// Forum: https://forum.arduino.cc/t/10-digit-down-counter/1083027
//

#define START_VALUE 9999999999ULL
uint64_t counter = START_VALUE;

void setup()
{
  Serial.begin(115200);
}

void loop()
{
  Print64(counter);
  
  if( counter == 0ULL)
    counter = START_VALUE;
  else
    counter--;

  delay(10);
}

void Print64(uint64_t x)
{
  char buffer[20];
  snprintf(buffer,sizeof(buffer),"%lld",x);
  Serial.println(buffer);
}