I found my old GameBoy Advance consoles and found that I’ve never done any development work on it and decided to play around with it.
Unfortunately, I don’t have a flashcart and it’s difficult to find decent ones to buy on the internet anymore. The only option to actually upload code was to use the multiboot protocol in the GBA BIOS to run the code.
If you don’t know, the GBA has a feature where it’s possible to multiplayer link multiple consoles even if the other consoles did not have the game cartridges. The master console can download 256K worth of game code to each of the slave units so they can each run a copy of the game without a game cartridge.
The first obstacle to bypass was to actually able to communicate with the console. There are actually many ways to communicate with the GBA. With a bit of searching around and debugging, I found that the GBA can actually use 3 different serial protocols to download code. There’s the normal serial mode that is really just 32bit SPI. Then you have your proprietary Joybus mode and 16bit multiplayer serial interface.
After reading through a lot of diagrams and code, I managed to set up a Teensy AVR to bitbang the multiplayer serial protocol and bridge it to USB serial. (Hint: don’t ever use digitalWrite for bitbanging. It’s too slow!).
Second obstacle is actually working out the protocol. I had to read through lots of reverse engineered code to work it out. It seems the GBA uses this weird as encryption and checksumming scheme which I cannot work out. I have managed to complete all the handshaking except encryption and checksum verification though.
I will post source code and documentation once I finish it completely. In the meantime, here’s a video.