RFID/NFC Readers: MFRC522 & PN532

Working Principle

The MFRC522 is a 13.56 MHz contactless reader/writer IC from NXP. It communicates with MIFARE-compatible RFID tags using ISO 14443A protocol. The reader generates a 13.56 MHz electromagnetic field via its antenna coil. When a passive tag enters this field, it draws power inductively and responds by modulating the field (load modulation). The MFRC522 demodulates this response to read tag data, UIDs, and memory sectors.

Electrical Characteristics

Interfacing with an MCU

The RC522 module uses SPI by default: connect SDA (SS), SCK, MOSI, MISO, RST, and GND. The module operates at 3.3 V — do not connect VCC to 5 V. Some modules have a 3.3 V regulator but the SPI lines are not 5 V tolerant. Use logic-level shifters with 5 V Arduinos, or use 3.3 V boards (ESP32, STM32).

Calibration

The MFRC522 is factory-calibrated for antenna tuning. For maximum read range, ensure the antenna coil has clear space (no metal within 20 mm). Adjust the RxGain register (default 0x48) to increase receiver sensitivity. The TModeReg and TPrescalerReg control timeout for slow-responding tags.

Code Example

// MFRC522 RFID — read UID and authenticate on Arduino
#include <SPI.h>
#include <MFRC522.h>

#define SS_PIN   10
#define RST_PIN   9

MFRC522 rfid(SS_PIN, RST_PIN);
MFRC522::MIFARE_Key key;

void setup() {
    Serial.begin(115200);
    SPI.begin();
    rfid.PCD_Init();

    // Default MIFARE key (factory default = 0xFF x 6)
    for (byte i = 0; i < 6; i++) key.keyByte[i] = 0xFF;

    Serial.println("MFRC522 RFID Ready");
    Serial.print("Firmware: 0x");
    Serial.println(rfid.PCD_ReadRegister(rfid.VersionReg), HEX);
}

void loop() {
    if (!rfid.PICC_IsNewCardPresent() || !rfid.PICC_ReadCardSerial())
        return;

    // Print UID
    Serial.print("UID: ");
    for (byte i = 0; i < rfid.uid.size; i++) {
        Serial.print(rfid.uid.uidByte[i] < 0x10 ? " 0" : " ");
        Serial.print(rfid.uid.uidByte[i], HEX);
    }
    Serial.println();

    // Print tag type
    Serial.print("Type: ");
    Serial.println(rfid.PICC_GetTypeName(rfid.PICC_GetType(rfid.uid.sak)));

    // Authenticate and read block 4
    byte block = 4;
    byte buffer[18];
    byte size = sizeof(buffer);
    MFRC522::StatusCode status;

    status = rfid.PCD_Authenticate(MFRC522::PICC_CMD_MF_AUTH_KEY_A, block, &key, &(rfid.uid));
    if (status == MFRC522::STATUS_OK) {
        status = rfid.MIFARE_Read(block, buffer, &size);
        if (status == MFRC522::STATUS_OK) {
            Serial.print("Block 4: ");
            for (byte i = 0; i < 16; i++) {
                Serial.print(buffer[i] < 0x10 ? " 0" : " ");
                Serial.print(buffer[i], HEX);
            }
            Serial.println();
        }
    }

    rfid.PICC_HaltA();
    rfid.PCD_StopCrypto1();
    delay(1000);
}

Real-World Applications

PN532: Full NFC Module

The NXP PN532 is a full-featured NFC controller that supports everything the MFRC522 does — plus much more. It implements the complete NFC specification with three operating modes:

• Reader/Writer: Read/write ISO 14443A/B tags, MIFARE Classic/Ultralight, FeliCa, and ISO 15693 vicinity cards
• Peer-to-Peer: Exchange data between two NFC devices (Android Beam, SNEP/LLCP protocols)
• Card Emulation: The PN532 itself acts as an NFC tag — useful for mobile payment terminals and access systems

Interfaces: The PN532 supports I2C, SPI, and HSU (High-Speed UART). Most breakout boards (Adafruit, Elechouse) use DIP switches to select the interface. I2C is the most common choice for Arduino projects (address 0x24).

// PN532 NFC — read ISO 14443A tag UID via I2C
#include <Wire.h>
#include <Adafruit_PN532.h>

#define PN532_IRQ   2
#define PN532_RESET 3

Adafruit_PN532 nfc(PN532_IRQ, PN532_RESET);  // I2C mode

void setup() {
    Serial.begin(115200);
    nfc.begin();

    uint32_t versiondata = nfc.getFirmwareVersion();
    if (!versiondata) {
        Serial.println("PN532 not found");
        while (1);
    }
    Serial.print("PN532 Firmware: ");
    Serial.print((versiondata >> 16) & 0xFF);
    Serial.print('.');
    Serial.println((versiondata >> 8) & 0xFF);

    nfc.SAMConfig();  // Configure Secure Access Module
    Serial.println("Waiting for NFC tag...");
}

void loop() {
    uint8_t uid[7];
    uint8_t uidLength;

    if (nfc.readPassiveTargetID(PN532_MIFARE_ISO14443A, uid, &uidLength)) {
        Serial.print("UID (");
        Serial.print(uidLength);
        Serial.print(" bytes): ");
        for (uint8_t i = 0; i < uidLength; i++) {
            Serial.print(uid[i] < 0x10 ? " 0" : " ");
            Serial.print(uid[i], HEX);
        }
        Serial.println();
        delay(1000);
    }
}

MFRC522 vs PN532 Comparison

Feature	MFRC522	PN532
Protocols	ISO 14443A only	ISO 14443A/B, FeliCa, ISO 15693
NFC Modes	Reader/Writer only	Reader/Writer + P2P + Card Emulation
Interfaces	SPI, I2C, UART	SPI, I2C, HSU (High-Speed UART)
Supply Voltage	2.5–3.3 V	3.3–5.5 V (on-board regulator)
Read Range	~5 cm	~5–7 cm (larger antenna)
7-Byte UID	Supported	Supported
Android/Phone NFC	✗	✓ (P2P + HCE)
Typical Price	~$1–2	~$4–7

Limitations

• Short range: 13.56 MHz RFID is limited to ~5–7 cm. UHF RFID (900 MHz) provides metres of range but requires different hardware entirely.
• Metal interference: Metallic surfaces near the antenna detune it and drastically reduce read range. Use ferrite shielding.
• Security: Default MIFARE Classic keys (0xFF) are well-known. Always change authentication keys for security-sensitive applications.
• Multi-tag collisions: Both readers handle anti-collision, but throughput drops with simultaneous tags.
• MFRC522 protocol limits: No ISO 14443B, FeliCa, or NFC P2P — upgrade to PN532 for these capabilities.