void xt_host_init(void)
{
XT_INT_INIT();
+
+ /* hard reset */
+#ifdef XT_RESET
+ XT_RESET();
+#endif
+
+ /* soft reset: pull clock line down for 20ms */
+ XT_INT_OFF();
+ data_lo(); clock_lo();
+ _delay_ms(20);
+ data_in(); clock_in();
XT_INT_ON();
}
ISR(XT_INT_VECT)
{
- static uint8_t state = 0;
+ /*
+ * XT signal format consits of 10 or 9 clocks and sends start bits and 8-bit data,
+ * which should be read on falling edge of clock.
+ *
+ * start(0), start(1), bit0, bit1, bit2, bit3, bit4, bit5, bit6, bit7
+ *
+ * Original IBM XT keyboard sends start(0) bit while some of clones don't.
+ * Start(0) bit is read as low on data line while start(1) as high.
+ *
+ * https://github.com/tmk/tmk_keyboard/wiki/IBM-PC-XT-Keyboard-Protocol
+ */
+ static enum {
+ START, BIT0, BIT1, BIT2, BIT3, BIT4, BIT5, BIT6, BIT7, END
+ } state = START;
static uint8_t data = 0;
- if (state == 0) {
- if (data_in())
- state++;
- } else if (state >= 1 && state <= 8) {
- wait_clock_lo(20);
- data >>= 1;
- if (data_in())
- data |= 0x80;
- if (state == 8)
- goto END;
- state++;
- } else {
- goto DONE;
+ // This is needed if using PCINT which can be called on both falling and rising edge
+ if (clock_in()) return;
+
+ switch (state) {
+ case START:
+ // ignore start(0) bit
+ if (!data_in()) return;
+ break;
+ case BIT0 ... BIT7:
+ data >>= 1;
+ if (data_in())
+ data |= 0x80;
+ break;
+ }
+ if (++state == END) {
+ pbuf_enqueue(data);
+ state = START;
+ data = 0;
}
- goto RETURN;
-END:
- pbuf_enqueue(data);
-DONE:
- state = 0;
- data = 0;
-RETURN:
return;
}