Synchronous pipelines usually have a fixed clock frequency determined by the worst-case process-voltage-temperature (PVT) analysis of the most critical path. Higher operating frequencies are possible under typical PVT conditions, especially when the most critical path is not triggered. This paper introduces a design methodology that uses asynchronous design to generate the clock of a synchronous pipeline. The result is a variable clock period that changes cycle-by-cycle according to the current operations in the pipeline and the current PVT conditions. The paper also presents a simple design flow to implement variable-clock systems with standard cells using conventional synchronous design tools. The variable-clock pipeline technique has been tested on a 32-bit microprocessor in 90 nm technology. Post-layout simulations with three sets of benchmarks demonstrate that the variable-clock processor has a two-fold performance advantage over its fixed-clock counterpart. The overhead of the added clock generation circuit is merely 2.6% in area and 3% in energy consumption, compared to an earlier proposal that costs 100% overhead.
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