This is an example project that shows how to do communication between an A-Star 32U4 and a Raspberry Pi over I2C. While I developed it before the A-Star Robot Controller was released, it is compatible with that controller.

I2C is a protocol for sending packets of bytes back and forth between a master and slave; it is left up to the user to decide what the bytes in each packet mean. In typical applications, I2C is used to access a shared region of memory, with the first byte of the packet representing an address and additional bytes being read or written starting at that location. Since writes and reads are controlled by the master, the slave must be ready to handle them at any time, which makes it a challenge to handle multi-byte data safely. Other challenges:

• There is no standard procedure for issuing a "command" rather than just updating some data.
• The processor on the Raspberry Pi has a bug that makes reliable I2C hard to establish except at very low speeds.
• The Arduino Wire library, which is the standard for I2C on the Arduino, is inefficient and does not allow the required workarounds for the I2C bug mentioned above.

The approach I used in this project was to a invent a way of interpreting data in the I2C shared memory region in terms of commands, which are defined as functions in the main .ino file, with arguments translated to bytes using some C++ template magic.

To make it possible work around the I2C bug, I implemented my own general-purpose I2C library FastTWISlave - the RPiSlave class extends this one, adding the delays necessary for communication with a Raspberry Pi.