//---------------------------------------------------------------------------------------
// Programm Nr.: 13
// Content     : blinking LED with timer0 interrupt
// Author      : digital AG Halle
// Date        : 27.07.2008
// System      : ATmega8-16PU - Dai Hoc Hue Development Board
//---------------------------------------------------------------------------------------

//*** Look at the usage of program memory of only 3,6 % ! ***

//--included Files-----------------------------------------------------------------------

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

//--declarations-------------------------------------------------------------------------

#ifndef F_CPU
#define F_CPU 8000000	//CPU frequency = 8 MHz
#endif

uint16_t count;
uint8_t reload;

//--INTERRUPT SERVICE ROUTINES (ISR)-----------------------------------------------------

SIGNAL (TIMER0_OVF_vect)		//event: timer0 overflow 255->0
{
	
	count++;			//counter in the ISR (to extent ISR-time)
	
	if (count < 61)	        	//condition time 1 reached	
	    	PORTD |= (1<<PD0);  	//set LED
	   else
		PORTD &= ~(1<<PD0); 	//reset LED    
	
	if (count >=122)		//condition time 2 reached: end of counter cycle
	    count=0;			//reset counter
          
}

//--MAIN PROGRAMM------------------------------------------------------------------------

int main(void)
{

	//--INITIALISATION---------------------------------------------------------------

	DDRD  = 1;	// LED0 output
	PORTD = 1;	// LED0 = off

	//INTERRUPTS:
		//Timer0

// target frequency = 1 Hz
// because the toggling of the output-pin in the ISR, the frequency is only 1/2 -> we need 2 Hz
// target_freq*(255-reload)=cpu_freq/prescaler
// 
// with a prescaler of 256, the ISR_freq. is 15,625 kHz
// by using the full range of timer0 (0...255) (=256 steps), we get 61 cycles to reach the 2 Hz.
// 
// 8000000/256/256/61 = 2 

		// reload is not needed - we use the full timer0 range 0->255->0->255....

		TIMSK |= (1<<TOIE0);	//(timer/counter interrupt mask)
					//set TOIE0 (timer0 overflow interrupt)

		TCCR0 |= (1<<CS02); 	//(timer/counter control register)
					//set CS02 -> prescaler = 256

	
	//--PROGRAMM CODE BEFORE MAIN LOOP (USED JUST ONE TIME)--------------------------
		
		sei();  //enable flag for all interrupts

	//--MAIN LOOP--------------------------------------------------------------------
	
	while(1)
	{
	}
	return 0;
}

//--MAIN PROGRAMM END--------------------------------------------------------------------

//--EXTERNAL ROUTINES--------------------------------------------------------------------

//--PROGRAMM END-------------------------------------------------------------------------