A Bi-Directional, Zero-Latency Adaptive Clocking Circuit in a 28-nm Wide AVFS System

Abstract

Resilient circuits based on in situ timing monitoring adaptive voltage-frequency scaling (AVFS) eliminate excess time margins caused by process, voltage, and temperature (PVT) variations but suffer from 50% throughput loss during error recovery when operating at a half frequency. We propose a bi-directional adaptive clocking circuit to provide fine frequency tuning with zero latency for AVFS system. It can either stretch the clock cycle when there are timing errors to ensure correct function or compress the cycle when there are excess timing margins. To support a wide frequency range, we generate multiple phase clocks based on two delay lines and select one appropriate phase clock to obtain a stretched output clock, where balanced clock paths are obtained by a time-to-digital converter and dynamic-OR gates. Applied to a wide-operating-range AVFS system of an SHA-256 accelerator with transition detector (TD) latches, the whole AVFS system is able to respond to errors in one clock cycle, with dynamic-OR gates collecting all the errors in half a cycle and adaptive clocking circuit stretching at the current cycle. Fabricated in 28-nm CMOS, chip measurement shows that it achieves 38.6%-69.4% power gains at near threshold while reducing throughput loss during error recovery.