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Capybara: Reconfigurable Energy-storage Architecture

Synopsis

Capybara (ASPLOS'18) is a hardware design with a software interface for creating energy-storage for energy-harvesting devices that provides bursts of energy matched to the application energy demands.

The hardware consists of a circuit for a variable-capacity energy storage. The storage is implemented by connecting multiple capacitor banks with non-volatile switches that connect and disconnect each bank at runtime. The hardware design is implemented in the Capybara board, which demonstrates the reconfigurable energy storage hardware within a general-purpose solar-powered energy-harvesting device with several sensors (magnetometer, accelerometer with gyroscope, gesture, gas) and a BLE radio. The design files (schematics, board layout, BOM) are available in the capybara repository. Assembled boards are available on request.

The software interface, provided by the libcapybara library, targets applications written using the task-based programming model, and has been tested with Chain. The interface gives the application an ability to associate energy modes to tasks. An energy mode describes the energy demand of the task. The energy mode declarations are made using a set of macros and function calls from libcapybara. The runtime automatically reconfigures the hardware energy storage bank to a capacity that matches the energy demand. The library also allows asynchronous tasks (that may be invoked based on an external event) to request a burst of energy to be available by the time the task runs. The runtime pre-charges a capacitor bank. The interface reference is available in the libcapybara documentation. To use the interface, the application must include and link against libcapybara, which is packaged as a dependency for the Maker build system.

The following example applications, written in Chain and built with Maker, demonstrate Capybara:

  • Gesture Recognition: recognizes proximity and gestures using the APDS sensor
  • TempAlarm: monitor temperature and broadcasts a BLE packet if temperature exceeds a predefined range
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