byte Ton = 0, Toff = 20;
byte Step = 1;

void setup()
{
  pinMode(9, OUTPUT);
}

void loop()
{

  for (int i = 0; i < 2500; i++)
  {
    if (Ton > 0) {
      digitalWrite(9, HIGH);
      delayMicroseconds(Ton);
    }

    if (Toff > 0) {
      digitalWrite(9, LOW);
      delayMicroseconds(Toff);
    }
  }

  Ton += Step;
  Toff -= Step;

  if (Ton > 20)
  {
    Step = -1;
    Ton = 20;
    Toff = 0;
  }
  else if (Ton == 0)
  {
    Step = 1;
    Toff = 20;
    Ton = 0;
  }

}