Improving the Forward Progress of Transient Systems

Emerging applications for Internet of Things (IoT) devices demand smaller mass, size, and cost whilst increasing capability and reliability. Energy harvesting can provide power to these ultra-constrained devices, but introduces unreliability, unpredictability, and intermittency. Schemes for wireless sensors without batteries or supercapacitors overcome intermittency through saving system state into nonvolatile memory before the supply drops below the minimum operating voltage, termed transient, or intermittent computing. However, this introduces significant time and energy overheads. This article presents two schemes that significantly reduce these overheads: entering a sleep mode to avoid saving state and utilizing direct memory access (DMA) when state saves are required. Time and energy previously wasted on state saves can instead be used to perform useful computation, termed “forward progress.” We practically validate the proposed approaches across a range of energy sources and IoT benchmarks and demonstrate up to 46.8% and 40.3% increase in forward progress and up to 91.1% and 85.6% reduction in overheads for each scheme, respectively.

I. Introduction

The Internet of Things (IoT) is a fast expanding and developing field, ranging from connected homes to concepts of smart cities with a unity of physical world and cloud [1]. A key area of growth is ultra low power sensor devices [2]. To maximize the potential deployment opportunities for these devices, they must be autonomously powered and long-running. Some deployments necessitate a small cost, size, and mass; examples include biomedical implants [3], data-rich radio frequency identification, and structural monitoring. Powering these types of devices is a significant challenge. Batteries alone incur high maintenance overheads with frequent replacement or charging, or must be much larger, which is impractical. This has prompted developments in energy harvesting (EH) [4].