C3Pi is our experimental tracked robot with an onboard Raspberry Pi and an Arduino.
The Pi drives the Arduino over I2C, and the Arduino drives a motor controller board using PWM.
The Python program that drives the Pi uses our I2C library; it's about four lines of code.
At the moment all we do is respond to a character sent by the Pi by doing a little dance like a bee that has just found some honey. The next step is to define a simple protocol that will allow the Pi to set C3Pi's speed and direction. After that we will add a Sharp distance sensor which will tell the Pi to stop the motor if C3Pi approaches an obstacle.
We're using a SparkFun logic level shifter to interface the 3.3v I2C levels on the Pi to the 5v on the Arduino. Here's a picture of the current configuration: