ExtFriend is a USB adapter for the WonderSwan's EXT port based on the RP2040 chip and the Raspberry Pi Pico board, providing the following features at a low cost:

• A USB serial port communicating with the WonderSwan at 9600 and 38400 baud,
• A USB audio device allowing digital stereo audio capture from the WonderSwan.

Both features can be active simultaneously - for example, this could allow one to control a WonderSwan as an audio synthesizer.

Special thanks to BluRaf for providing support and advice through my first journey into TinyUSB lands.

• If you mute the USB audio device, the WonderSwan will consider the headphones as disconnected, re-enabling the internal speaker.

• Copy pico_sdk_import.cmake from your Pico-SDK installation to the repository's root.
• Run mkdir build, cd build, cmake .., make.

• Flash the program to the Pico: for example, picotool load -v -x extfriend.uf2
• Connect the Raspberry Pi Pico to the WonderSwan's EXT port. The default build of ExtFriend uses seven consecutive Pi Pico pins, as follows:
  • EXT GND - Pico GND (for example pin 3)
  • EXT SER_MOSI - Pico pin 2 (GP1)
  • EXT SER_MISO - Pico pin 1 (GP0)
  • If you want to use digital audio capture:
    • EXT HDPN_BCLK - Pico pin 7 (GP5)
    • EXT HDPN_LRCK - Pico pin 6 (GP4)
    • EXT HDPN_SDAT - Pico pin 5 (GP3)
    • EXT /HDPN_DETECT - Pico pin 4 (GP2)

If you don't have an EXT port plug or cable handy, you can use an HDMI breakout board. An example instruction guide is provided here (in Japanese), for a $2 AliExpress board.

GPLv3+.