I2C

I2C, short for inter-integrated circuit, is a two-wire serial bus used for short-distance communication within integrated circuits. It allows multiple masters and slaves on a single shared bus and was developed by Philips (now NXP) in the 1980s. The standard uses two wires: Serial Data (SDA) and Serial Clock (SCL). Both lines are open-drain and require pull-up resistors to a common supply.

Communication begins with a start condition (SDA falls while SCL is high). Data is transferred in sequences of eight bits, with the data valid when SCL is high. After every byte, the transmitter releases the SDA line and the receiver may pull it low during the ninth clock to acknowledge. A stop condition (SDA rises while SCL is high) ends the transfer; repeated starts are allowed to transition to another address without releasing the bus.

Addressing: Each slave is addressed by a seven-bit address (followed by a Read/Write bit in the first byte). Acknowledgments manage bus access. A ten-bit addressing scheme exists for scarce addresses. The protocol supports multi-master operation with arbitration on the SDA line; if two masters drive different bits, the one that writes a high while the other writes low loses and releases the bus. Slaves may clock-stretch by holding SCL low to delay the master.

Speeds: Standard-mode (100 kbps), Fast-mode (400 kbps), Fast-mode Plus (1 Mbps), High-speed mode (3.4 Mbps). Ultra-fast mode (5 Mbps) exists but is master-only and not widely used. Bus capacitance and pull-up strengths affect actual speed, rise times, and reliability.

Typical uses include sensors, EEPROMs, ADCs, DACs, and RTCs in microcontroller-based systems. I2C is widely supported in microcontrollers and peripherals; it is often compared with SMBus, a derivative with stricter timing and power-management features.