+ /*
+ * 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
+ */