DIY Arduino Eurorack clock divider / multiplier using interrupt

unosyncer.ino

 
// Timer library, https://github.com/dniklaus/wiring-timer, 
//   add it by using the Arduino IDE Library Manager (search for wiring-timer)
#include <Timer.h>

// LcdKeypad, https://github.com/dniklaus/arduino-display-lcdkeypad, 
//   add it by using the Arduino IDE Library Manager (search for arduino-display-lcdkeypad)
#include <LcdKeypad.h>

LcdKeypad* myLcdKeypad = 0;

const byte interruptPin = 2;
unsigned long time = NULL;
unsigned long time_1x = NULL;
unsigned long time_2x = NULL;
unsigned long time_diff = NULL;
unsigned long tickcount = 0;
unsigned long bpm = 0;

void setup() {
  // Timer0 is already used for millis() - we'll just interrupt somewhere
  // in the middle and call the "Compare A" function below
  OCR0A = 0xAF;
  TIMSK0 |= _BV(OCIE0A);

  Serial.begin(115200);
  pinMode(interruptPin, INPUT);
  pinMode(12, OUTPUT);
  pinMode(13, OUTPUT);
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(A0, INPUT);
  attachInterrupt(digitalPinToInterrupt(interruptPin), tick, FALLING);

  myLcdKeypad = new LcdKeypad();  // instantiate an object of the LcdKeypad class, using default parameters

  myLcdKeypad->setCursor(0, 0);   // position the cursor at beginning of the first line
  myLcdKeypad->setBacklight(1);

}
void tick() {
  tickcount++;
  unsigned long currentTime = millis();
  if(time != NULL)
  {
    time_diff = currentTime - time;
  }
  time_1x = time = currentTime;
  time_2x = time_1x + floor(time_diff / 2);
}

 // Interrupt is called once a millisecond, 
SIGNAL(TIMER0_COMPA_vect) 
{
  digitalWrite(12, LOW);
  digitalWrite(13, LOW);
  unsigned long currentMillis = millis();
  if(time_1x != NULL)
  {
    if(tickcount % 2 == 0){
      digitalWrite(13, HIGH);
    }
    bpm = floor(15000 / time_diff);
    //Serial.print("clone ");
    //Serial.print(30000 / time_diff);
   // Serial.print("bpm ");
  //  Serial.println(time_1x);
    time_1x = NULL;
    digitalWrite(12, HIGH);
  }
  if(time_2x != NULL && currentMillis >= time_2x) {
    //Serial.print("extra ");
    //Serial.print(30000 / time_diff);
   // Serial.print("bpm ");
  //  Serial.println(currentMillis);
    time_2x = NULL;
    digitalWrite(12, HIGH);
  }
}

void loop() {
  myLcdKeypad->setCursor(0, 0);   // position the cursor at beginning of the first line
  myLcdKeypad->print("BPM:");   // print a Value label on the first line of the display
  myLcdKeypad->setCursor(5, 0); 
  myLcdKeypad->print(bpm);   // print a Value label on the first line of the display
  
}