MFRC522 RFID Reader: Read a Tag UID with ESP32

Before you start

Board and software

Classic ESP32-WROOM-32/32E DevKit with the listed GPIOs exposed and unused. Arduino IDE 2.x; esp32 by Espressif Systems 3.3.2 (review baseline, not a latest-release claim). Select the documented board or ESP32 Dev Module for a generic WROOM board. Other families need their own pin map. Serial Monitor: 115200 baud.

Supported hardware

A documented RC522/MFRC522 board configured for SPI with 3.3 V, GND, SCK, MOSI, MISO, SS (often SDA), RST and optional IRQ. Supply and host logic must both be 3.3 V. The illustration does not prove genuine NXP silicon or a board schematic.

Prerequisites and parts

Complete IDE setup and Blink. Reader, short wires and a known ISO/IEC 14443 Type A test tag, such as MIFARE Classic. A generic RFID card/fob or phone is not automatically compatible.

Libraries

Install MFRC522 by GithubCommunity 1.4.12 from Library Manager. SPI is bundled with Arduino-ESP32 3.3.2.

Expected result

A version byte appears, then UID bytes when a compatible card is presented. Remove and present it again for another read.

Verification status

Documentation and source review only. Not compiled or tested on hardware. The approved product-style illustration identifies a family, not a verified physical pin layout or manufacturer-authenticated board. Follow the logical diagram and documentation for your actual part.

Overview

02Technical Specifications

Arduino library: MFRC522 1.4.12

03Pinout

• 3.3 V Supply ESP32 3.3 V Only a documented 3.3 V board; no 5 V.
• GND Return ESP32 GND Common ground.
• SCK SPI clock GPIO18 Explicit in SPI.begin.
• MISO Reader to ESP32 GPIO19 3.3 V signal.
• MOSI ESP32 to reader GPIO23 3.3 V signal.
• SDA / SS SPI select GPIO27 Active LOW; not I2C SDA.
• RST Reset GPIO26 Library-controlled reset.
• IRQ Optional interrupt Not connected Polling example.

04Wiring Diagram

Reader 3.3 V and common GND; SCK18, MISO19, MOSI23, SS27 and RST26. IRQ unused.

Open wiring diagram at full size (new tab)

1. 1

   Disconnect USB and verify SPI configuration and 3.3 V power requirements.

2. 2

   Connect named signals, install MFRC522 1.4.12 and upload.

3. 3

   Open Serial Monitor at 115200 and present a known compatible tag; remove it before another presentation.

Wiring and matching Arduino code

SPI reader check and tag UID

Reader 3.3 V and common GND; SCK18, MISO19, MOSI23, SS27 and RST26. IRQ unused.

#include <Arduino.h>
#include <SPI.h>
#include <MFRC522.h>
constexpr uint8_t SS_PIN = 27;
constexpr uint8_t RST_PIN = 26;
MFRC522 reader(SS_PIN, RST_PIN);
bool ready = false;
void setup() {
  Serial.begin(115200);
  if (!SPI.begin(18, 19, 23, SS_PIN)) {
    Serial.println("SPI initialization failed");
    return;
  }
  reader.PCD_Init();
  delay(4);
  byte version = reader.PCD_ReadRegister(MFRC522::VersionReg);
  Serial.print("Reader version byte: 0x");
  Serial.println(version, HEX);
  if (version == 0x00 || version == 0xFF) {
    Serial.println("No usable reader response; check power, SS and SPI wiring");
    return;
  }
  ready = true;
  Serial.println("Present a compatible Type A test card");
}
void loop() {
  if (!ready) { delay(100); return; }
  if (reader.PICC_IsNewCardPresent() && reader.PICC_ReadCardSerial()) {
    Serial.print("UID:");
    for (byte i = 0; i < reader.uid.size; ++i) {
      Serial.print(' ');
      if (reader.uid.uidByte[i] < 0x10) Serial.print('0');
      Serial.print(reader.uid.uidByte[i], HEX);
    }
    Serial.println();
    reader.PICC_HaltA();
    reader.PCD_StopCrypto1();
  }
  delay(10);
}

The version read detects a missing SPI response, not authenticity. A halted stationary card need not produce repeated lines. This reads UID only, with no authentication or protected data access.

How it works

Technical references

• NXP MFRC522 datasheet Rev. 3.9 — RF protocols, supply and host interfaces; not authenticity of the artwork.
• MFRC522 library 1.4.12 — SPI API, cleanup and security limitations.
• Arduino-ESP32 3.3.2 SPI implementation — Explicit pins and boolean begin result.