Phyduino boards use their own core (platform/cores/phyduino), written against the pico-sdk and ArduinoCore-API. It does not depend on another RP2040 Arduino core such as arduino-pico or ArduinoCore-mbed.

The board package is a teaching tool that must stay small and understandable for one maintainer coming back after months away. A thin core over the SDK is easy to read end to end, and students can use the SDK directly from sketches. Depending on a large third-party core would tie releases to its schedule and internals.

Every Arduino API feature has to be implemented here. The scope is kept to what the course needs; the SDK covers the rest.