const byte led = 9;
byte x, y = 0;
void setup() {
}

void loop() {
  if (y == 0) {
    x = x + 1;
    analogWrite(led, x);
    delay(10);
   } 
    {
    if (x == 255)
    }
   { 
    y = y - 1;
    analogWrite(led, y);
    delay(10);
   
   }
}