twi_demo(3avr) on Linux
The text baca5f0ab433, as Debian 8 (Jessie) shipped it.
Some newer devices of the ATmega series contain builtin support for interfacing the microcontroller to a two-wire bus, called TWI. This is essentially the same called I2C by Philips, but that term is avoided in Atmel's documentation due to patenting issues. For further documentation, see: http://www.nxp.com/documents/user_manual/UM10204.pdf