Leakage power minimisation techniques for embedded processors

Abstract

Leakage power is a growing concern in modern technology nodes. In some current and emerging applications, speed performance is uncritical but many of these applications rely on untethered power making energy a primary constraint. Leakage power minimisation is therefore key to maximising energy efficiency for these applications. This thesis proposes two new leakage power minimisation techniques to improve the energy efficiency of embedded processors. The first technique, called sub-clock power gating, can be used to reduce leakage power during the active mode. The technique capitalises on the observation that there can be large combinational idle time within the clock period in low performance applications and therefore power gates it. Sub-clock power gating is the first study into the application of power gating within the clock period, and simulation results on post layout netlists using a 90nm technology library show 3.5x, 2x and 1.3x improvement in energy efficiency for three test cases: 16-bit multiplier, ARM Cortex-M0 and Event Processor at a given performance point. To reduce the energy cost associated with moving between the sleep and active mode of operation, a second technique called symmetric virtual rail clamping is proposed. Rather than shutting down completely during sleep mode, the proposed technique uses a pair of NMOS and PMOS transistors at the head and foot of the power gated logic to lower the supply voltage by 2Vth. This reduces the energy needed to recharge the supply rails and eliminates signal glitching energy cost during wake-up. Experimental results from a 65nm test chip shows application of symmetric virtual rail clamping in sub-clock power gating improves energy efficiency, extending its applicable clock frequency range by 400x. The physical layout of power gating requires dedicated techniques and this thesis proposes dRail, a new physical layout technique for power gating. Unlike the traditional voltage area approach, dRail allows both power gated and non-power gated cells to be placed together in the physical layout to reduce area and routing overheads. Results from a post layout netlist of an ARM Cortex-M0 with sub-clock power gating shows standard cell area and signal routing are improved by 3% and 19% respectively. Sub-clock power gating, symmetric virtual rail clamping and dRail are incorporated into power gating design flows and are compatible with commercial EDA tools and gate libraries.

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