//
// Simple divide-by-10,000,000 program for ATtiny13
//
// QRP Labs, 2016
//
#include <avr/io.h>
#include <avr/interrupt.h>

static uint16_t counter;

int main(void)
{
	DDRB = (1<<PB4);				// Enable pin 3 of ATtiny13 as output
	TCCR0B = (1<<CS00);				// Start counter, prescale = 1
	TIMSK0 = (1<<OCIE0A);			// Switch on output compare A interrupt
	
									// A rising edge on INT0 (pin 6) generates an interrupt							
	MCUCR |= (1<<ISC00) | (1<<ISC01);
	GIMSK = (1<<INT0);				// Enable interrupt on INT0
	
	sei();							// Enable interrupts
	
	for (;;);
	
	return 0;
}
									// Interrupt handler for output compare A interrupt
SIGNAL(TIM0_COMPA_vect)
{
									// Switch on output pulse at 0
	if (counter == 39062) PORTB = (1<<PB4);	
	
	counter++;						// Increment the counter
	
	if (counter == 3907)			// Switch off output pulse at 3907
		PORTB = 0;
	else if (counter == 39062)
		OCR0A ^= 0x80;				// 10,000,000 / 256 = 39062.5 - so change this cycle to divide-by-128
	
	else if (counter == 39063)
	{								// Next interrupt is 128 clock cycles later instead of 256
		counter = 0;				// Reset counter to 0
		OCR0A = 0;					// Reset OCR0A to zero (divide-by-256 again)
	}
}

SIGNAL(INT0_vect)
{
	TCNT0 = 0;						// Set timer counter to 0
	counter = 0;					// set overflow counter to 0
	OCR0A = 0;						// Make sure OCR0A is zero (to ensure divide-by-256)
	PORTB = (1<<PB4);				// Switch on output pulse
}