A little SNES controller simulator

snescontroller.rs

trait ControllerPins {
    // inputs:
    // latch pin (normally low)
    fn latch_rising(&mut self);
    fn latch_falling(&mut self);
    // clock pin (normally high)
    fn clock_rising(&mut self);
    fn clock_falling(&mut self);
    
    // output:
    // data pin (high = 0, low = 1)
    fn data(&self) -> bool;
}

struct SnesController {
    buttons: u16,
    shift_reg: u16,
    data: bool
}

impl SnesController {
    fn new() -> Self {
        SnesController {
            buttons: 0,
            shift_reg: 0,
            data: false
        }
    }

    /// This only represents the state of the actual buttons, not
    /// any sort of latches in the controller.
    fn set_buttons(&mut self, buttons: u16) {
        self.buttons = buttons;
    }
    
    /// Shifts 1 into right end, putting leftmost bit in self.data.
    fn shift(&mut self) {
        self.data = (self.shift_reg & 0x8000) != 0;
        self.shift_reg <<= 1;
        self.shift_reg |= 0x0001;
    }
}

impl ControllerPins for SnesController {
    fn latch_rising(&mut self) {
        self.shift_reg = self.buttons & 0xfff0;
        // bits 12...15 are controller id, which happens to be
        // 0b0000 for a normal controller
    }
    
    fn latch_falling(&mut self) {
        // Because the clock pin is normally high, the controller
        // shifts the first bit on the falling edge of latch,
        // so that the first bit will be read correctly.
        self.shift()
    }
    
    fn clock_rising(&mut self) {
        self.shift()
    }
    
    fn clock_falling(&mut self) { }
    
    fn data(&self) -> bool {
        self.data
    }
}

fn main() {
    let mut controller = SnesController::new();
    controller.set_buttons(0b0101_0100_1010_0000);
    
    // read buttons from controller
    controller.latch_rising();
    controller.latch_falling();
    for _ in 0..16 {
        controller.clock_falling();
        print!("{}", controller.data() as u8);
        
        controller.clock_rising();
    }
}