Raspberry Pi slave library for Arduino

Version: 1.0.1
Release date: 2017 January 23

www.pololu.com

Summary

This is an Arduino library that helps establish I²C communication with a Raspberry Pi, with the Arduino acting as the I²C slave. It should work with most Arduino-compatible boards, but we designed it for use with the A-Star 32U4 Robot Controller, which mounts conveniently to the Pi's GPIO header. The idea is that the Raspberry Pi can take care of high-level tasks like video processing or network communication, while the A-Star takes care of actuator control, sensor inputs, and other low-level tasks that the Pi is incapable of.

There are a few reasons we made a library for this instead of just recommending the standard Arduino I²C library, Wire.h:

• Wire.h just gives you access to raw I²C packets; you have to decide how they should relate to your data, and that's not trivial. This library implements a specific protocol allowing bidirectional access to a buffer, with atomic reads and writes.
• The processor on the Raspberry Pi has a major bug that corrupts data except at very low speeds. The standard Arduino I²C library, Wire.h, is not flexible enough to allow us to follow the suggested workarounds (delays inserted at key points).

We have included example Arduino code for the A-Star and Python code for the Raspberry Pi. Together, the examples set up a web server on the Raspberry Pi that will let you remotely control and monitor a robot from your smartphone or computer.

Getting started

See this blog post for a complete tutorial including step-by-step build instructions for an example robot.

Benchmarking

The included script benchmark.py times reads and writes of 8 bytes, to give you an idea of how quickly data can be transferred between the devices. Because of a limitation in the AVR's I²C module, we have slowed down reads significantly.

Bus speed	Reads	Writes
100 kHz	21 kbit/s	53 kbit/s
400 kHz	43 kbit/s	140 kbit/s

Version history

• 1.0.1 (2017 Jan 23): Added and adjusted delays necessary for reliable operation on the Pi 3.
• 1.0.0 (2016 Feb 16): Original release.