keyboard.c

volatile unsigned char kbd_clock_status = 0;
volatile unsigned char kbd_bit = 0;
volatile unsigned char kbd_code = 0;

void read_keyboard() {
  // clock was before low and clock is now high, thus rising
  if (kbd_clock_status == 0 && bit_is_set(PIND, PIND6) == 0) {
    kbd_clock_status = 1;
    return;
  }

  // clock wasn't high before or clock isn't currently low, thus not falling and
  // we've probably been triggered by the data line changing - we don't want to
  // react yet
  if (kbd_clock_status == 0 || bit_is_set(PIND, PIND6) == 0) {
    return;
  }

  // by process of elimination, we're now at the clock's falling edge

  // reset set the clock status, since the line is now low
  kbd_clock_status = 0;

  if (kbd_bit == 0) {
    // start bit, should be 0
    // light onboard led if it's not 0
    if (bit_is_set(PIND, PIND7) == 0) {
      PORTB |= _BV(PORTB5);
    }

    // reset the keyboard code
    kbd_code = 0;
  } else if (kbd_bit >= 1 && kbd_bit <= 8) {
    if (bit_is_set(PIND, PIND7) == 0) {
      kbd_code += (1 << (kbd_bit - 1));
    }
  } else if (kbd_bit == 9) {
    // parity bit
    // TODO: handle parity bit, lol
    if (bit_is_set(PIND, PIND7) == 0) {
      PORTB |= _BV(PORTB5);
    }
  } else if (kbd_bit == 10) {
    // stop bit, should be 1
    // light onboard led if it's not 1
    if (bit_is_set(PIND, PIND7) != 0) {
      PORTB |= _BV(PORTB5);
    }
  }

  // 10th bit is the end, reset the bit counter and act on the input
  if (kbd_bit == 10) {
    kbd_bit = 0;

    write_leds(kbd_code);
  } else {
    kbd_bit++;  
  }
}