STM32F103 I2C (Inter-Integrated Circuit) or sometimes called TWI (Two Wire Interface) is a synchronous serial protocol that only needs 2 wires for communication. The wires are SCL for clock line and SDA for data line. With I2C, you can connect devices like temperature sensor, EEPROM, RTC, and etc (up to 112 devices) just using 2 wires (plus GND wire). For accessing these devices, I2C use 7-bit address. Theoretically, 7-bit address space allows 128 addresses, however some addresses are reserved for special purpose. Thus, only 112 addresses can be used.