#define PERIOD 1    // control loop period [ms] - max 4194 ms !!!

boolean state = LOW;
unsigned int heater_time = 0;

void setup() {
  cli();  // disable interrupts

  pinMode(2, OUTPUT);
  digitalWrite(2, LOW);
  pinMode(A0, INPUT);
  Serial.begin(115200);
  Serial1.begin(115200); // Serial1 is connected to the custom chip

  // Configure Timer 4 to generate interrupt each PERIOD
  // (CTC mode; 8 prescaler; enable interrupt on compare match)
  TCCR4A = 0;
  TCCR4B = (1 << WGM42) | (1 << CS41);
  TCNT4  = 0;
  // output compare register
  OCR4A = (16000000 / 8) * PERIOD / 1000;
  // Output Compare A Match Interrupt Enable
  TIMSK4 = (1 << OCIE4A);

  sei();  // enable interrupts
}

void loop() {
  // nothing to do
  delay(100);
}

// Control loop goes here
ISR(TIMER4_COMPA_vect)
{
  char heater;
  ++heater_time;
  if (heater_time == 20000)  // 20s cycle
  {
    heater_time = 0;
  }

  if (heater_time < 10000)   // turn the heater on for 10s, power increased lineary
  {
    heater = heater_time / 100;
  }
  else
  {
    heater = 0;
  }

  // print analog input every 0.5 s
  if (heater_time % 500 == 0)
  {
    Serial1.write(heater);
    float temperature = (float)analogRead(A0) * 50 / 1024;
    Serial.println(temperature, 1);   // print to serial console, one decimal place
  }

}