Power-Optimal Pipelining in Deep Submicron Technology

This paper explores the effectiveness of pipelining as a power saving tool, where the reduction in logic depth per stage is used to reduce supply voltage at a fixed clock frequency. We examine power-optimal pipelining in deep submicron technology, both analytically and by simulation. Simulation uses a 70 nm predictive process with a fanout-of-four inverter chain model including input/output flip-flops, and results are shown to match theory well. The simulation results show that power-optimal logic depth is 6 to 8 FO4 and optimal power saving varies from 55 to 80% compared to a 24 FO4 logic depth, depending on threshold voltage, activity factor, and presence of clock-gating. We decompose the power consumption of a circuit into three components, switching power, leakage power, and idle power, and present the following insights into power-optimal pipelining. First, power-optimal logic depth decreases and optimal power savings increase for larger activity factors, where switching power dominates over leakage and idle power. Second, pipelining is more effective with lower threshold voltages at high activity factors, but higher threshold voltages give better results at lower activity factors where leakage current dominates. Lastly, clock-gating enables deeper pipelining and more power saving because it reduces timing element overhead when the activity factor is low.