I2Cs

I2Cs, or Inter-Integrated Circuits, is a synchronous two-wire serial bus designed for short-distance communication between integrated circuits on a single board. It supports multiple masters and multiple slaves on the same bus, enabling simple interconnects with a limited number of pins.

The bus uses two wires: Serial Data (SDA) and Serial Clock (SCL). Both lines are open-drain and require pull-up resistors to a supply voltage. Devices on the bus can pull a line low, but neither can drive it high directly, which allows multiple devices to share the same lines without collision.

Communication is initiated by a start condition, followed by a 7-bit or 10-bit address and a read/write bit. The addressed device acknowledges, after which data bytes are transferred with an acknowledge bit after each byte. Transfers can be stopped with a stop condition, and a repeated start can be issued to switch devices without releasing the bus. In multi-master configurations, arbitration ensures that only one master drives the bus at a time.

I2C supports several speed modes. Standard mode operates up to 100 kilobits per second, Fast mode up to 400 kilobits per second, Fast-mode Plus up to around 1 megabit per second, and High-speed mode up to 3.4 megabits per second. Ultra-fast mode exists in some implementations but is less common. High-speed mode additions require devices and circuitry that support the extended signaling.

Electrical and electrical-physical considerations include bus capacitance, pull-up resistor sizing, and voltage levels. I2C devices may run at different voltages (commonly 3.3V or 5V), in which case level shifting may be necessary.

I2C is widely used for peripherals such as sensors, EEPROMs, and real-time clocks, offering low pin count and simple wiring at the expense of distance and maximum throughput. It originated with Philips (NXP) in the 1980s and is known as TWI in some ecosystems.