/*
 * TIMERwithIRQ.c
 *
 * Created: 5.6.2014 15:27:41
 *  Author: maticpi
 */ 
#include <avr/io.h>
#include <avr/interrupt.h>

void Init();

volatile long v_systick;	//system tick (counter of milliseconds)
long systick;				//non-volatile copy of the system tick (safe to use outside of interrupts)

int main(void)
{
	Init();
	sei();
	
	while(1)
	{
		cli();		//disable interrupts to prevent any interruptions to the following instruction
		systick=v_systick;	//safely copy the system tick to its non-volatile copy
		sei();		//re-enable interrupts
		PORTB=systick/1000;	//display the system tick (in seconds) on PORTB
	}
}

ISR(TIMER0_COMPA_vect)	//every ms increment the system tick
{
	v_systick++;
}

void Init()
{
	//IO port
	PORTA=0x08; //0000 1000
	PORTB=0x00; //0000 0000
	PORTC=0x3E; //0011 1110
	PORTD=0x7D; //0111 1101
	//1-output, 0-input
	DDRA=0xF0; //1111 0000
	DDRB=0xFF; //1111 1111
	DDRC=0xC1; //1100 0001
	DDRD=0x82; //1000 0010
	
	//Init timer0
	TCCR0A=(1<<WGM01);	//Output Compare A and B disconnected (normal port operation), Timer mode CTC
	TCCR0B=(1<<CS02);	//Select prescaler 1:256 (18432000/256=72000) 
	OCR0A=71;			//Set top counter value to 71 (0 to 71 equals 72 steps or 1ms at 72kHz timer clock)
	TIMSK0=(1<<OCIE0A);	//Enable Output compare A interrupt
}