Basics

The I²C (Inter-Integrated Circuit) protocol, referred to as I-squared-C, I-two-C, or IIC) is two wire serial communication protocol for connecting low speed peripherals to a micrcontroller or computer motherboard.

You can go through I²C /TWI Basics tutorials to understand how I²C works.

Registers

TWBR ( TWI Bit Rate Register )

EEDR
7 6 5 4 3 2 1 0
TWBR7 TWBR6 TWBR5 TWBR4 TWBR3 TWBR2 TWBR1 TWBR0

Bits [7:0] – Bit Rate

It selects the division factor for the bit rate generator. The bit rate generator is basically a frequency divider. It generates the SCL clock frequency in the Master modes.

TWCR

TWSR

TWDR

Practical Example

AVR Interfacing: RTC DS1307