Steady state driven power gating for lightening always-on state retention storage

Abstract

It is generally known that a considerable portion of flip-flops in circuits is occupied by the ones with mux-feedback loop (called self-loop), which are the critical (inherently unavoidable) bottleneck in minimizing total (always-on) storage size for the allocation of non-uniform multi-bits for retaining flip-flop states in power gated circuits. This is because it is necessary to replace every self-loop flip-flop with a distinct retention flip-flop with at least one-bit storage for retaining its state since there is no clue as to where the flip-flop state, when waking up, comes from, i.e., from the mux-feedback loop or from the driving flip-flops other than itself. This work breaks this bottleneck by safely treating a large portion of the self-loop flip-flops as if they were the same as the flip-flops with no self-loop. Specifically, we design a novel mechanism of steady state monitoring, operating for a few cycles just before sleeping, on a partial set of self-loop flip-flops, by which the expensive state retention storage never be needed for the monitored flip-flops, contributing to a significant saving on the total size of the always- on state retention storage for power gating.