Failure Sentinels: Ubiquitous Just-in-time Intermittent Computation via Low-cost Hardware Support for Voltage Monitoring

Energy harvesting systems support the deployment of low-power microcontrollers untethered by constant power sources or batteries, enabling long-lived deployments in a variety of applications previously limited by power or size constraints. However, the limitations of harvested energy mean that even the lowest-power microcontrollers operate intermittently-waiting for the harvester to slowly charge a buffer capacitor and rapidly discharging the capacitor to support a brief burst of computation. The challenges of the intermittent operation brought on by harvested energy drive a variety of hardware and software techniques that first enabled long-running computation, then focused on improving performance. Many of the most promising systems demand dynamic updates of available energy to inform checkpointing and mode decisions.

Unfortunately, existing energy monitoring solutions based on analog circuits (e.g., analog-to-digital converters) are ill-matched for the task because their signal processing focus sacrifices power efficiency for increased performance-performance not required by current or future intermittent computation systems. This results in existing solutions consuming as much energy as the microcontroller, stealing energy from useful computation. To create a low-power energy monitoring solution that provides just enough performance for intermittent computation use cases, we design and implement Failure Sentinels, an on-chip, fullydigital energy monitor. Failure Sentinels leverages the predictable propagation delay response of digital logic gates to supply voltage fluctuations to measure available energy. Our design space exploration shows that Failure Sentinels provides 30-50mV of resolution at sample rates up to 10kHz, while consuming less than 2µA of current. Experiments show that Failure Sentinels increases the energy available for software computation by up to 77%, compared to current solutions. We also implement a RISC-V-based FPGA prototype that validates our design space exploration and shows the overheads of incorporating Failure Sentinels into a system-on-chip.

• Failure Sentinels must minimize power consumption and provide just enough resolution and sample rate to serve software's needs. Section II shows that existing voltage monitors fail at this, whereas Failure Sentinels provides enough resolution to support frequent, voltage measurements, while avoiding the design and power concerns associated with DSP-focused ADCs.