Fast All-Digital Clock Frequency Adaptation Circuit for Voltage Droop Tolerance

In classical synchronous designs, supply voltage droops can be handled by accounting for them in clock margins. However, this results in a significant performance hit even if droops are rare. In contrast, adaptive strategies detect such potentially hazardous events and either initiate a rollback to a previous state or proactively reduce clock speed in order to prevent timing violations. The performance of such solutions critically depends on a very fast response to droops. State-of-the-art solutions incur synchronization delays in the order of several clock cycles to avoid, with sufficient probability, that the clock signal is affected by metastability. We present an all-digital circuit that can respond to droops within a fraction of a clock cycle. This is achieved by using potentially metastable measurement values to delay clock signals while they undergo synchronization, instead of after they are synchronized. The challenge is to ensure that this strategy does not lead to harmful glitches or metastable upsets within the circuit. To this end, we verify our solution by formally proving correctness. We complement our findings by simulations of a 65-nm ASIC design confirming the results of our analysis.