A scan-chain based state retention methodology for IoT processors operating on intermittent energy

Future IoT systems are tightly constraint by cost and size and will often be operated from an energy harvester's output. Since these batteryless systems operate on intermittent energy they have to be able to retain their state during the power outages in order to guarantee computation progress. Due to the lack of large energy buffers the state needs to be saved quickly using residual energy only. In related work, the state is retained in-place by replacing all flip-flops with state retentive flip-flops (SRFF), which are powered by auxiliary supplies for retention or incorporate non-volatile memory cells. However, these SRFFs increase the power consumption during active operation impairing the overall systems efficiency. In this paper, we present a scan-chain based state retention approach, where the state is moved to memory using only 4.5pJ/b. Since our approach does not introduce any power overhead, this energy cost pays off after an on-time of just 100us compared to state-of-the-art in-place solutions. Moreover, compared to a software mechanism, our approach requires 6.6x less energy to move the state and is 5.8x faster.