/*
 * Use the timer to produce a sound in a speaker.
 *
 * Formular for the frequency:
 * (F_CPU/Prescaler)/(wanted_frequency*2)+1
 *
 * Cut off the decimal part and see which is
 * closer to the wanted frequency.
 */
#define F_CPU 16000000UL
#define __AVR_ATmega328P__
#include <avr/interrupt.h>
#include <avr/io.h>
#include <util/delay.h>

#define MINUTE 60 * 1000
const int TIME = 45 * MINUTE;

ISR(TIMER0_COMPA_vect) { PORTB ^= (1 << PORTB5); }

int main(void) {

  DDRB = 0xff; // write to B
  DDRD = 0;    // read from D

  // Timer 0
  // Mode: Overflow, Interrupt
  // Prescaler: /256
  // max: 70
  TCCR0A |= (1 << WGM01);  // CTC Mode
  OCR0A = 70;              // max value for timer
  TIMSK0 |= (1 << OCIE0A); // enable interrupt ISR_COMPA_vect

  sei(); // magic interrupt macro

  TCCR0B |= (1 << CS02); // enable with prescaler 256
  while (1) {
  }
}