gcio-nes

gcio-nes is a project bringing GameCube input to the NES/Famicom through an Atmel AtMega8 microcontroller adaptor as third party successor to the ‘raphnet’ GameCube to NES Adaptor using the same technology.

The Vision

1. Full report and Backwards Compatibility

   1. The ability to report all the information from the controller relevant to the system its connected to.

   2. Triple buffered to allow for instant controller reading formatted to imitate a NES, then SNES controller

   3. 100% Backwards compatibility support for games that correctly use a standard American NES/SNES Controller.

2. Console Reprogrammable behavior

   1. Sticks can report raw deltas to centre or a combination of angle with magnitude.

   2. Stick Sync options, permitting L to transfer to C or C to L on the adaptor..

   3. Option for creating an angle with magnitude for either or both of L and C stick from d pad inputs.

   4. Adaptor input skipping if not all inputs are required.

   5. Built-in input bitflips for runtime input conversion.

How it works

Every 6ms the adaptor commands the controller for its ID, fetching the necessary information to correct its output (such as stick neutral location). Then immediately sends a command for the controller to get the current state. While The controller is generating the information for the adaptor to receives it modified its internal state to correctly receive the inputs from the controller and preparing itself for reads from the controller.

Given the code:

lda #$01
sta buttons     ; (user defined memory location)
sta $4016       ; as long as the the reads on $4016 occurred while OUT was deasserted (typical)
                ; this does nothing to the adaptor as its polls the controller continuously
lda #$00
sta $4016       ; since no reads were performed to $4016 while OUT was asserted, this either:
                ; (1) sets the adaptor task to 'compatability report' mode
                ; (0) does nothing, as its latched into an advanced task

:   lda $4016   ; if not latched into an advanced task, will read bits like a NES Controller
                ; otherwise, will return zeroes until the task is exhausted.
    lsr a
    rol buttons
    bcc -

Note

If the adaptor is latched into an advanced task as its being used in a game that does not use the feature. It will at most return empty inputs for eight reads which typically is eight frames. This is because reads are used to accept bits to build a buffer when latched. Once the task is unlatched it will set the task index to 0 (legacy) in which it will increment the task again on each $4016 read when OUT is asserted.

Nearly all games will not read while OUT is asserted which will guarantee after 8 cycles the backwards compatibility mode will be selected.

But to use the full capability, the correct report code is:

lda #$01
sta buttons
sta $4016
bit $4016
lsr a
sta $4016

ldx #$08
:   dex
    bne ++
:   lda $4016
    lsr
    rol buttons, x
    bcc -
    clc
    bcs --
:

This code reports 64 bits of data from the controller such as long as the mask is set up properly. It knows to use the full capability mode as the bit $4016 performs a read while OUT is asserted increment the task index to 1 which is responsible for full capacity responses.