Clockless unidirectional serial link between a Raspberry Pi and Arduino.

serial link.txt

Clockless unidirectional serial link between a Raspberry Pi and Arduino.

Inspired by every Commodore 64 disk fastloader ever :).

Motivation: I want to send commands from a Raspberry Pi to an Adafruit
Gemma (a tiny Arduino-compatible). Gemma only has three GPIO pins, and one
of them is in use to make LEDs glow pretty colours. I don’t want to have
to worry about tricky timing constraints.

Gemma signals at 3.3V, which is the same level as the Pi. Most other
Arduinos signal at 5V and will require a conversion circuit to talk to the
Pi.

There are two GPIO lines, D0 and D1.

"pull up" = connect a line to +3.3V via a pull-up resistor.
"short to ground" = connect a line directly to ground.
"wait" = poll the state of a line every so often until it changes to the
         expected state.

1.  Arduino pulls up both lines and waits for one of the lines to go low.
2.  Pi pulls up both lines and waits until it wants to send a bit.
3.  Pi shorts one of the lines to ground. To send a zero, it shorts D0. To
    send a one it shorts D1. Whichever line is shorted is temporarily
    designated the 'signal' line. The other line is temporarily designated
    the 'ack' line.
4.  Pi waits for the ack line to go low.
5.  Arduino detects that the signal line has gone low. If D0 is the signal
    line, Arduino receives a zero. If D1 is the signal line, Arduino
    receives a 1.
6.  Arduino acknowledges the bit by shorting the ack line to ground.
7.  Arduino waits for the signal line to go high.
8.  Pi detects that the ack line has gone low and pulls up the signal line.
9.  Arduino detects that the signal line has gone high and pulls up both
    lines.
10. Go to 1.