Power neutral performance scaling for energy harvesting MP-SoCs

Using energy `harvested' from the environment to power autonomous embedded systems is an attractive ideal, alleviating the burden of periodic battery replacement. However, such energy sources are typically low-current and transient, with high temporal and spatial variability. To overcome this, large energy buffers such as supercapacitors or batteries are typically incorporated to achieve energy neutral operation, where the energy consumed over a certain period of time is equal to the energy harvested. Large energy buffers, however, pose environmental issues in addition to increasing the size and cost of systems. In this paper we propose a novel power neutral performance scaling approach for multiprocessor system-on-chips (MP-SoCs) powered by energy harvesting. Under power neutral operation, the system's performance is dynamically scaled through DVFS and DPM such that the instantaneous power consumption is approximately equal to the instantaneous harvested power. Power neutrality means that large energy buffers are no longer required, while performance scaling ensures that available power is effectively utilised. The approach is experimentally validated using the Samsung Exynos5422 big.LITTLE SoC directly coupled to a monocrystalline photovoltaic array, with only 47mF of intermediate energy storage. Results show that the proposed approach is successful in tracking harvested power, stabilising the supply voltage to within 5% of the target value for over 93% of the test duration, resulting in the execution of 69% more instructions compared to existing static approaches.