Software ("bit-bang") UART Transmitter ( 8 data bits, 1 stop bit, no parity ) for Attiny24A this can easily be ported to other AVR8 µC's or might even work without changes on many of them
the Baud rate is calculated as follows: BAUD = F_CPU / ( TIMER0_PRESCALER * (OCR0A + 1) so we can modify the prescaler and/or the OCR0A value to achieve a certain baud rate.
In this example I am using the internal 8MHz oscillator as clock source, so F_CPU=8000000 and a baud rate of 9600.
The program can be compiled with avr-gcc and the avr-libc libraries.