This project puts the Elektor ATM18 AVR board to use as a tester for the Inter-Integrated Circuit bus. This two-wire bus goes by various names, including IIC, I2C, and (at Atmel) TWI (for ‘two-wire interface’). As this bus is very widely used, many microcontrollers have a built-in interface for it. The tool described here can be used to perform quick, simple tests on I2C ICs and modules.