Sensor Watch Accessory Boards (aka Sensor Boards)

You may have noticed that there are no sensors on this board. That is by design: rather than pick sensors for you, the goal is to add a tiny flexible PCB with the sensors YOU want, and interface them over the nine-pin connector. The connector provides the following options for power and connectivity:

• 3V power (3.08 V nominal; can drop to ~2.7V near end of life)
• An I²C interface with built-in pull-up resistors
• Five general purpose IO pins, which can be configured as:
  • Five analog inputs
  • Five interrupt-capable digital inputs, with internal pull-up or pull-down resistors
  • Five digital outputs
  • SPI controller (with one spare analog / GPIO pin leftover)
  • One UART TX/RX pair (with three GPIO leftover)
  • Up to four PWM pins on two independent TC instances
  • Two external wake inputs that can wake from the ultra-low-power BACKUP mode

These tiny “sensor boards” have a set outline, and the available area for your electronics is quite small (5.7 × 5.7 × 1 mm). Still, this is plenty of room for an environmental sensor, MEMS accelerometer or magnetometer and a couple of decoupling capacitors. Note that you will likely be limited to QFN and LGA type parts; SOICs are too large, and even SSOP packages are generally too thick. You can find reference designs for several sensor boards in the PCB/Sensor Boards directory in the Sensor Watch repository.