hardware//I/O//serial bus//I2C

2025-07-16

I2C (Inter-Integrated Circuit) is a two-wire serial bus on which a controller talks to many chips on the same board, each selected by an address, and it is the usual way to read slow sensors such as a barometer, a magnetometer or a temperature chip. One wire carries the clock (SCL) and the other the data (SDA). Both are open-drain lines held high by pull-up resistors, so any device can pull a line low and none drives it high, which is what lets a dozen chips share two wires without short-circuiting each other.

Each transaction starts with a 7-bit address and a read or write bit, often followed by the number of the register to read, and every byte is answered by an acknowledge bit from the receiver, so a byte costs nine clock periods. Standard mode runs at 100 kbit/s, fast mode at 400 kbit/s and fast-mode plus at 1 Mbit/s. The overhead of addresses and acknowledgements, and the fact that only one transfer happens at a time, is why I2C suits quantities read tens of times per second and why fast inertial sensors go to SPI (the arithmetic is the bus budget of the parent note).

The classic field failure is a hung slave. A chip that resets or browns out in the middle of a byte can keep SDA low, and then every sensor on the bus goes silent at once. Drivers recover by clocking SCL until the line is released, and a robust board adds a timeout and a switch that can cut the power of the sensor rail.

A silent bus is dangerous because the software rarely says so. A driver that keeps returning the last good value turns a dead barometer into a perfectly steady altitude, a frozen value that looks like physics (sensor error model); counting failed transfers and checking that a noisy reading still moves are the cheap defences.

Its reach is the board. Capacitance grows with trace length and with each chip, the rising edges slow down, and beyond some tens of centimetres the bus needs buffers. Between boxes the job belongs to CAN or RS-485.