/*
fcpu = 16MHz
generovana frekvence = 4kHz
*/

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

#define port_pipaku PORTB
#define ddr_pipaku DDRB
#define pipak PB5

void tim0_start(void);

int main(void)
{
	//----------------------------------------------------- mcu init
	ddr_pipaku |= 1<<pipak;	//nastavi pin jako vystup

	tim0_start();		//inicializace citace

	sei();				//povolit global. prer.

	for(;;);
	return 0;
}		

void tim0_start(void)
{
	TCNT0 = 48;				//prednastaveni viz nize
	TCCR0 |= 1<<CS00;		//16MHz, 8bit, bez delicky -> f=62.5kHz, T=16us
	TIMSK |= 1<<TOIE0;		//poreruseni T0 overflow
}

ISR(TIMER0_OVF_vect)
{
	/*
	4kHz -> T=250us, zmena pinu kyzdych 125us
	perioda citace 16us -> citac se musi otocit 7.8125 krat
	tedy 7 x bez prednastaveni a 1x s prednastavenim 1-0.8125 z rozsahu citace
	tj. (1-0.8125)*256 = 48
	*/
	static unsigned char count = 0;
		//static urcuje, ze promenna zustane v pameti nezmenena i po opusteni obsluhy preruseni
	//
	if(++count > 7)
	{
		TCNT0 = 48;		//prednastaveni, ve skutecnosti by to melo byt
						//mensi cislo (doba reakce na preruseni a predchozi instrukce)
		count = 0;
		port_pipaku ^= 1<<pipak;	//xor s cislem = negace na mistech, kde je "1"
	}
}