I2C on the Raspberry Pi

Tue 12 March 2013 by Lars Kellogg-Stedman Tags hardware raspberrypi arduino i2c

I've set up my Raspberry Pi to communicate with my Arduino via I2C. The Raspberry Pi is a 3.3v device and the Arduino is a 5v device. While in general this means that you need to use a level converter when connecting the two devices, you don't need to use a level converter when connecting the Arduino to the Raspberry Pi via I2C.

The design of the I2C bus is such that the only device driving a voltage on the bus is the master (in this case, the Raspberry Pi), via pull-up resistors. So when "idle", the bus is pulled to 3.3v volts by the Pi, which is perfectly safe for the Arduino (and compatible with it's 5v signaling). To transmit data on the bus, a device brings the bus low by connecting it to ground. In other words, slave devices never drive the bus high. This means that the Raspberry Pi will never see a 5v signal from the Arduino...unless, of course, you make a mistake and accidentally digitalWrite a HIGH value on one of the Arduino's I2C pins. So don't do that.

Note that the built-in pull-up resistors are only available on the Pi's I2C pins (Pins 3 (SDA) and 5 (SCL), aka BCM GPIO0 and GPIO1 on a Rev. 1 board, GPIO2 and GPIOP3 on a Rev. 2 board):

Raspberry Pi Pins

On the Arduino Uno, the I2C pins are pins A4 (SDA) and A5 (SCL):

Arduino Uno Pins

For information about other boards and about the Arduino I2C API, see the documentation for the Wire library.