esp32 chapa inteligente con rfid

/*
 * RFID_Chapa.cpp
 *
 *  Created on: 08/06/2019
 *      Author: Jhonathan
 */

#include <Arduino.h>
#include <SPI.h>
#include <MFRC522.h>

#define SS_PIN 10
#define RST_PIN 9

MFRC522 mfrc522(SS_PIN, RST_PIN);

void setup()
{
    Serial.begin(9600);
    while (!Serial);

    SPI.begin();
    mfrc522.PCD_Init();
    Serial.println("RFID Chapero");
}

void loop()
{
    if (mfrc522.PICC_IsNewCardPresent() && mfrc522.PICC_ReadCardSerial())
    {
        Serial.print("UID tag: ");
        String content = "";
        byte letter;
        for (byte i = 0; i < mfrc522.uid.size; i++)
        {
            Serial.print(m