Game Boy Advance interface. Only the master mode is supported.
The link to the Game Boy Advance is the Multiboot cable (parallel to Gameboy) also called the XBoo cable.
The general connection is very simple (only needs four wires), the only problem is that you need a special GBA plug or otherwise need to solder wires directly to the GBA mainboard.
The 'docs/gba' directory contains the test programs to test the GBA link. To compile, you need the devkitPro development kit. The compiled binaries are included.
The Gameboy Advance has 5 serial protocols:
The GBA interface supports the 8-bit/32-bit normal mode and the 16-bit multi-player modes. The JOY Bus protocol is not available yet. I have some documentation about the registers for the GBA side, but for bit banging the protocol itself, I don't have the specs for this. It's proprietary to Nintendo.