/* Example code with timer intyerrutp that will create an interruption each 
 *  500ms using timer1 and prescalar of 256.
Calculations (for 500ms): 
  System clock 16 Mhz and Prescalar 256;
  Timer 1 speed = 16Mhz/256 = 62.5 Khz    
  Pulse time = 1/62.5 Khz =  16us  
  Count up to = 500ms / 16us = 31250 (so this is the value the OCR register should have)*/  
bool LED_STATE = true;

#define M_MS 60000 // ms/us

void setTimer(int bpm) {
  unsigned int ocr;
  ocr = M_MS * 1000 / bpm / 32;
  Serial.println(bpm);
  Serial.println(M_MS/bpm);
  Serial.println(ocr);
  stopTimer();
 
  /*2. We set the prescalar to the desired value by changing the CS10 CS12 and CS12 bits. */  
  TCCR1B |= B00000100;        //Set CS12 to 1 so we get prescalar 256  
  
  /*3. We enable compare match mode on register A*/
  TIMSK1 |= B00000010;        //Set OCIE1A to 1 so we enable compare match A 
  
  /*4. Set the value of register A to 31250*/
  OCR1A = ocr;             //Finally we set compare register A to this value  
  sei();                     //Enable back the interrupts
}

void stopTimer() {
  LED_STATE = true;
  digitalWrite(13,LED_STATE);  //Write new state to the LED on pin D5
  cli();                      //stop interrupts for till we make the settings
  /*1. First we reset the control register to amke sure we start with everything disabled.*/
  TCCR1A = 0;                 // Reset entire TCCR1A to 0 
  TCCR1B = 0;                 // Reset entire TCCR1B to 0
}

void setup() {
  Serial.begin(9600);
  pinMode(13, OUTPUT);        //Set the pin to be OUTPUT
  // stopTimer();
  setTimer(120);
}

void loop() {
  // put your main code here, to run repeatedly:
}

//With the settings above, this IRS will trigger each 500ms.
ISR(TIMER1_COMPA_vect){
  TCNT1  = 0;                  //First, set the timer back to 0 so it resets for next interrupt
  LED_STATE = !LED_STATE;      //Invert LED state
  digitalWrite(13,LED_STATE);  //Write new state to the LED on pin D5
}