Abstract:
An improved cell circuit for data readout for use in a multiport memory is provided. The multiport memory stores write data signals. The cell circuit includes a plurality of multiplexers each coupled to a discharge device. Each of the multiplexers 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. Each of the discharge devices are coupled to one of the multiplexers for receiving the output enable signal to generate a drive signal for driving one or more bitlines of the multiport memory.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     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. 
    
    
     BACKGROUND OF THE INVENTION 
     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. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 . 
    
    
     DETAILED DESCRIPTION 
     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.