I²C
Initialism of Inter-Integrated Circuit, a standard for integrated circuit communication using two lines.
I²C: two-wire protocol for talking to chips on a board
I²C (Inter-Integrated Circuit) is a synchronous serial bus that lets integrated circuits communicate with each other over just two wires: a clock line (SCL) and a data line (SDA). Both lines are open-drain, pulled high by resistors, and devices pull them low to transmit. This two-wire constraint makes I²C cheap to implement on circuit boards and allows multiple devices (slaves) to be addressed by a single master controller.
The protocol uses 7-bit or 10-bit addressing, with data transmitted in 8-bit frames, one bit per clock cycle. Standard mode runs at 100 kHz, Fast mode at 400 kHz, and Fast Plus at 1 MHz; higher speeds exist but require shorter traces. The slave being addressed acknowledges receipt of each byte by pulling SDA low during the ninth clock bit. Communication is half-duplex: one device transmits while others listen.
I²C sits everywhere in industrial and consumer electronics. Temperature sensors, EEPROM memory chips, real-time clocks, IO expanders, and display drivers all commonly use I²C. A single master (often a microcontroller) can talk to dozens of slaves on the same pair of wires, making it ideal for distributed sensing and control where minimizing wiring is valuable.
Common failure modes and design traps
Pull-up resistor choice is critical: too weak and edges are slow, too strong and current draw climbs. Typical values are 4.7 kΩ to 10 kΩ for standard mode, lower for faster speeds. Long runs (over 1 metre) suffer capacitance buildup that slows rise time and causes bit errors. Adding buffering, intermediate pull-ups, or switching to CAN (which uses differential signalling) solves this on longer buses. Address collisions occur when two devices have the same factory-programmed address; some chips offer jumper-selectable address bits to avoid this.
I²C remains dominant in industrial control because it is simple to debug, requires no special connectors, and most microcontroller vendors provide hardware I²C controllers. Software-only (bit-banged) I²C is common in legacy systems and works but is less reliable. For systems demanding higher speed or longer distance, SPI or CAN are preferred, but neither can address multiple slaves as cheaply from a wiring perspective.