// Increment 64 bits counter as fast as possible.
// Press the button to show the value.
// Forum: https://forum.arduino.cc/t/sum-of-float-variables-differing-by-10-8/1253425/18
// This Wokwi project: https://wokwi.com/projects/396560196465457153

uint64_t counter;

unsigned long previousMillis;
const unsigned long interval = 1000;
volatile bool flag;

void setup() 
{
  Serial.begin(115200);
  Serial.println("Press button to show counter value");
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2),showNow,FALLING);
}

void loop() 
{
  counter++;
  
  if(flag)
  {
    byte buffer[21];
    buffer[20] = '\0';
    uint64_t copy = counter;
    for(int i=19; i>=0; i--)
    {
      int digit = copy % 10;
      copy /= 10;
      buffer[i] = (byte) (digit + '0');
    }
    Serial.println((char *)buffer);
    flag = false;
  }
}

void showNow()
{
  flag = true;
}