volatile uint16_t ovf = 0;
volatile uint16_t ovf3 = 0;

ISR(TIMER3_OVF_vect)
{
  //ovf3++;

  ovf++;

  //digitalWrite(13, !digitalRead(13));

}


void startTimer1 ()
{
  cli();

  // Timer 1
  TCCR1A = 0;
  TCCR1B = 0;

  TCNT1 = 0; // reset counter
  TIFR1 |= bit (TOV1);  // clear overflow flag
  //TIFR1 = 0;
  //OCR1A = 65535;
  //OCR1B = 0;

  // start Timer 3
  //TCCR3B =  bit (CS10) | bit (CS12);  //  prescaler of 1024
  //TCCR1B =  bit (WGM12) | bit (CS10);  //  CTC, no prescaler
  //TCCR1B =  bit (CS10);  //  no prescaler

  // Timer3

  TCCR3A = 0;
  TCCR3B = 0;

  TCNT3 = 0; // reset counter
  TIFR3 |= bit (TOV3);  // clear overflow flag

  
  //TIFR1 = 0;
  //OCR3A = 15625;
  //OCR1B = 0;

  // start Timer 1
  //TCCR3B =  bit (CS30) | bit (CS32);  //  prescaler of 1024
  //TCCR3B =  bit (WGM12) | bit (CS10);  //  CTC, no prescaler
  //TCCR3B =  bit (CS30);  //  no prescaler

//  TCCR3B =  bit (CS30) | bit (CS32);  //  prescaler of 1024

  TIMSK3 |= bit(TOIE3); // | bit(OCIE3);

  TCCR3B =  bit (CS10) ;  //  no prescaler
  TCCR1B =  bit (CS10) ;  //  no prescaler

  sei();
}

uint32_t getTimer1Reading ()
{
  uint32_t elapsed = TCNT1;
  if (TIFR1 &  bit (TOV1))
    elapsed += 65536;
  return elapsed;
}

void setup ()
{
  Serial.begin (1000000);
  Serial.print (F("Frequency: "));
  Serial.print (F_CPU);
  Serial.println (F("Hz"));

  Serial.println ("Cycles, ns");
}


void loop ()
{

  ovf = 0;

  uint32_t us = micros();

  //cli();


  startTimer1 ();

  //delay (7560);

  //delay(10);


  //delayMicroseconds(1);

  //  int i = 10000;
  //  while (i--)
  //  {
  //    _NOP();
  //  }
  // 16 x NOP 102 cycles
  // 100 x NOP 606 cycles
  // 160 x NOP 966 cycles
  // 1000 x NOP 6006 cycles
  // 1600 x NOP 9606 cycles
  // +6 per loop - 6, above some number of loops

  // while (1)
  // {
  //   _NOP();
  //   if (!(--i))
  //     break;
  // }

  // -5 per loop and -2 cycle correction for real ATmega2560!


  volatile uint32_t i = 1000000;
  while (1)
  {
    //_NOP();

    /*
        if (TIFR1 &  bit (TOV1))
        {
          TIFR1 |= bit (TOV1);  // clear overflow flag

          //TIFR1 = 0;
          ovf++;
        }
    */

    if (!(--i))
      break;
  }




  volatile uint16_t cy = TCNT1;

  //sei();

  us = micros() - us;

  volatile uint32_t cy_total = (int32_t)ovf << 16;
  cy_total += cy;


  // correction on overflow
  //if (cy == 65535 && ovf)
  //cy_total -= 1;

  uint32_t ns10 = cy_total * 625;

  Serial.print (ovf);
  Serial.write (' ');

  Serial.print (cy);
  Serial.write (' ');

  Serial.print (ns10 / 10);
  Serial.write ('.');
  Serial.print (ns10 % 10);

  Serial.write (' ');
  Serial.print (us);

  Serial.println ();
  Serial.flush ();

  delay(1000);
}