The full form of I2C is Inter – Integrated Circuit.

The Inter-Integrated Circuit (I²C) Protocol is a protocol intended to allow multiple “peripheral” digital integrated circuits (“chips”) to communicate with one or more “controller” chips. Like the Serial Peripheral Interface (SPI), it is only intended for short distance communications within a single device. Like Asynchronous Serial Interfaces (such as RS-232 or UARTs), it only requires two signal wires to exchange information.

I2C is ordinarily pronounced “I-two-C”, though it is also sometimes written as IIC (and pronounced “I-I-C”) or I²C (pronounced “I-squared-C”). The acronym stands for Inter-Integrated-Circuit. It is a type of serial computer bus and communications protocol that was first introduced to the market by Philips Semiconductor in 1982.

I2C is a way of allowing multiple electronic devices (most often low-speed, peripheral integrated circuits) to communicate with each other over a single pair of wires. These wires are also called data lines, or buses. The first of these buses is the data line and is called the SDA (Serial DAta) line, and the other bus is the clock, or SCL (Serial CLock) line. Since all devices on any I2C circuit are hooked to these two lines to communicate, most I2C-compatible devices have pins labeled SDA and SCL, as well as VIN and GND pins for positive and ground connections.

Both the clock and data buses are open-drain lines. A resistor must be connected between each line and the circuit’s positive voltage supply (also referred to as Vcc) in order for the bus to work correctly. The size of these resistors can vary, from 1 kΩ all the way up to 47 kΩ, but they must be present between the bus and the system’s HIGH voltage. If they are not present, all lines will be pulled low, and the I2C bus will fail to work. Luckily, only one pair of resistors (one for each line) is necessary for the entire system, not a pair for each device–that could quickly get messy with a lot of devices.

Quite a few devices (also called nodes) can be attached to these buses. In fact, the number of devices that can practically be connected to any I2C circuit is normally limited only by space, the inherent capacitance of the lines, and the addresses of the connected devices. Most experts agree that this limit falls around 1008 devices.