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In the previous work, the problem of finding gate delays to eliminate glitches has been solved by linear programs (LP) requiring an exponentially large number of constraints. By introducing two additional variables per gate, namely, the fastest and the slowest arrival times, besides the gate delay, we reduce the number of the LP constraints to be linear in circuit size. For example, the 469-gate c880 circuit requires 3,611 constraints as compared to the 6.95 million constraints needed with the previous method. The reduced constraints provably produce the same exact LP solution as obtained by the exponential set of constraints. For the first time, we are able to optimize all ISCAS'85 benchmarks. For the c7552 circuit, when the input to output delay is constrained not to increase, a design with 366 delay buffers consumes only 34% peak and 38% average power as compared to an unoptimized design. As shown in previous work, the use of delay buffers is essential in this case. The practicality of the design is demonstrated by implementing an optimized 4-bit ALU circuit for which the power consumption was obtained by a circuit-level simulator.

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This paper provides a theoretical basis for eliminating or reducing the energy consumption due to transients in a synchronous digital circuit. The transient energy is minimized when every gate has no more than one output transition per clock cycle. This condition is achieved for a gate when the gate delay equals or exceeds the maximum di erence between path delays at gate inputs. In practice, path delays are adjusted either by increasing gate delays or by inserting delay bu ers. The minimum transient energy design is obtained when no delay bu er is added. This design requires possible increases in gate delays to meet the minimum energy condition at all gates. However, the delay of the critical path may be increased. In an alternative design, where the critical path delay is not allowed to increase, delay bu ers may have to be added. The theory in this paper allows trade-o s between minimum transient energy and critical path delay. We formulate the problem as a linear program to obtain the minimum transient energy design with the smallest number of delay bu ers for a given overall delay of the circuit. An optimized four-bit ALU circuit is found to consume 53 peak and 73 average power compared t o the original circuit.

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Modern digital circuits consist of logic gates implemented in the complementary metal oxide semiconductor (CMOS) technology. The time taken for a logic gate output to change after one or more inputs have changed is called the delay of the gate. A conventional CMOS gate is designed to have the same input to output delay irrespective of which input caused the output to change. We propose a new gate design that has different delays along various input to output paths within the gate. This is accomplished by inserting selectively sized "permanently on" series transistors at the inputs of the logic gate. We demonstrate the use of the variable input delay CMOS gates for a totally glitch-free minimum dynamic power implementation of a digital circuit. Applying a previously described linear programming method to the c7552 benchmark circuit, we obtained a power saving of 58% over an unoptimized design. This power consumption was 18% lower than that for an alternative low power design using conventional CMOS gates. All circuits had the same overall delay. Since the overall delay was not allowed to increase, the glitch elimination with conventional gates required insertion of delay buffers on non-critical paths. The use of the variable input delay gates drastically reduced the required number of delay buffers.

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