void setup()
{
}

void loop()
{
  uint32_t now = millis();
  static uint32_t past = now;

  if ( now - past >= 5 )
  {
    past = now;

    static uint16_t frequency = 440;

    tone( 12, frequency );

    if ( ( frequency += 25 ) >= 1000 )
    {
      frequency = 440;
    }
  }
}