Fully interrupt driven soft-uart receiver on an ATtiny24

By accident I stumbled across my 'system-ticker' interrupt today. As it happens it conveniently ticks at 1MHz! One tick of timer0 is one microsecond. Microsecond timing is exactly what is needed for receiving serial data.

The sampling interrupt (TIM0_COMPA_vect) is now invoked at the right time by setting OCR0A = TCNT0 + FULL_BIT_DELAY.