Stateful Energy Management for Multi-Source Energy Harvesting Transient Computing Systems

The intermittent and varying nature of energy harvesting (EH) entails dedicated energy management with large energy storage, which is a limiting factor for low-power/cost systems with small form factors. Transient computing allows system operations to be performed in the presence of power outages by saving the system state into a non-volatile memory (NVM), thereby reducing the size of this storage. These systems are often designed with a task-based strategy, which requires the storage to be sized for the most energy consuming task. That is, however, not ideal for most systems since their tasks have varying energy requirements, i.e., energy storage size and operating voltage. Hence, to overcome this issue, this paper proposes a novel energy management unit (EMU), tailored for multi-source EH transient systems, that allows selecting the storage size and operating voltage for the next task to be performed at run-time, thereby optimizing task-specific energy needs and startup times based on application requirements. For the first time in literature, we adopted a hybrid NVM+VM approach allowing our EMU to reliably and efficiently retain its internal state, i.e., stateful EMU, under even the most severe EH conditions. Extensive empirical evaluations validated the operation of the proposed stateful EMU at a small overhead (0.07mJ of energy to update the EMU state and a $\simeq 4\mu\mathrm{A}$ of static current consumption of the EMU).