const byte servoPin = 13; 
byte dutyCycle = 0;

const byte freq = 50, chan = 0, res = 8;

void setup() {  
  Serial.begin(9600);
  ledcSetup(chan, freq, res);
  ledcAttachPin(servoPin, chan);
  ledcWrite(chan, dutyCycle);
}

void loop() {
  for (dutyCycle = 5; dutyCycle <= 32; dutyCycle++) {
    ledcWrite(chan, dutyCycle);
    delay(60);    
  }
  for (dutyCycle = 32; dutyCycle >= 5; dutyCycle--) {
    ledcWrite(chan, dutyCycle);
    delay(60);    
  }
}