I2C
A two-wire serial bus that lets a microcontroller talk to several sensors and peripherals over shared clock and data lines, each identified by an address.
I2C (Inter-Integrated Circuit) connects a controller to many peripheral chips using only two signals, a clock and a bidirectional data line, with pull-up resistors on each. Every peripheral has a 7-bit address, so a temperature sensor, an IMU, a real-time clock and an EEPROM can share the same pair of pins. That economy of wiring is why most small digital sensors offer an I2C interface and why it is the default bus on development boards.
Its limits follow from the same design. Speed is modest, the bus is meant for short distances on a single board, and two parts with the same fixed address cannot coexist without a multiplexer. When a peripheral needs high throughput, such as a display or a flash chip, designers switch to SPI, which uses more pins but runs much faster.
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