For the digital section I went with the Atm… erm, Microchip ATSAML21E18B as I already had a previous project centered around it. It’s capable of low power operation and is packed with both analog and digital peripherals that make it quite versatile. It’s got USB and I adapted this UF2 bootloader to run on my board which makes development a lot easier.

Features

• Most of the pins broken out to headers
• On-board Li-Ion battery charger - Microchip MCP73832. It can be disabled for use with non-rechargeable batteries
• Power jumper. Can be used to measure current on the 3V3 line
• Type-C port with TVS diode protection
• User LED and button
• Reset button & circuitry
• Header close to MCU for pins connected to the internal opamps

Schematic