Patent Publication Number: US-6037804-A

Title: Reduced power dynamic logic circuit that inhibits reevaluation of stable inputs

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates in general to electronic circuitry and, in particular, to dynamic logic digital circuitry. Still more particularly, the present invention relates to reduced power dynamic logic circuitry that inhibits the reevaluation of logic inputs during clock cycles in which the logic inputs remain unchanged. 
     2.Description of the Related Art 
     As integrated circuits have achieved higher clock speeds and are fabricated with ever increasing densities, power dissipation has become a primary consideration in integrated circuit design. While the need for reduced power integrated circuits is of particular concern in the design of battery-powered devices such as portable computers, applications for reduced-power integrated circuits extend to desktop computer systems and other electronic devices due to power supply and heat dissipation considerations. 
     As is well-known to those skilled in the art, a conventional dynamic logic integrated circuit divides a clock cycle into precharge and evaluate phases. During the precharge phase, the dynamic logic circuit is preset to a known logic state, and during the evaluate phase, the logic inputs of the dynamic logic circuit are evaluated by circuitry that implements a logic function to determine a logic output. Importantly, a conventional dynamic logic circuit evaluates its logic inputs during the evaluate phase of each cycle, and accordingly consumes switching power during each cycle. 
     Therefore, a need arises for a reduced-power dynamic logic circuit. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the present invention to provide improved electronic circuitry. 
     It is another object of the present invention to provide reduced-power dynamic logic digital circuitry. 
     The foregoing objects are achieved as is now described. A reduced-power integrated circuit includes a circuit data input, a circuit data output, and at least one row of dynamic logic. The row of dynamic logic includes a row clock input, a row data input, and a row data output coupled to the circuit data output, where a value received at the row data input is derived from the value at the circuit data input. The integrated circuit further includes a comparator that compares current and previous values at the circuit data input and a switch that selectively sets the row clock signal received at the row clock input to an inactive state and temporarily maintains the row clock signal in the inactive state in response to the comparator detecting that the current previous values of at the circuit data input are equivalent. Consequently, the row of dynamic logic does not (and need not) reevaluate the circuit data input value, and power dissipation is reduced. 
     Thus, in contrast to prior art dynamic logic circuits that drive their output loads during clock cycles in which the logic inputs have remained stable, the present invention advantageously reduces the power consumed by a dynamic logic circuit through inhibiting the dynamic logic circuit from driving the output load during clock cycles in which the logic inputs remain unchanged from the previous clock cycle. In this manner, switching power is not unnecessarily consumed during cycles in which the logic inputs have remained stable. 
     The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1A illustrates a conventional domino dynamic logic circuit; 
     FIG. 1B depicts a two-input AND gate implemented utilizing domino logic; 
     FIG. 1C illustrates a two-input OR gate implemented utilizing domino logic; 
     FIG. 1D depicts a two-input multiplexer implemented utilizing dynamic logic; 
     FIG. 1E is a timing diagram illustrating the operation of the conventional domino dynamic logic circuit depicted in FIG. 1A; 
     FIG. 2A is a block diagram representation of a row of domino logic; 
     FIG. 2B illustrates a conventional multiple-row domino logic circuit; 
     FIG. 2C is a timing diagram depicting the operation of the conventional multiple-row domino logic circuit shown in FIG. 2B; 
     FIG. 3A is a block diagram of an illustrative embodiment of a multiple-row domino logic circuit in accordance with the present invention; and 
     FIG. 3B is a timing diagram depicting the operation of the multiple-row domino logic circuit illustrated in FIG. 3A. 
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     With reference now to the figures and in particular with reference to FIG. 1A, there is illustrated a schematic diagram of a conventional dynamic logic circuit implemented utilizing domino logic. Circuit 10 includes a precharge transistor 12, a discharge transistor 14, a domino logic block 16, an inverter 18, and a feedback transistor 22. As illustrated in FIG. 1E, precharge transistor 12 is turned on and precharges node X to a logic high state during the precharge phase of clock signal 8. Then, in response to the rising edge (i.e., evaluate phase) of clock signal 8, discharge transistor 14 is turned on and discharges node Y to a logic low state. During the evaluate phase, node X will either remain at a logic high state or discharge to the logic low state of node Y depending upon the logic states of the logic inputs and the logic function implemented by domino logic block 16. Examples of various implementations of domino logic block 16 are depicted in FIGS. 1B, 1C, and 1D, which respectively illustrate a two-input AND gate 30, a two-input OR gate 32, and a two-input multiplexer 34. During both of the precharge and evaluate phases of clock signal 8, the logic state of node X is inverted by inverter 18 to obtain output signal 20, which is latched by the operation of feedback transistor 22. 
     Referring now to FIG. 2A, a block diagram of a conventional row of domino logic circuitry is depicted. As illustrated, row 40 includes M domino logic circuits 10, which may implement the same or different logic functions. The M domino logic circuits 10 forming row 40 each receive a respective set of input signals (IN M-1 ) and generate a respective output signal (OUT M-1 ) in response to the evaluate phase of clock signal 42. In order to implement a complex logic function, such as that performed by the arithmetic logic unit (ALU) of a processor, a series of rows 40 can be coupled together sequentially, as shown in FIG. 2B. 
     With reference to FIG. 2B, there is illustrated a conventional multiple-row dynamic logic circuit 50 including an input register 52, domino logic rows 1 through A, and an output register 54. As shown, input register 52 latches and stores register input values in response to a first clock signal 56, while domino logic rows 1 through A and output register 54 operate in response to a second clock signal 58 that is 180° out of phase with the first clock signal 56. 
     Referring now to FIG. 2C, which is an exemplary timing diagram of the operation of circuit 50, input register 52 latches the register input in response to first clock signal 56 making a transition from a logic low state to a logic high state. When first clock signal 56 makes a transition to a logic high state, second clock signal 58 makes a transition to a logic low state to initiate a precharge phase in which the domino logic circuits within each of domino logic rows 1 through A are precharged to a known state. As explained supra with respect to FIG. 1E, the output of each individual domino logic circuit is set to a logic low state during the precharge phase. Accordingly, FIG. 2C illustrates each of row output signals RO 1  -RO A  making a transition from a logic high state to a logic low state in response to second clock signal 58 entering the precharge phase. 
     Thereafter, in response to second clock signal 58 entering the evaluate phase, input register 52 outputs its contents as the logic input of domino logic row 1. In response to receipt of the logic input and the evaluate phase of second clock signal 58, domino logic row 1 evaluates the logic input and generates row output 1 (RO 1 ), which is fed to domino logic row 2 as a logic input. In response to receipt of RO 1  and the evaluate phase of second clock signal 58, domino logic row 2 evaluates RO 1  to generate R0 2 , which is feed to domino logic row 3. This process continues until all of domino logic rows 1 through A have evaluated their respective inputs. Finally, RO A  is stored in output register 54 in response to second clock signal 58 entering the precharge phase and first clock signal 56 returning to a logic high state. During the next cycle, the contents of output register 54 are output as output data 60. Importantly, each of the logic rows within conventional multiple-row dynamic logic circuit 50 illustrated in FIG. 2B evaluates its inputs during each clock cycle, regardless of whether or not the contents of input register 52 change. Thus, when the contents of input register 52 remain stable across multiple clock cycles, circuit 50 needlessly dissipates power by evaluating each of domino logic rows 1 through A. 
     Referring now to FIG. 3A, there is depicted an illustrative embodiment of a reduced-power multiple-row dynamic logic circuit in accordance with the present invention. Like circuit 50 of FIG. 2B, circuit 80 of FIG. 3A includes an input register 82, A rows of sequentially coupled domino logic, an output register 84, a first clock signal 86, and a second clock signal 88. In addition, circuit 80 includes a history register 90 that stores the contents of input register 82 from the immediately previous cycle of first clock signal 86, a comparator 92 that compares the contents of history register 90 and input register 82, and an AND gate 94 that generates a third clock signal 96 in response to second clock signal 88 and the equal signal 98 output by comparator 92. Importantly, third clock signal 96, which controls the evaluation of inputs by the last X of the A domino logic rows, is inactive during the evaluate phase of second clock signal 88 when comparator 92 indicates that the contents of input register 82 and history register 90 are equivalent. In this manner, the contents of output register 84 remain unchanged (as would be the case if the last X domino logic rows did evaluate their inputs), and power dissipation by circuit 80 is reduced. 
     As shown in FIG. 3A, input register 82, history register 90, comparator 92, and AND gate 94 of circuit 80 together form comparing circuitry 99; the first A-X domino logic rows 84 of circuit 80 form a first evaluation circuit 85; and the last X domino logic rows 84 of circuit 80 form a second evaluation circuit 87. 
     With reference now to FIG. 3B, there is illustrated a timing diagram of the operation of circuit 80 of FIG. 3A. As depicted, when first clock signal 86 rises, register input state 1 is loaded into input register 82. In parallel, input state 0, the previous state of input register 82, is loaded into history register 90. In addition, the row output (RO) of each of domino logic rows 1 through A is set to a logic low state in response to the falling edge of second clock signal 88. When second clock signal 88 rises at the beginning of the evaluate phase, domino logic rows 1 through A-X-1 evaluate sequentially. That is, domino logic row 1 evaluates and generates RO 1 , which is received as an input by domino logic row 2. Domino logic row 2 similarly evaluates RO 1  to generate R0 2 . This process continues until domino logic row A-X is encountered. 
     In parallel with the evaluation of inputs by domino logic rows 1 through A-X-1, comparator 92 compares the contents of history register 90 and input register 82. If the contents of history register 90 and input register 82 are equivalent, equal signal 98 output by comparator 92 is logic high. Alternatively, if the contents of history register 90 and input register 82 are not equivalent, equal signal 98 is logic low. If equal signal 98 is logic low, third clock signal 96 follows second clock signal 88, and domino logic rows A-X through A evaluate sequentially until RO A  generated by domino logic row A is loaded into output register 84 on the falling edge of third clock signal 96. 
     On the other hand, if equal signal 98 is logic high (as illustrated in FIG. 3B), third clock signal 96 is forced to a logic low state before RO A-X-1  is received as an input by domino logic row A-X. Consequently, the last X rows of domino logic remain in a precharge state for the remainder of the current cycle of second clock signal 88. Because output register 84 does not receive a falling edge of third clock signal 96, the previous output state (i.e., output state 1) is retained. Thus, circuit 80 conserves power by eliminating the evaluation of X rows of logic. 
     In order for domino logic rows A-X through A to subsequently correctly evaluate new input data, equal signal 98 must return to a logic low state before RO A-X-1  is received at domino logic row A-X. Consequently, circuit 80 is subject to a timing requirement that the evaluation time of comparator 92 plus the delay of AND gate 94 must be less than the worst case (i.e., fastest) evaluation time of domino logic rows 1 through A-X-1. In order to ensure proper operation, an additional time margin is preferably taken into account to compensate for process and operating temperature variations. An equation summarizing this relationship is given below: 
     
         T.sub.e &gt;T.sub.c +T.sub.a +T.sub.m, 
    
     where T e  is the evaluation time of dynamic logic rows 1 through A-X-1, T c  is the evaluation time of comparator 92, T a  is the delay associated with AND gate 94, and T m  is the timing margin. 
     As has been described, the present invention provides a reduced power dynamic logic circuit that selectively inhibits evaluation of one or more rows of logic in order to reduce unnecessary power dissipation in cases in which the circuit inputs remain unchanged. Importantly, the ability of a dynamic logic circuit in accordance with the present invention to inhibit evaluation of one or more rows of logic is not dependent upon a prior determination that the circuit inputs remain unchanged. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. For example, the present invention is not limited to embodiments that utilize clocked dynamic logic as shown in FIG. 3A. Rather, the present invention encompasses alternative embodiments that replace the first and second clock signals shown in FIG. 3A with signals generated by self-timed circuitry.