int intensidad = 9;
int i=0;
int val=0;
int flag_up=1;
int flag_down=0;
unsigned long time=0;

void setup() 
{ 
  Serial.begin(19200); 
  delay(100); 
  analogWrite(9,0);
  time=millis();
  i=0;
} 

void loop() 
{ 
 
 if (Serial.available()) { 
   val=Serial.read();
 }
 
//if (val==0x01) analogWrite(9,255);
//else analogWrite(9,0);

 if ((0x01==(val&0x01))&&((millis()-time)>=5)&&(flag_up))
 {
   analogWrite(9,i);
   i=i+1;
   if (i==256)
   {
     i=254;
     flag_up=0;
     flag_down=1;
   }
   time=millis();
 }
 else if ((0x01==(val&0x01))&&((millis()-time)>=5)&&(flag_down))
 {
   analogWrite(9,i);
   i=i-1;
   if (i==0)
   {
     i=0;
     flag_up=1;
     flag_down=0;
   }
   time=millis();
 }
 
else if (0x00==(val&0x01))
        {
          analogWrite(9,0);
          i=0;
        }
}