void setup() {
  Serial.begin(115200);
  pinMode(3, OUTPUT); 
}

void loop() {
  int c;
  c = Serial.read();
  if (c == 'P') {
    if (Serial.read() == 'C') {
      c = Serial.read();
      if (c == 'S')  {
        int val = Serial.parseInt();
        int pwm = val * 255 / 100;
        analogWrite(3, pwm);
      }
    }
  }
  delay(100);
}