TVTAC: Triple Voltage Threshold Approximate Cache for Energy Harvesting Nonvolatile Processors

Energy harvesting is considered to be a substitute for batteries in many modern systems. Systems based on energy harvesting receive environmental energies from sources, such as sun, radio frequency, wind, vibration, etc, and convert them to electrical energy to be used by the capacitor of the system or feed the cyber-physical system (CPS) system directly. Despite its advantages, energy harvesting comes with some limitations, such as the instability of the received energy, which means that the energy may not be received for a moment due to environmental conditions during energy harvesting. Therefore, due to not receiving enough energy, the system function may face problems, which can lead to system shutdown and data loss. To prevent program execution interruption caused by frequent power interruptions in systems based on energy harvesting, these systems use nonvolatile processor (NVP). Saving the state in the NVP is done through nonvolatile registers and memories that can hold the contents until the power is restored. However, systems based on energy harvesting and NVPs also have challenges, such as frequent backups' energy consumption, slow program forward progress, and loss of data. In this article, we propose TVTAC, a framework for energy harvesting-based NVP CPS systems. TVTAC modifies conventional NVP's cache architecture to efficiently work with newly introduced operational mode to prevent unnecessary backup operations. Furthermore, TVTAC is equipped with an NVP's specific approximation unit that controls the approximation knobs during the approximate data cache accesses in order to save more energy. The simulation results show that TVTAC improves forward progress by 28% in the best case and 12% on average, compared to similar methods. From an energy consumption perspective, TVTAC reduces energy consumption by 43.5% in the best case and 28.5% on average.