Abstract:
An improved pull-down latch circuit is provided for better latch performance. Previous pull-down latch circuit performance is compromised during pull-up operation since weak PFETs are employed to pull up latch nodes. A pull up assist circuit is incorporated to assist weak PFET when latch node is being pulled up. The assist circuit is isolated from latch circuit when latch node is being pull down to guarantee that pull down circuit can overcome pull-up circuit (for correct latch operation).

Description:
FIELD OF THE INVENTION 
     This invention relates to local clock distribution and low power circuit design 
     1. Trademarks 
     IBM® is a registered trademark of International Business Machines Corporation, Armonk, N.Y., U.S.A. Other names may be registered trademarks or product names of International Business Machines Corporation or other companies. 
     2. Background 
     In modern microprocessors, an important new design obstacle has begun to emerge. Now, instead of a designer spending most of his time maximizing the speed of his logic, power consumption must be considered a more critical parameter. Recent studies have shown that the primary problem with power distribution lies in the clock, more specifically, the local clock and latch power, and with feature sizes decreasing and scale of integration increasing, this problem will continue to worsen. Thus, it can be concluded that improvements in clock distribution techniques, especially local clock distribution, and latch design, have the potential to lead to major power savings overall. 
     SUMMARY OF THE INVENTION 
     Many prior-art latch designs employ a simple complementary pull down network to write data into the latch. Unfortunately, due to the small transistor sizes, these designs can be, slow compared to the pass gate based latch designs. While it would be possible to improve the performance by increasing transistor size, it is important to consider power when doing so. Since larger transistors mean more power consumption, this solution is unacceptable. Instead, an additional small logic structure can be added to the latch to increase the performance of the typically slow pull-up of the complementary latch with a minimal increase in power consumption. Additional improvements also make it possible to save much of the clock power dissipated in driving these latches. 
    
    
     These and other improvements are set forth in the following detailed description. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a prior art complementary pull down latch. 
     FIG. 2 illustrates the pull-up assisted complementary pull down latch. 
     FIG. 3 compares the performance of the prior art and pull-up assisted complementary pull down latches. 
    
    
     Our detailed description explains the preferred embodiments of our invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a schematic of the prior-art complementary pull down latch is shown. When clock  10  is high, the latch stores the data din  11  on the node state_r and its complement on node stated_ 1 . If, for example, din  11  is a logical 1 (Vdd potential), the NFET  12  will turn ON and node stated_ 1  will be pulled down to a logical zero (ground potential.) This will subsequently turn on PFET  17 , pulling node state_r up to Vdd potential. Subsequently when clock  10  is pulled low, the pull down network (consisting of NFETs  12 ,  14  and  15 ) is disabled and the data on nodes state_ 1  and state_r are held. 
     This latch posesses a number of flaws. First, this particular latch design is dynamic. In other words, when clock  10  is low, either PFET  16  or PFET  17  is responsible for keeping either state_ 1  or state_r at a logical 1, but no device holds state_ 1  or state_r at a logical 0. As a result, either state_ 1  or state_r (whichever is supposed to be at 0) could switch due to FET gate leakage, FET source/drain leakage, a charge sharing event (when NFET  12  or NFET  14  turns ON) or an event which capacitively couples charge into nodes stated_ 1  or stated_r. 
     Second, in order to ensure a quick pull down, and prevent unnecessary power consumption when the latch is in its steady state, the strengths of PFETs  16  and  17  are made significantly smaller than the strengths of NFETs  12  and  14 . This way, the NFET stack will always be strong enough to change the state of nodes stated_ 1  and starer. Unfortunately, weak PFETs  16  and  17  yield poor performance when they need to pull either state 1  or state_r up to a logical 1. As an illustration of this poor performance consider the following sequences of events. Initially nodes din  21 , state_ 1  and state_r are at logical 1, 1 and 0 respectively, and NFET  12  is ON. Next clock  10  switches to a logical 1 turning NFET  15  ON. State_ 1  is pulled low through NFETs  12  and  15 . Subsequently PFET  17  is turned ON, which in turn pulls up node state_r to a logical 1. Recall that PFET  17  is weak however; node state_r is thus pulled up slowly and severely increases the time to propagate a logical 1 from node din  1  to a logical 1 on node state_r. 
     Referring to FIG. 2, the first problem is easily fixed. NFET  29  and NFET  210  have been added to the latch design in order to keep the appropriate node at a logical 0 when the latch is not being written. This makes the latch immune to any charge sharing, coupling, or leakage events that could disturb its state. Like PFET  26  and PFET  27 , these transistors are small in order to avoid unnecessary power consumption. 
     The second problem is slightly more difficult. In order to improve latch write performance, a pull-up assist network has been added to the design. Initially nodes din  21 , state 1  and state_r are at logical 1, 1 and 0 respectively, inverter  214  is driving a  1  onto the gates of PFET  213  and NFET  211 , NFET  211  is ON and PFET  213  is OFF. Next clock  20  switches to a logical 1 turning NFET  25  ON. State_ 1  is pulled low through NFETs  22  and  25 . Subsequently PFET  27  and PFET  212  are turned ON, which in turn pulls up node state_r to a logical 1. Recall that PFET  27  is weak; however, PFET  212  is a much stronger device. This significantly reduces the time required to propagate a logical 1 from node din  1  to a logical 1 on node state_r. PFET  212  can be mush stronger than PFET  27  since it is electrically isolated from node state_r whenever is a logical 1 (and must be pulled down.) The electrical isolation works as follows. When node state_r is a logical 1, the output of inverter  214  is a logical 0, NFET  211  is OFF and PFET  213  is ON. The gate of PFET  212  is pulled to a logical 1 and PFET  212  is thus electrically isolated from the rest of the circuit. 
     FIG. 3 contains wave forms illustrating the latch performance improvement. As can be seen, the output node  215  of the pull-up assisted latch (represented by the dotted line in the graph) evaluates much faster than output node  19  of the prior art latch (represented by the solid line). It should also be noted that this additional hardware results in virtually no increase in overall power consumption of the latch. 
     It is possible to improve this design even further. Not all applications are timing critical. In these cases, it is acceptable to trade performance for power savings. Local clock power can be reduced by three-fours by a half swing clock  20 . (Power is proportional to voltage squared.) When clock  20  is at a logical 0 (ground) NFET  25  is OFF as usual. When clock  20  transitions to Vdd/ 2 , NFET  25  is partially ON. Node state_ 1  or node state_r are still pulled to ground, but more slowly than when NFET  25  is fully ON. 
     While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.