Reliable Timekeeping for Intermittent Computing

Abstract

Energy-harvesting devices have enabled Internet of Things applications that were impossible before. One core challenge of batteryless sensors that operate intermittently is reliable timekeeping. State-of-the-art low-power real-time clocks suffer from long start-up times (order of seconds) and have low timekeeping granularity (tens of milliseconds at best), often not matching timing requirements of devices that experience numerous power outages per second. Our key insight is that time can be inferred by measuring alternative physical phenomena, like the discharge of a simple 𝑅𝐶 circuit, and that timekeeping energy cost and accuracy can be modulated depending on the run-time requirements. We achieve these goals with a multi-tier timekeeping architecture, named Cascaded Hierarchical Remanence Timekeeper (CHRT), featuring an array of different 𝑅𝐶 circuits to be used for dynamic timekeeping requirements. The CHRT and its accompanying software interface are embedded into a fresh batteryless wireless sensing platform, called Botoks, capable of tracking time across power failures. Low start-up time (max 5 ms), high resolution (up to 1 ms) and run-time reconfigurability are the key features of our timekeeping platform. We developed two time-sensitive batteryless applications to demonstrate the approach: a bicycle analytics tool-where the CHRT is used to track time between revolutions of a bicycle wheel, and wireless communication-where the CHRT enables radio synchronization between two intermittently-powered sensors.

Battery-Free Game Boy

“…Numerous explorations of battery-free smart devices address the calls for sustainable/carbon-neutral electronic device interaction and electronic design and computing [12,65,67,85,145] while preparing human-interactive electronics for the "post-collapse society" [131]. Other work has developed core systems [44], hardware [23,28], and programming languages [82,150] for serious systems focusing on solving the intermittent computing problem caused by energy harvesting and battery-free operation, where frequent power failures prevent a program from finishing a task (see Figure 2). In all cases the electronic device is powered by harvested energy from the environment [107] and stored in (super-)capacitors of much smaller energy density and size than batteries.…”

“…While a seemingly trivial technology advancement, with this approach we increase compute speed but increase our I/O burden for checkpointing, as the traditional MSP430 FRAM-enabled MCUs have internal FRAM memory accessible at CPU speeds. This is a different tradeoff space than any other intermittent hardware platform [23,28,43]. This addresses Challenge 2 and Challenge 4.…”

“…active, communication techniques also become actively researched. These include simple sense and transmit sensor powered by ambient temperature differences [156], UFoP [42] and Capybara [23]-energy-harvesting storage-adaptive sensors, Battery Free Phone [126], SkinnyPower-wearable sensor powered by intra-body power transfer [121], Camaropteraimage-inferring sensor [96], SoZu-battery-free activity detector [155], or Botoks-time-aware wireless sensor [28]. Non-wireless/non-communicating battery-free sensors include CapHarvester-local energy monitor powered by harvesting stray voltage from AC power lines- [40], self-powered step motion counter [60], Saturn-battery-free microphone [6], and active radio battery-less eye tracker [73].…”

Can Crystal Oscillators Keep Time Without Power?

“…For all other uses, contact the owner/author(s). ENSsys '20, November 16-19, 2020, Virtual Event, Japan © 2020 Copyright held by the owner/author(s [4] short CUSTARD [4] long CHRT [1] very long This work short of memory locations (a group of bits) are initialized to a certain value, and when power is lost, those bits return to their default state over time. When power returns, the number of remaining bits can be used to estimate how long the system was without power.…”

“…CusTARD [4] and CHRT [2], for instance, use an external capacitor with known discharge (RC) characteristics to improve both timing duration and resolution. CusTARD uses a single capacitor that charges quickly during power availability and discharges slowly using a resistor during outages.…”