Abstract:
A method is provided for precharging a node in an integrated circuit in which the node is precharged a first predetermined delay after the node evaluates and, thereafter, the precharge ceases after a second shorter predetermined delay.

Description:
BACKGROUND 
     The present invention relates to a reset controller for a domino circuit characterized by improved turn off times. 
     As is known, integrated circuits may include domino circuits that carry active data on only one phase of a driving clock, called the “evaluation phase.” During another phase of the clock, the “precharging phase,” the domino circuit precharges its output to a predetermined value. A reset circuit in the domino circuit controls the precharging. 
     An evaluation circuit also is coupled to the output terminal having a data input terminal. If active data is input to the evaluation circuit during the evaluation phase, the evaluation circuit may drive the output terminal from the precharge voltage. The active data typically is removed from the evaluation circuit prior to the precharge phase. The reset circuit precharges the output terminal in preparation for another evaluation phase. 
     Known reset circuits may include a propagation path that extends from the output terminal to a precharge transistor. An output of the reset circuit drives the gate of the precharge transistor. Such reset circuits typically are characterized by a propagation delay that is sufficient to guarantee that the reset circuit will not cause the precharge transistor to precharge the output terminal at the same time that the evaluation terminal causes the output terminal to be driven to a different potential. If two transistors were permitted to drive the same terminal to two different potentials, it would cause contention and damage to the circuit. Thus, the delay of the reset circuit typically is designed to be large enough so that the precharge transistor is turned on only after the data signal that is input to the evaluation circuit is deactivated. 
     In known self-resetting domino circuits, the reset circuit that turns on the precharge transistor also turns it off. Thus, after the precharge circuit is activated, it remains activated for the same propagation delay that was designed into the reset circuit to avoid contention. 
     This feature of reset circuits may be disadvantageous. Although a relatively long delay in turning the precharge transistor on may be necessary to avoid contention at the output terminal, a long delay in turning off the precharge transistor is not necessary. An output terminal may be precharged very quickly relative to the length of the data pulse input to the domino circuit. No known reset circuit provides a different delay for activating a precharge transistor than for deactivating a precharge transistor. 
     Accordingly, there is a need in the art for a reset circuit in a domino circuit that provides activates a precharge transistor after a first delay but deactivates the precharge transistor after a second, shorter delay. 
     SUMMARY 
     According to an embodiment, the present invention provides a method of precharging a node in an integrated circuit in which the node is precharged a first predetermined delay after the node evaluates and, thereafter, the precharge ceases after a second shorter predetermined delay. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of self-timed atomic circuit constructed in accordance with an embodiment of the present invention. 
     FIG. 2 is a timing diagram of the atomic circuit of FIG. 1 operating in accordance with an embodiment of the present invention. 
     FIG. 3 is diagram of a self-timed atomic circuit constructed in accordance with another embodiment of the present invention. 
     FIG. 4 is diagram of a self-timed atomic circuit constructed in accordance with yet another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide domino circuit having a self-timed reset circuit in which the reset circuit is characterized by a long delay path to enable the reset precharge and also a short delay path to disable the reset precharge once the reset is activated. An understanding of these embodiments may be facilitated by reference to the figures and the following description. 
     FIG. 1 is a circuit diagram of a domino circuit  100  according to an embodiment of the present invention. The domino circuit  100  includes an output terminal  110  that is precharged to a predetermined potential (the “precharge” or “standby” potential). In the example of FIG. 1, the output terminal  110  is precharged to V cc . The domino circuit  100  includes an evaluate circuit  120  having an input terminal  130 . The evaluate circuit  120  couples the output terminal  110  to a second predetermined potential, shown as ground in FIG. 1 (the “evaluation” potential). During the evaluation phase, the evaluate circuit  120  may cause the output terminal  110  to discharge to ground based upon the state of the signal at input terminal  130 . 
     In the example of FIG. 1, the domino circuit  100  is shown as a latch circuit. Of course, as is understood by those of skill, the domino circuit  100  may be designed to accommodate a host of logical functions. As is known, different applications of the present invention may cause the nature and character of the evaluate circuit  120  to deviate from the structure shown in the present invention. Such deviations are within the spirit of the present invention. 
     The domino circuit  100  also may include a precharge transistor  140  that couples the output terminal  110  to the precharge potential across a source to drain path. A reset circuit  150  couples the gate  141  of the precharge transistor  140  to the output terminal  110 . The precharge transistor  140  may be a PMOS transistor that is conductive when the signal applied at the gate  141  goes low. When the precharge transistor  140  is conductive, it pulls the voltage at the output terminal to the precharge potential. 
     According to an embodiment of the present invention, the reset circuit  150  may be populated by two circuit paths, a “long delay path”  160  and a “short delay path”  170 , each extending from the output terminal  110  to the gate  141  of the precharge transistor  140 . The two paths each are input to a common NAND gate  151 . 
     The long delay path  160  and the short delay path  170  each may be populated by one or more inverter buffers  161 - 163 ,  171 . The inverter buffers of each path  160 ,  170  are interconnected in a cascaded relationship. As is known, each inverter buffer imposes a propagation delay upon an input data signal; the number of inverter buffers in each path  160 ,  170  determines how much delay the respective path imposes upon a signal as it propagates from the output terminal  110  through the respective path to the NAND gate  151 . The NAND gate  151  itself may impose a propagation delay upon an input signal. 
     In the example of FIG. 1, only one inverter buffer  171  is shown in the short delay path  170  and three inverter buffers  161 - 163  are shown in the long delay path  160 . These numbers are merely exemplary. Typically, the number of inverters in a particular domino circuit  100  will be tuned to the application for which the circuit  100  is to be used. 
     For notational purposes, the input from the long delay path  160  to the NAND gate  151  is labeled node “A” and the input from the short delay path  170  to the NAND gate  151  is labeled node “B.” An output of the NAND gate  151  is input to the gate of the precharge transistor  140  at a node “C.” 
     FIG. 2 is a timing diagram illustrating the state of signals in the domino circuit  100  of FIG. 1 according to an embodiment of the present invention. In the example of FIG. 2, the inverter buffers  161 - 163 ,  171  are assumed to impose an identical propagation delay upon a signal. FIG. 2 illustrates signals at the input terminal  130 , at nodes A-C and at the output terminal  110 . The dashed lines represent time samples measured in units of delay imposed by a single inverter buffer. 
     During a rest state, the data signal at terminal  120  is precharged to the precharge potential. Assume that the output terminal  110  is precharged to a high state but that no external source maintains the output terminal  110  at such a state. Nodes A and B therefore are low. The input to the precharge transistor  140  (node C), therefore, is high. Thus, both the evaluate circuit  120  and the precharge transistor  140  are nonconductive. 
     The data signal is shown as evaluating in sample 1. When the data signal evaluates, the evaluate circuit  120  conducts and discharges the output terminal  110  to ground. Thus, the inputs to both the long delay path  160  and the short delay path  170  are low. The exemplary data signal is shown as being low for over three samples. It drives the output terminal  110  to ground during the time that the data signal is in the evaluate state. 
     At sample 3, the signal at the output terminal  110  will have propagated through the short delay path  170 . Thus, node B is shown as being high. But the data signal will not have propagated through the long delay path  160  (Node A remains low). The output of the NAND gate  151  (node C) does not change. The precharge transistor  140  remains nonconductive. The output terminal  110  remains driven to ground by the data signal. 
     As shown in FIG. 2, sometime during the duration of sample 4, the data signal ceases to evaluate and returns to its high state. The evaluate circuit  120  no longer drives the output terminal  110  low. Although no longer driven to ground, the output terminal  110  will remain at ground until driven by some other potential. 
     At sample 5, the data signal that was input to the long delay path  160  in sample 1 will have propagated through the long delay path  160 . Thus, nodes A and B both are high. The NAND gate  151  goes low and the precharge transistor  140  conducts. When the precharge transistor  140  conducts, the output terminal  110  is driven to the precharge potential. The precharge potential is input to the two paths  160 ,  170  of the reset circuit  150 . 
     At sample 8, the state change at the output terminal (sample 7) will have been inverted by inverter  171  and input to NAND gate  151 . The input from the long delay path  160  does not change. Thus, node B will be low but node A will remain high. The output of the NAND gate  150  (node C) goes high and the precharge transistor  140  ceases to conduct. The output terminal  110  remains at the precharge potential but is no longer driven so. It is precharged and ready for the next evaluation phase. 
     As shown in FIG. 2, the delay of the long delay path  160  (in combination with delays that may be introduced by the NAND gate  151  and precharge transistor  140 ) determines the time when the precharge transistor  140  precharges the output terminal  110 . Typically, the delay path  160  may be tuned to a period that is longer than the duration of the input data pulse so as to ensure there will be no contention between the evaluate circuit  120  and the precharge transistor  140 . Such tuning may require calibrating a number of inverter buffers in the delay path  160  to introduce a desired propagation delay to the path. 
     Also as shown in FIG. 2, the delay of the short delay path  170  (again, in combination with delays that may be introduced by the NAND gate  151  and precharge transistor  140 ) determines the time after the precharge begins when the precharge transistor  140  ceases to precharge the output terminal. In an embodiment, this path may be tuned to maintain the precharge transistor  140  conductive only so long as may be required to precharge the output terminal  100 . Such an embodiment increases the speed at which the domino circuit  100  may receive a new data signal and, therefore, increases the throughput of the system as a whole. 
     FIG. 3 illustrates a domino circuit  200  constructed in accordance with another embodiment of the present invention. The domino circuit  200  may be populated by an output terminal  210  and input terminal  220 . An evaluate circuit  230  couples the output terminal to an evaluation potential (such as ground) and a precharge transistor  240  couples the output terminal to a precharge potential (such as V cc ). A reset circuit  250  couples the output terminal to the gate of the precharge circuit. 
     According to an embodiment of the present invention, the reset circuit  250  provides a short delay path  260  and a long delay path  270  from the output terminal  210  to the gate  241  of the precharge transistor  240 . A NAND gate  251  receives inputs from the two delay paths  260 ,  270  and has an output coupled to the gate  241  of the precharge transistor  240 . The long delay  270  path includes a cascaded chain of inverter buffers  271 - 273  extending from the output terminal  210  to the NAND gate  251 . The short delay path  260  provides a shunt path from an intermediate point in the chain of inverter buffers to a second input of the NAND gate  251 . In the exemplary reset circuit  250  of FIG. 3, an output of the first inverter buffer  273  is input directly to the NAND gate  241 . Thus the short delay path  260  includes a fewer number of inverter buffers than would the long delay path  270 . 
     The embodiment of FIG. 3 operates in a similar manner to the embodiment of FIG. 1 particularly as it relates to the signals and timing shown in FIG.  2 . However, the embodiment of FIG. 3 includes fewer inverter buffers than that of FIG.  1 . Thus, the embodiment of FIG. 3 may be preferable for use in integrated circuits where it is desired to conserve elements and chip area. 
     FIG. 4 illustrates a domino circuit  300  according to yet another embodiment of the present invention. FIG. 4 illustrates use of the present invention in an embodiment where the precharge potential is V ss  (ground) and the evaluation potential is V cc . The domino circuit  300  may include an output terminal  310 , an evaluation circuit  320 , an input terminal  330  and a precharge circuit  340 . 
     The domino circuit further may include a reset circuit  350  having a short delay path  360  and a long delay path  370 . Each delay chain may include a chain of cascaded inverter buffers, each chain having an input coupled to the output terminal  310 . The short delay path  360  may include a smaller number of inverter buffers than the long delay path. Outputs of the two delay chains  360 ,  370  may be input to a NOR gate  351 . An output of the NOR gate may be input to the precharge circuit  340 . 
     Several embodiments of the present invention are specifically illustrated and described herein. However, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.