Arduinoboy download: https://github.com/trash80/Arduinoboy
Arduinoboy is software for the Arduino hardware platform that allows serial communication (MIDI) to the Nintendo Gameboy for music applications such as LittleSoundDJ, Nanoloop., and mGB
Current Features
Affordable and easily accessible parts for assembly.
Accurate MIDI Sync, Start and Stop commands.
Push Button selector sets the sync/state modes (7 modes available)
mGB Mode: Full MIDI in support across all Gameboy Channels, including a unique "poly" mode allows you to play your Game Boy like a synthesizer.
Midi Out Doubles as a Midi Thru
"Filtering" data for only sync messages, no dedicated MIDI line required.
Can be powered by the Game Boy's gamelink port.
USB upgradeable via Arduino.
Midi settings configurable using a Mac/PC editor built in Max.
Tested and works with DMG (Original), Gameboy Color, and Advance/SP.
Read instructions on the website link.
USB smart card: https://store.kitsch-bent.com/product/usb-64m-smart-card (discontinued)
Other reference: http://millionmachinemarch.com/page_id285/
This is what a Nanoloop cartridge can do: http://www.nanoloop.com/one/index.html watch the video