Allocation of Always-On State Retention Storage for Power Gated Circuits--Steady-State-Driven Approach

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 is the critical (inherently unavoidable) bottleneck in minimizing total (always-on) storage size for the allocation of nonuniform multibits 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 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 is 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. Through experiments with benchmark circuits, it is shown that our proposed method is able to reduce the total number of retention bits by 27.12% on average when at most 2-bit retention flip-flop is used, saving standby power by 19.41% compared with the state-of-the-art conventional method.