void init_port(void);
void init_timer(void);

void setup() {
  // put your setup code here, to run once:
  volatile long i = 0;
  volatile long j = 0;
  init_port();
  init_timer();
  Serial.begin(9600);
  while(1){
    for(i = 0; i < 500000; i++);

    // Serial.println(j, OCT);
    // Serial.print(j);
    Serial.println(j);

    // NOTE:
    // For the Serial.println(); function, if we havn't mentioned the print"ln", it will
    // just print the character continously. To print in new line, we have to mention,
    // println.
    // Also, in the arguments of println(); we can see that, we have mentioned HEX, which
    // means, in the Serial monitor, we will be having the output in "Hexadecimal" format.
    // We can also have our output in octal form by mentioning "OCT" as the argument after "j".
    // If we havn't mentioned any extra argument, it means we are taking decimal, but we
    // can explicitly mention "DEC" for decimal.

    j++;
  }
}

void init_port(void){
  volatile char *dirf = nullptr;
  dirf = 0x30;
  *dirf = 0x01;
}

void init_timer(void){
  volatile short *Timer1_TCNT1 = 0x84; // This is a 16 - bit register.
                                       // Here, we have gave the address
                                       // of the Lower Byte.
                                       // TCNT1H AND TCNT1L ARE, "0x85" AND "0x84".
  volatile char *Timer1_TCCR1A = 0x80;
  volatile char *Timer1_TCCR1B = 0x81;
  volatile short *Timer1_OCR1A = 0x88; // This is a 16 - bit register.
                                       // Here, we have gave the address
                                       // of the Lower Byte.
                                       // OCR1AH AND OCR1AL ARE, "0x89" AND "0x88".
  volatile char *Timer1_TIMSK1 = 0x6f;

  *Timer1_TCNT1 = 0;
  *Timer1_TCCR1A = 0;
  *Timer1_TCCR1B = 0;

  // NOTE:
  // Here, we have used TCCR1B register instead of TCCR1A register,
  // Because, we need to set up the CTC (mode of operation) and 256 - prescaler (clock selection).
  // In TCCR1A register, we can perform certain operations such as, set / clear / toggle on the 
  // certain port pin based on the direction of the port.

  *Timer1_OCR1A = 50000; // TCNT1 register value increases till 50000 and 
                         // after reaching this, the compare register will be cleared.
                         // We can see in the below, we have set the timer1 in CTC mode.
                         
  *Timer1_TCCR1B = 0x0c; // CTC mode and 256 - prescaler

  *Timer1_TIMSK1 = 0x02; // This register is used to mask the 
                         // output compare register that we need 
                         // to use for comparison purpose to enable
                         // the corresponding interrupt
}

ISR(TIMER1_COMPA_vect){
  volatile char *outf = nullptr;
  outf = 0x31;
  *outf ^= 0x01;
}

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

}