I'd just like to say thanks a lot for this firmware, it took me a moment to fathom it out and understand it, but I have successfully mapped all the buttons as a gamepad but left the side buttons as vol up/down and enter/escape. I used Putty to achieve this via USB serial connection.
The reason I did this is because I use RetroPie, and there are a couple of issues I found with simultaneous keyboard button presses not working reliably in certain libretro cores (you have to hold one button and then press the other to get the desired output, eg. pressing jump+attack to perform a special move in Final Fight), and lr-mame2003 just acts strange because it doesn't enjoy the default bindings of the Picade keyboard (pressing ctrl triggers ctrl+shift, etc), and some games don't control correctly as a result. Remapping as a gamepad and configuring Retropie accordingly has solved both of these issues, the only thing it messed up is the Amiga emulator because that requires a keyboard and is hardcoded - but that's ok. If anyone is planning on doing something similar I had to change the joypad driver to SDL2 in retroarch.cfg (input_joypad_driver = "sdl2") otherwise the "controller" did not respond in any games. It does produce some SDL warnings on the console output as mentioned above which is stupid, but this can be ignored.
The only issue I have, is volume is not saved to EEPROM on shutdown anymore, and as previously mentioned in another thread you have to tap the volume keys to increase/decrease volume, which isn't an issue and i'm not fussed about that. Any plans to save the volume to EEPROM on shutdown though like on the stable firmware? As a workaround I set mine to about 70% over serial and saved it, which for me is a nice default on boot.
If anyone is interested, my config looks like:
a 220 221 222 223 230 231 232 233 234 235 238 239 176 177 250 251 105 111 112
Don't forget to use the + key to increase your volume over serial and save it, otherwise as mentioned above the Picade seems to start at minimum volume. I set mine to 20 (or something like that) so it defaults to this on boot, the max is 24.
Thanks!