void setup()
{
}

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

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

    static bool invert = false;
    static uint16_t frequency = 440;

    tone( 12, frequency );

    if ( ( invert == false && ++frequency == 1000 )
      || ( invert == true  && --frequency ==  440 ) )
    {
      invert = !invert;
    }
  }
}