//---------------------------------------------------------------------------------------
// Programm Nr.: 15
// Content     : sound output 1 kHz with timer0 interrupt
// Author      : digital AG Halle
// Date        : 27.07.2008
// System      : ATmega8-16PU - Dai Hoc Hue Development Board
//---------------------------------------------------------------------------------------

// programm memory usage = 3,9 %

//--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
{
	TCNT0 = reload;			//load timer0 count-register for the next run
	
	count++;				//counter in the ISR (to extent ISR-time)
	
	if (count < 10)		//condition time 1 reached
		
	    PORTB |= (1<<PB1);  //set bit 1 (PIEZO) to high-level
	else
	    PORTB &= ~(1<<PB1); //pause
	
	if (count >=20)		//condition time 2 reached: end of counter cycle
		count=0;			//reset counter
}

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

int main(void)
{

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

	DDRB  = 2; //PORTB.BIT1 output (PIEZO)
	PORTB = 2; //PIEZO start value = off (high-level (because it is between MC-output and Vcc))

	//INTERRUPTS:
		//Timer0

// target frequency = 1 kHz
// because the toggling of the output-pin in the ISR, the frequency is only 1/2 -> we need 2 kHz
// target_freq*(255-reload)=cpu_freq/prescaler
//
// 8000000/8=1000000 Hz (prescaler=8)
// 1000000/50= 20000 Hz (ISR-frequency)
// count value 1 = 10  -> 
// count value 2 = 20  -> we need 2 cycles for 1 wave -> factor 1/2 -> 1 kHz

// reload = 255-50 = 205

		reload=205;

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

		TCNT0 = reload;		//pre load timer0 count register 
					//from here the timer counts up to 255 and breaks 
					//over to 0, creating an overflow interrupt

		//**It must be reloaded in every ISR cycle or it will start the next from 0!**


		TCCR0 |= (1<<CS01); 	//(timer/counter control register)
					//set CS01 -> prescaler = 8
					//CPU-freq. = 8 MHz / 8 -> 1 MHz timer freq.

	
	//--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-------------------------------------------------------------------------