Patent Publication Number: US-6657912-B1

Title: Circuit for optimizing power consumption and performance

Description:
FIELD OF THE INVENTION 
     The present invention relates to driver circuits, and more particularly to a circuit for optimizing power consumption and performance of driver circuits. 
     BACKGROUND OF THE INVENTION 
     Low power requirements in a circuit often conflict with high performance requirements. As a result, the voltage ranges at which circuits can operate can be limited in order to address this conflict. For example, circuit configurations that are efficient with regard to power, performance, and area are limited in that they can operate at one voltage but often cannot operate properly at other voltages, e.g., lower voltages. Conversely, circuit configurations that are designed for broad voltage operations often do not exhibit optimal power, performance, and area characteristics. 
     A classic example of the trade off between low power requirements and high performance requirements is that of a register file. A register file often includes an array of memory elements. All memory elements within a bit slice are connected to one another through a multiplexor. Each memory element within a bit slice could include, for example, one or more pass gates, each associated with a different word line. Incorporating the pass gates in each memory element and then coupling the pass gate outputs together within the bit slice effectively distribute the multiplexor. 
     FIG. 1 is a diagram of a first conventional multiplexed memory bit-line circuit. As is seen, a plurality of memory cells  201  are coupled to respective nfet devices  202   a - 202   n . The nfet device  206  which is controlled by a write through read (!WTR) signal, is active during a normal read operation and node- 2  will follow node- 1 , but it is delayed in time and will not achieve the same voltage uplevel. The inverter  212  whose output is connected to node- 3 , has a very high nfet to pfet ratio to cause a better response to the weak/poor rising response on node- 2 . As node- 3  falls, it activates the pfet feedback device  210 , which causes node- 2  to rise to Vdd eliminating any leakage current in inverter  212  associated with the weak “1” input. 
     As Vdd is reduced, the response of node- 2  is greatly degraded as the nfet device  206  connecting node- 1  and node- 2 , is on very weakly. In fact, as the Vdd is lowered further, node- 2  cannot achieve a voltage high enough to switch the inverter  212 . This is clearly a failure. One approach to resolving this failure is to apply the same boot-strap technique to the !WTR controlled device as is applied to the wordlines. This can and will work, but will raise the power associated with both the increased current demand on the voltage “doubler” and the power that will result on the bit lines being allowed to charge to a higher potential as the feedback device  210  raises node- 3  and ‘reverse’ loads the bit lines. 
     FIG. 2 illustrates a second conventional multiplexed memory bit-line. In this circuit  300 , the pass transistor connecting node- 1  and node- 2  is replaced by a transmission gate, T 1 , (nfet device  305   a  controlled by !WTR and pfet device  305   b  controlled by WTR) to improve performance and expand the Vdd operating range. 
     With the transmission gate configuration, node- 2  will now follow node- 1 , albeit rather slowly, and result in node- 2  having the identical voltages as node- 1 . The transient response of node- 2  will naturally be slow as the pfet device  305   b  portion of the transmission gate, while active, is on rather weakly. A major drawback of this configuration is the fact the once the inverter  312 , switches low, the output of the pfet feedback device  310  will now charge both node- 2  and node- 1 . Since node- 1  is the bitline that is highly capacitive, this will result in a slower falling response, but more importantly significantly higher power. 
     Accordingly, what is needed is a means to minimize power consumption while not compromising performance. The present invention addresses such a need. 
     SUMMARY OF THE INVENTION 
     A memory bit line multiplexor circuit is disclosed. The circuit comprises at least one memory cell arrangement. The circuit includes a first active device coupled to the at least one memory cell arrangement and for coupling a first node to a second node within the circuit. The first active device is controlled by a write through read (!wtr) signal. The circuit includes a second active device coupled to the second node and a gate. The gate has a first input coupled to the first node; a second input coupled to the !WTR signal, and a third input being controlled by an inversion of the output of the circuit. The gate and the second active device provide a pulsed self-timed response that minimizes power consumption while optimizing performance of the circuit. 
     A circuit in accordance with the present invention is disclosed in which the most power, and area efficient realizations can be employed for applications requiring broad power supply operating ranges. The circuit employs a simple logic gate system configured to create a self-timed pulsed stimulus for allowing improved performance over a very broad Vdd operation range while simultaneously reducing power. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a first conventional multiplexed memory bit-line circuit. 
     FIG. 2 illustrates a second conventional multiplexed memory bit-line. 
     FIG. 3 illustrates a multiplexed memory bit-line circuit in accordance with the present invention. 
     FIG. 4 illustrates the response for the three circuits shown in FIGS. 1,  2  and  3 , respectively. 
     FIG. 4A illustrates the actual power/performance improvements associated with the circuit of FIG.  3 . 
     FIG. 5 illustrates power versus Vdd at Tj=125C. 
     FIG. 6 illustrates Tplh performance versus Vdd at Tj=125C. 
     FIG. 7 illustrates power versus Vdd at Tj=−55C. 
     FIG. 8 illustrates performance versus Vdd at Tj=−55C. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to driver circuits, and more particularly to a method and circuit for optimizing power consumption and performance of driver circuits. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     A multiplexed memory bit-line circuit in accordance with the present invention utilizes a logic gate system configured to create a self-timed pulse stimulus which allows for improved performance over conventional circuits. For a more particular understanding of the features of the present invention, refer now to the following description and the accompanying figures. 
     FIG. 3 illustrates a multiplexed memory bit-line circuit in accordance with the present invention. The circuit  400  shares some of the characteristics of both FIG.  1  and FIG. 2 realizations. From FIG. 1, the nfet device  406   a  is employed for connecting node- 1  and node- 2 . From FIG. 2, an n/p pair transmission gate (pfet device  405   a  and nfet device  405   b ) is associated with the condition for an active WTR. 
     A key feature of the present invention is the use of a gate, in this embodiment a NAND gate  420 , and an additional active device, in this embodiment a pfet device  422 . The pfet device  422  in turn is coupled to node- 2  and the output of the NAND gate  420 . The NAND gate  420  has one of its inputs connected to node- 1 . A second input of the NAND gate  420  is connected to the !WTR signal and a third input is connected to node- 3  or any down stream node with the same phase as node- 3 . The output of the NAND gate  420  is connected to the gate of pfet device  422 , whose source is connected to Vdd and whose drain is connected to node- 2 . 
     The circuit  400  operates as follows. For the case of transferring/reading a “1” from the bit-line, the wordline driver signal (Win) activates, thereby transferring the logical “1” content to the bit-line, node- 1 . For the situation wherein a low Vdd is applied and a boot-strapped word-line circuit is enabled, the node- 1  will charge to somewhere between Vdd+Vtp minus Vtn and Vdd. 
     As stated earlier, due to the body effect of the boosted nfet device pass gate, the bit-line voltage will generally be less than Vdd. Now, as node- 1  charges, node- 2  begins to follow, but node- 2  will not achieve the full voltage of node- 1  due to the body effect of the nfet device  406 . The gate of NAND gate  420  connected to node- 1  however will ‘see’ the full node- 1  voltage and will switch. 
     Since, the !WTR signal is active (“1”) and node- 3  is also active (“1”), the NAND gate  420  switches quickly to ground. Since the nfet device  406  is essentially shut off by the time node- 2  reaches the lesser of node- 1  voltage or Vdd-Vtn, node- 1  is isolated from node- 2 . The pfet device  422 , receiving the logical “0” from the NAND gate  420 , will begin to readily charge node- 2  to Vdd. This forces inverter  412  to switch rapidly, which in turn shuts off the NAND gate  420 , thereby disabling the pfet device  422 . The duration of the NAND gate  420  output being low and hence the duration of pfet device  422  charging node- 2  is a function of the response of node- 3 . 
     For increased active duration of these elements, the signal connected to the third input to the NAND gate  420  must be delayed. This can be accomplished by adding another inverter (not shown) after the output inverter  414  and connecting its output to the third input of the NAND gate  420 . The duration of the improved response circuit is essentially self-limited. As the response improves, the duration reduces and hence limits any further improvement. Since the accelerated response of node- 2  is defined by the duration of the pfet device  422  being active, the feedback device  424  will continue to charge the node completely to Vdd to eliminate the static leakage. Accordingly, the NAND gate  420  and pfet device  422  provide for a “pulsed,” self-timed response. 
     The reason for using a pulsed, self-timed configuration of the NAND gate  420  and pfet device  422  is that when the bit-lines need to be discharged the supplied current of pfet device needs to be held off. The pfet device  422  being held off after it completes its task, allows the falling response to be the same as in the original configuration of FIG.  1 . This allows a designer to ‘size’ the pfet device  422  for the best power/performance associated with the rising output transition without worry of adversely affecting the falling performance. Compared to the n/p pair configuration illustrated in FIG. 2, the circuit  400  demonstrates improved falling response and reduced power, as the bit-lines are not charged to Vdd. FIG. 4 illustrates the response for the three circuits  200 ,  300  and  400  shown in FIGS. 1,  2  and  3 , respectively. As is seen, the output of the circuit (node- 3 ) has a significantly faster response in the circuit  400  of the present invention. 
     This rapid charging of node- 2  greatly improves the response time of the output and also reduces the short-circuit current of inverter  412 . In fact, compared with an n/p pair configuration of FIG. 2 in a 130 nm CMOS realization being supplied with a Vdd of 0.7 volts at −55C, this configuration improves the total response time of the entire memory by 25% and reduced the power by 5%! 
     Another advantage of this configuration is that the bit line is only charged to Vdd-Vt which reduces overall bit power and improves the falling response. Even when the Vdd is at 1.4 volts and t=125C this configuration improves the output response of the entire memory by approximately 9% and reduces power 9%! 
     FIG. 4A illustrates the actual power/performance improvements associated with the circuit  400  of FIG.  3 . In FIG. 4A, Vdd is the supply voltage; Tplh is the rising output response relative to the rising word line. The rising response is the dominant performance limiter. Power L-H is the normalized power from a low voltage to high voltage. All content has been normalized to the n/p pair configuration. Note, in reviewing this table, that while having a 3% lower power, the delay of this new inventive circuit configuration relative to the n/p realization is 35% less at 0.7 volts. At 1.4 volts, this configuration has a 3% delay improvement along with a 6% power reduction. 
     FIGS. 5 through 8 graphically illustrate the power and performance improvements at various temperatures. FIG. 5 illustrates power versus Vdd at Tj=125C. FIG. 6 illustrates Tplh performance versus Vdd at Tj=125C. FIG. 7 illustrates power versus Vdd at Tj=−55C. FIG. 8 illustrates performance versus Vdd at Tj=−55C. Note that throughout the temperature range (−55 C. to 125 C.), the circuit  400  in always represents improvements over the prior art. 
     Hence, a technique has been described wherein the most power and area efficient realizations can be employed for applications requiring broad power supply operating ranges. The technique employs a simple logic gate system configured to create a self-timed pulsed stimulus allowing improved performance over a very broad Vdd operation range while simultaneously reducing power. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.