The primary source for this work is the Atmel application note AVR312: Using the USI module as a I2C slave that explains how to use the built-in USI unit as I2C slave.
The source code is available at https://bitbucket.org/tinusaur/usitwix.
User the USITWIX Library
Here are some references to sources that were used while developing this library.
AVR312: Using the USI module as a I2C slave
C-code driver for TWI slave, with transmit and receive buffers; Compatible with I2C protocol; Interrupt driven, detection and transmission/reception; Wake up from all sleep mode, including Power Down.
TINY USI Interface in I2C mode and the AVR312 Appnote
What’s wrong with the AVR Appnote?
ATTiny USI I2C Introduction – A powerful, fast, and convenient communication interface for your ATTiny projects!
I2C, it’s a standard that’s been around for around 20 years and has found uses in nearly every corner of the electronics universe. It’s an incredibly useful technology for us microcontroller hobbyists but can seem daunting for new users. This tutorial will solve that problem, first by reviewing what I2C is and how it works, then by going in-depth on how to implement I2C in Atmel’s ATTiny USI (Universal Serial Interface) hardware.
I2C Bus for ATtiny and ATmega
This two wire interface is formally known as the Inter-Integrated Circuit bus, or just the I2C bus and was invented by NXP when it was still Philips Semiconductors. If you’re reading this Instructable then you’ve probably heard of the I2C bus and may even have used it on a PIC or other microcontroller. While conceptually very simple, and supported by hardware resources on the AVRs, software drivers are still necessary to use the I2C bus. Atmel provides Application Notes (see the Resources later in this Instructable), but these are incomplete and don’t show any examples beyond communicating with another AVR device.