A proof-of-concept PICO-8 controller adapter over the GPIO SERIAL interface, allowing for extended controller input support including analogue sticks and more buttons.

Building the p8-controller process requires:

1. A C11 compiler
2. GNU make
3. SDL2 (make sure sdl2-config is correctly configured in your path)

Then, once you have all of the required dependencies, run:

make

Run the p8-controller binary. This will create controller.data and controller.clock "serial" lines that can be "connected" to a PICO-8 console:

pico8 -i controller.data > controller.clock

For example, you can:

1. make run to start the controller handler process and create the "serial" lines
2. make demo to start the demo cart attached to the controller "serial" lines
   • Be sure to also try out make poom 😈

Just make sure to load the controller JS file in your page hosting the exported PICO-8 game.

<script src="p8-controller.js"></script>

Take a look at the Makefile to see how we do this automatically for the exported demo and POOM cartridges.

It may be confusing that there is a src/controller.js and a target/p8-controller.js. This was done to allow the Makefile to potentially bundle multiple JavaScript files in the future. Currently it just cps the file over.

GPIO works very differently on various PICO-8 targets. Because of this, the way we support Desktop (Running PICO-8 locally on your computer) and Web (so HTML/JS exported PICO-8 games) are very different.

For desktop targets, additional controller data is sent over a named pipe attached to PICO-8 process's -i input file (serial channel 0x806). PICO-8 SERIAL command allows scheduling reading a certain amount from the named pipe with:

serial(0x806, target_address, data_length)

The basic concept is to periodically send controller data over the named pipe for it to be read with SERIAL commands. The tricky bit is synchronizing the reading and writing of controller data. For this, we use the PICO-8 process's standard output. Specifically the controller application will wait for some well-formed message indicating that the game is requesting controller data. Once this message is received, controller state is read, encoded and sent over PICO-8's standard input. We use a single digit numerical value alone on a line representing the controller index being polled to request controller data. For example, to get the controller data for controllers index 2 and 5, you could:

printh"2\n5" -- request new controller data for controllers 2 and 5 over stdout
serial(0x806,0x9a00,60) -- read 60 bytes from input file, each controller state is 30 bytes long

In theory, it should be possible to use serial channels for both input and output lines:

pico8 -i controller.data -o controller.clock

This would have a minor advantage over piping PICO-8's standard output into the controller.clock serial line of not losing the PICO-8 cart's standard output. This allows, for example, for PRINTH to be used for debugging.

Unfortunately, this also has a small issue and introduces a frame of controller input delay. This additional frame delay is interesting as it appears, in part, to be a PICO-8 issue. Specifically, PICO-8 will always wait for SERIAL reads before writes, even if the write was queued first. For example:

serial(0x807, ...) -- write to file specified in `-o` parameter
serial(0x806, ...) -- read from file specified in `-i` parameter

In this case, unintuitively, PICO-8 will first read from the file spcified by the -i parameter before writing to the file specified by the -o parameter. Therefore, we need to request controller data one frame early. This leads to a total of 2 frames of delay for receiving controller input:

• Frame 0: request controller data
  • FLIP causes the request to get flushed over the SERIAL interface
• Frame 1: read controller data
  • The controller data gets written to memory between frames
• Frame 2: controller data is in memory and can be used in the _UPDATE function

-- Frame 0
serial(0x807, ...) -- signal that we want controller data
flip() -- flush the serial output

-- Frame 1
serial(0x807, ...) -- already request controller data for next frame
serial(0x806, ...) -- queue read of the controller data
flip() -- flush the serial output and read serial data to memory

-- Frame 2
peek(...) -- we can now read controller data from memory

By using the fact that PRINTH flushes immediately and piping PICO-8's standard output to the controller.clock serial line, we only have 1 frame of delay.

Another alternative solution would have been to use PICO-8's standard input and output instead of named pipes for sending and synchronizing controller data. In fact, this project used to implement a wrapper binary that started PICO-8 and connected to its standard input and output (9765128).

This method has one major issue. If you were to start a game with extended controller support without properly attaching a controller to the PICO-8 process's standard input and output will cause PICO-8 to freeze. This is because the SERIAL command would get stuck waiting for 30 bytes of controller data over PICO-8's standard input that will never come. On the other hand, if you read from serial line 0x806 without specifying a file with the -i command line option, PICO-8 will just not read any bytes. This means that you can even detect if no controller is attached:

poke(0x9a0c, 0x9a) -- write marker value to memory location of first button
printh"0" -- request controller 0 data
serial(0x806, 0x9a00, 30) -- queue read of the controller data
flip() -- read serial data to memory
if @0x9a0c == 0x9a then
  -- controller not connected
end

I don't have a Windows machine, so I can't develop a Windows version. In theory, the same support can be implemented using Win32 named pipes (using CreateNamedPipe and ConnectNamedPipe). PRs are welcome!

For web targets, it is very straight forward. On each animation frame, we first set the pico8_gpio[0] pin to a marker value, so that the PICO-8 cart can detect that it is in web mode. Then we read controller state and write it directly to pico8_gpio. Easy-peasy-lemon-squeezy.