Contador de segundos

  #include "TimerOne.h"
  byte hora = 0;
  int hora1 = 0;
  byte minu = 0;
  int minu1 = 0;
  byte segu = 0;
  int segu1 = 0;
  unsigned long tempo = 0;

  //---------------------------------
  void setup()
  {
    Serial.begin(9600);
    pinMode(13, OUTPUT);
    Timer1.initialize(1000000);         // initialize timer1, and set a 1 second period
    Timer1.attachInterrupt(callback);  // attaches callback() as a timer overflow interrupt
  }
  //--------------------------------------
  void callback()
  {
    digitalWrite(13, digitalRead(13) ^ 1);
    tempo++;
  }
  //--------------------------------------
  void loop()
  {
    hora = tempo / 3600;
    hora1 = hora * 3600;
    minu1 = tempo - hora1;
    minu = minu1/60;
    segu1 = minu*60 + hora1;
    segu = tempo - segu1;
    Serial.print(hora);
    Serial.print(":");
    Serial.print(minu);
    Serial.print(":");
    Serial.println(segu);
  }