The Two-wire serial Interface (TWI) is compatible with Philips' I2C protocol. The bus was developed to allow simple, robust, and cost effective communication between integrated circuits in electronics. The strengths of the TWI bus includes the capability of addressing up to 128 devices on the same bus, arbitration, and the possibility to have multiple masters on the bus.
The Universal Serial Interface (USI) module on devices such as Atmel® ATmega169, ATtiny26, and ATtiny2313, has a dedicated two-wire mode. The USI provides the basic hardware resources needed for synchronous serial communication. Combined with a minimum of control software, the USI allows higher transfer rates and uses less code space than solutions based on software only. Interrupts are included to minimize the processor load.
This document describes how to use the USI for TWI slave communication. Source code for communication drivers for transmission and reception is provided. The code is complete with both data buffer handling and combined transmitter and receiver.