I²C, prounounced I-Squared-C, is short for the “Inter-Integrated-Circuit bus”. It was invented in 1982 by Philips, although the patent is now expired.

More than you'll ever need to know is on the I²C Wikipedia page.

Sometimes it is also known as the “two-wire” bus, which is also descriptive, because it has two signal wires. Those two wires are called “SCL” and “SDA, for “serial clock” and “serial data.”

Some basic facts about I²C:

• It was originally intended to stay on the same PC board, but works over a few feet of cable
• It runs at 100Kbps or 400Kbps
• Devices only pull SDA and SCL low
• Pullup resistors are required on SDA and SCL, but most boards have the installed already.
• Each “slave” device has an address, and each slave on the bus must have a unique address.
• Addresses are 7 bits long, therefore there are only 128 possible addresses.
• Some devices have pins that can chose from several addresses. Others are limited to one per bus because their address can't be changed.

The I²C standard defines the basic signaling on the two wires for sending data back and forth to the device. The primitives available are:

• Claim the bus and start a transaction: pull SDA low while leaving SCL high
• Stop and release the bus: release SDA to go high, while leaving SCL high
• Send a byte (and get an acknowlegement)
• Read a byte

This figure shows the basic idea: start, write one byte, acknowledge, stop:

All transfers start as shown here, with the master sending one byte onto the bus containing the 7 bit address and one bit that indicates whether the next byte will be a write (master to slave) or a read (slave sends to master).

A good, relatively simple explanation of data transfer is https://i2c.info/i2c-bus-specification

These get combined in various ways depending on the device. Reading the datasheet is a must while writing software for an I2C device.

Since the introduction of the RoboRio, there has been an easy way to use I²C for FIRST robots. The connector on the Rio looks like this. The connector has four pins, but otherwise looks like the common “PWM” connectors. It uses the same housings and headers that hold pins on 0.1 inch centers.

In addition to SCL and SDA, there is a pin for 3.3v power, and a pin for the common ground for signal and power. This means that no external power source is needed or recommended for attaching an I2C device.

Around 2018-2019 we started to see packaged color and distance sensors from Rev Robotics and others.

• http://www.revrobotics.com/rev-31-1505/
• http://www.revrobotics.com/rev-31-1557/

For 2020 Rev introduced a cable to make using one of them easy with the RoboRio.

• http://www.revrobotics.com/rev-11-1729/

Nearly every I²C device has unique requirements from the software to communicate with it. Fortunately, the success of the Arduino family of microcontroller boards means that almost every interesting I²C device has example code ready to use with the arduino board and software.

This is often the best place to find such examples. Reading the sample code can help you understand the part's datasheet.

By 2021 there appeared FRC Java libraries for some of the I²C sensors that we've used. But going back to the Arduino C code is necessary for some of the interesting ones that are less common in FRC.

Sparkfun has many sensor boards and controllers that use a tiny 4-pin connector they call QWIIC. It carries the SDA and SCL signals, ground, and +3.3v power.

Adafruit wants to call the same thing STEMMA-QT.

In 2019 we had some flaky i2c operation, so we built this wiring tester testing_i2c_wiring