide-silicon (MOS) transistor having a gate terminal 404 a , drain terminal 404 b , and source terminal 404 c . The gate terminal 404 a is coupled to the output of the multiplexer 402 for receiving the DS cell output. The drain terminal 404 b is coupled to a bitline 406 for driving the bitline 406 . The source terminal 404 c is coupled to ground.

It will be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit. This description is intended for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the following claims.