/*
 * Use the timers pwm to dim a light on demand.
 * The light will change continously until you
 * press the button to stop.
 */
#define F_CPU 16000000UL
#define __AVR_ATmega328P__
#include <avr/interrupt.h>
#include <avr/io.h>
#include <util/delay.h>

#define PWM_LOW 0
#define PWM_HIGH 96
#define DELAY 25

void change();

typedef enum {
  UP,
  STOP,
  DOWN
} State;
State state = UP;
int brightness = PWM_HIGH;

// code for INT0
ISR(INT0_vect) {
  PORTB |= (1 << PORTB5);
  state = UP;
}

// code for INT1
ISR(INT1_vect) {
  PORTB &= ~(1 << PORTB5);
  state = STOP;
}

int main(void) {

  // read from D2 and D3, write to D6
  DDRD |= (1 << PORTD6);
  DDRB = 0xff; // used for internal led only

  PORTD |= (1 << PORTD2) | (1 << PORTD3); // pull up D2 and D3

  // enable external interruprs
  EIMSK |= (1 << INT0);
  EIMSK |= (1 << INT1);

  // trigger when anything changes
  EICRA |= (1 << ISC00); // for INT0
  EICRA |= (1 << ISC10); // for INT1

  // Timer 0
  // Mode: Fast PWM
  // Prescaler: /8
  // max: dynamic depending on pushed button
  TCCR0A |= (1 << WGM01) | (1 << WGM00); // Fast PWM Mode
  OCR0A = brightness;                    // max value for timer
  TCCR0A |= (1 << COM0A1); // set non inverting mode, start initialized

  TCCR0B |= (1 << CS02) | (1 << CS00); // enable with prescaler 1024

  sei(); // magic interrupt macro
  while (1) {
    change();
    _delay_ms(DELAY);
  }
}

void change() {
  // start looping through brightnesses
  if (state == UP) {
    OCR0A = brightness++;
    if (brightness >= PWM_HIGH) {
      state = DOWN;
    }
  }
  if (state == DOWN) {
    OCR0A = brightness--;
    if (brightness <= PWM_LOW) {
      state = UP;
    }
  }
}