gistfile1.c

#include <avr/io.h>
#include <avr/interrupt.h>

volatile uint8_t rwflags = 0b00000001;
volatile uint8_t bit = 0;

int main(void)
{
	volatile uint8_t byte = 0b10101010;
	DDRD |= (1 << 0) | (1 << 1); //portd0 portd1 output
	PORTD |= (1 << 0); //portd0 high
	PORTD &= ~(1 << 1); //ensure portd1 low
	DDRB &= ~(1 << 0); //ensure portb0 (pcint0) input
	PORTB |= (1 << 0); //portb0 (pcint0) pull-up resistor
	cli(); // disable interrupts for prescaler set
	CLKPR = (1 << CLKPCE); // set prescaler 1x
	CLKPR = 0;
	sei();
	TCCR0A |= (1 << WGM01); // timer0 ctc ocr0a
	OCR0A = 157; //5 ms
	//TCCR0B |= (1 << CS02); // /256 timer prescale
	PCMSK0 = (1 << PCINT0); //set pcint mask to pin pcint0 only (db0)
	PCICR |= (1 << PCIE0); //enable pcint0 interrupts
	TCCR1B |= (1 << WGM12); // timer1 ctc ocr1a
	OCR1A = 62500; // 0.5 seconds
	TCCR1B |= (1 << CS11) | (1 << CS10); // /64 timer prescale
	for (;;) {
		if (TIFR1 & (1 << OCF1A)) {
			if (rwflags & (1 << 0)) {
				if (byte & (1 << bit)) { PORTD |= (1 << 1); } else { PORTD &= ~(1 << 1); };
				// set portd1 to bit in byte
				if (bit == 7) { 
					rwflags &= ~(1 << 0); //stop writing
					
				} else { bit++; }; //next bit
			}
			PORTD ^= (1 << 0); //flip portd0
			if (!(rwflags & (1 << 0))) { DDRD &= ~(1 << 0); }; //disable clock
			TIFR1 = (1 << OCF1A);
		}
	}
}

ISR(PCINT0_vect) {
	// use timer0 to see if pin state is same after 5 ms
	uint8_t state = (PINB & (1 << 0));
	PCIFR |= (1 << PCIF0); //clear interrupt flag
	TIFR0 |= (1 << OCF0A); //clear ctc match flag
	TCNT0 = 0;
	TCCR0B |= (1 << CS02); // /256 clock prescale
	while (!(TIFR0 & (1 << OCF0A))) {};
	if ((PINB & (1 << 0)) == (state & (1 << 0))) { //if still same state
		rwflags |= (1 << 0); //enable another round
		PORTD &= ~(1 << 1); //reset data pin
		DDRD |= (1 << 0); //enable clock pin
		bit = 0; //reset data bit
		TCCR0B &= ~(1 << CS02); //stop timer0
	}
}