HARC: A Heterogeneous Array of Redundant Persistent Clocks for Batteryless, Intermittently-Powered Systems

Batteryless sensing devices powered solely by ambient energy sources are expected to operate in an intermittent manner, since they do not have a predictable, or even continuous, energy supply. When such an intermittent system is powered off, it cannot keep track of time using conventional means. However, a continuous sense of time is critical for any system running real-time or time-sensitive applications. In this paper, we present HARC (Heterogeneous Array of Redundant Persistent Clocks), a novel solution to the problem of timekeeping for batteryless, intermittently-powered systems. HARC uses a heterogeneous, redundant array of capacitor-based persistent clocks that each decay in parallel, but at different rates, to provide variation-resilient high accuracy over a wide range of power off-times. We demonstrate the feasibility and effectiveness of HARC using experimental evaluations on a HARC prototype, and trace-based simulations of HARC-supported communication directly between two devices intermittently-powered by RF harvesting.