int num = 0; // count value
void setup() {
  // put your setup code here, to run once:
  pinMode(4,OUTPUT); //four digit BCD output
  pinMode(5,OUTPUT);
  pinMode(6,OUTPUT);
  pinMode(7,OUTPUT);
  
  pinMode(9,OUTPUT); // RBI pin
  attachInterrupt(digitalPinToInterrupt(2), resetNum, FALLING); //Interrupt initialization
//  Here the interrupt is enabled on pin-2, the ISR function assigned is resetNum and 
//  mode is FALLING means when a falling edge occurs on pin-2, the interrupt is counted.
  
  digitalWrite(9,HIGH); //Set RBI to HIGH to turn ON the display
}

void loop() {
  
  digitalWrite(4,bitRead(num,0)); //display 4 BCD digits of number
  digitalWrite(5,bitRead(num,1));
  digitalWrite(6,bitRead(num,2));
  digitalWrite(7,bitRead(num,3));

  delay(1000); //1 Second Delay

  digitalWrite(9, LOW); //Turn OFF seven-segment display for blinking
  delay(500);           // 500 ms delay
  digitalWrite(9, HIGH); //Again turn ON the display
  
  num = num + 1; //Count next
  if(num == 10) //If count exceeds 9, reset to 0
  {
    num = 0;
  }
}

void resetNum() { //Interrupt ISR function; Called when interrupt occurs
  
  num = 0; //Reset count to zero
    
  digitalWrite(4,bitRead(num,0)); //display reset value on display
  digitalWrite(5,bitRead(num,1));
  digitalWrite(6,bitRead(num,2));
  digitalWrite(7,bitRead(num,3));
}