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I²C

The Inter IC bus (usually abbreviated to I²C) is an industry standard system for simple communication between ICs. The official standard is defined by Philips.

Physical interface

I²C uses two signals:

SDA = serial data.
SCL = serial clock.

I²C is a multidrop bus: the signals are common to all devices on that bus:

Both signals float high (i.e. require a pull-up). This is the quiescent state.
Each device must be able to drive the signal low, using an open collector or open drain output.
Each device must also be able to read the state of the signal.

Transfer protocol

A master device initiates a data transfer to or from a slave device. Each device can be the master for one transfer, and slave for another transfer (initiated by a different device acting as master), but a common implementation is to have a single master device that can read/write a number of slave-only devices.

All transfers consist of a sequence of 8-bit bytes, with each byte followed by an acknowledge bit.

The master device initiates a transfer by generating the START condition, i.e. by driving SDA low.

The first byte transmitted by the master contains the 7-bit address of the slave to be accessed:

Each slave device is identified by a unique 7-bit address, often documented as an 8-bit hexadecimal value with the least significant bit indeterminate (for example, "C0/C1"). Addresses 00…0F and F0…FF are reserved.

Followed by the data direction bit:

0 = write data from the master to the slave.
1 = read data from the slave to the master.

If the slave is present it must respond by transmitting one acknowledge bit.

Data is then transferred (in the direction specified by the R/W bit) one byte at a time:

The master device terminates the transfer (after the last byte) by generating the STOP condition.