The Raspberry Pi and the Arduino are both amazing, and each has its particular strengths. There are lots of projects that use one or the other, but there's growing interest in solutions that combine the strengths of the two platforms.
We use both on C3Pi (our experimental robot). At present we use I2C to link the two, and today we found a way to simplify the way they are connected. Since there seems to be a lot of interest in this topic, I thought we'd share our approach and explain why the solution is a little simpler that we thought!
The I2C software is easy
We've been using our Python library on C3Pi to drive the I2C bus that we currently use to connect the Pi to the Arduino. We've also adapted the Arduino I2C slave code which is one of the Arduino examples.
Both programs are very simple. You can find the current versions on github. (Warning: expect the code to change rapidly as we are experimenting with new ideas!)
Linking the Pi and Arduino hardware is easier than we thought
Until today, we were using a sparkfun level shifter to connect the 3v3 I2C bus used by the Pi to the 5v I2C bus on the Arduino.
It turns out that we don't need to, because the pull-up resistors on the Pi keep the SDA and SCL lines close enough to 3v3 to be safe, even if they are directly connected.
Until today I thought that wasn't the case. I dimly remembered that the Arduino has pull-up resistors that are enabled by default when you use the Wire library. However, it turns out that these are weak (high-resistance) pull-ups; the Pi has strong pull-ups which dominate those on the Arduino, making direct connection safe.
This only applies to I2C. If you want to connect the serial port on the Pi to the serial port on the Arduino, you will need some protective circuitry between them. That's why we have a level shifter on our interface board.
But for I2C, just connect the Pi and Arduino as shown below and everything will be fine!