I want to be able to exchange data between two units (in my case, two AVR processors), and I want to get away with using no more than 4 wires.
So what are the existing methods for doing this? I know there's USB, but that seems a bit overkill here since I don't need to for example support multiple connections over the same cable.
The requirements are not entirely set but I can at least give some approximations. The AVR processors I intent to use are ATTiny2313 and ATTiny13 which will run at 16 MHz (the ATTiny2313 will also run V-USB). Data throughput is extremely low since I will use this to implement a custom keyboard, so it will probably be something like a few hundred bits/s. The length of the wires will be something like up to one or two meters, maybe three.