There’s no UART on ATtiny85 so we had to write our own library that takes advantage on the built-in USI unit – the library is called USIUARTX.

This work is largely influenced by Atmel application note AVR307: Half Duplex UART Using the USI Module.

UART communication looks like this:

References

The source code available at https://bitbucket.org/tinusaur/usiuartx

Atmel application note AVR307: Half Duplex UART Using the USI Module

Discussion about “Using USI as a UART in the ATtiny 25/45/85”
http://www.avrfreaks.net/forum/using-usi-uart-attiny-254585

Discussion about “rs232 / microcontroler” (in German)
http://www.mikrocontroller.net/topic/51347

Source code that solves some issues:
newer: http://www.mikrocontroller.net/attachment/18859/USI_UART.c
older: http://www.mikrocontroller.net/attachment/18831/USI_UART.c

Simple ATtiny USI UART: http://www.technoblogy.com/show?RPY