Overview

The MFRC522 is a highly integrated 13.56 MHz contactless communication card reader/writer IC designed by NXP Semiconductors, most commonly sold on low-cost RC522 breakout modules. It provides full support for ISO/IEC 14443 A protocol standards and MIFARE card families (including MIFARE Mini, 1K, 4K, Ultralight, and DESFire).

The chip directly drives an onboard PCB loop antenna using high-frequency magnetic field modulation (13.56 MHz) with no external active driver circuitry required. It communicates with nearby passive RFID tags and smart cards via electromagnetic induction, exchanging framed commands and responses.

Host microcontrollers communicate with the module primarily over SPI (up to 10 Mbit/s), though I2C (up to 3.4 Mbit/s) and UART (up to 1228.8 kBd) interface modes are also supported by the underlying silicon. The module features an internal 64-byte FIFO buffer, hardware CRC engine, and programmable interrupt capabilities to offload timing-critical RFID transactions from the host MCU.

Quick reference

Pinout

Specifications

Communication

• Protocol: SPI Mode 0 (CPOL = 0, CPHA = 0), MSB first. Also supports I2C (up to 3.4 Mbit/s) and UART (up to 1228.8 kBd) when enabled on the IC.
• Max clock: 10 Mbit/s (SPI).
• Frame format: Each SPI transfer consists of an address byte followed by one or more data bytes:
  • Address byte format:
    • Bit 7 (MSB): Read / Write flag (1 = Read, 0 = Write).
    • Bits 6:1: Register address (6-bit offset 0x00–0x3F).
    • Bit 0 (LSB): Reserved, must always be 0.
  • SPI Write framing: (Address << 1) & 0x7E followed by data byte(s).
  • SPI Read framing: (Address << 1) | 0x80 followed by clocking dummy bytes to read response byte(s).

Register map

0x01 — CommandReg

0x14 — TxControlReg

0x26 — RFCfgReg

0x37 — VersionReg

Wiring

Example

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

#define RST_PIN  9
#define SS_PIN   10

MFRC522 mfrc522(SS_PIN, RST_PIN); // Create MFRC522 instance

void setup() {
  Serial.begin(9600);
  while (!Serial);
  
  SPI.begin();           // Init SPI bus
  mfrc522.PCD_Init();    // Init MFRC522 reader
  
  // Verify SPI communication by reading VersionReg
  byte v = mfrc522.PCD_ReadRegister(mfrc522.VersionReg);
  Serial.print(F("MFRC522 Version: 0x"));
  Serial.println(v, HEX);
}

void loop() {
  // Look for new RFID card
  if (!mfrc522.PICC_IsNewCardPresent()) return;
  
  // Select one of the cards
  if (!mfrc522.PICC_ReadCardSerial()) return;
  
  // Print UID to Serial Monitor
  Serial.print(F("Card UID:"));
  for (byte i = 0; i < mfrc522.uid.size; i++) {
    Serial.print(mfrc522.uid.uidByte[i] < 0x10 ? " 0" : " ");
    Serial.print(mfrc522.uid.uidByte[i], HEX);
  }
  Serial.println();
  
  // Halt PICC and stop crypto on PCD
  mfrc522.PICC_HaltA();
  mfrc522.PCD_StopCrypto1();
}

Common mistakes

• If reader fails to initialize (VersionReg returns 0x00 or 0xFF): Check wiring, especially 3.3V power and GND. Verify SPI pin assignments (MOSI, MISO, SCK, CS) and ensure power supply voltage is stable.
• If module dies or behaves erratically after connecting to Arduino Uno: Check logic levels. Direct 5 V GPIO signals on SCK, MOSI, SDA, or RST can degrade or burn out the MFRC522 inputs. Add level shifters.
• If card detection range is short (< 10 mm) or intermittent: Check antenna tuning and receiver gain. Verify RFCfgReg (RxGain) is set to 0x48 (38 dB) or 0x70 (48 dB), and ensure no metal objects or noise sources are near the PCB antenna.
• If SPI bus conflicts occur when using multiple devices: Check MISO output. Some cheap RC522 clone modules fail to properly tri-state MISO when SDA/CS is high. Add a tri-state buffer or dedicated SPI bus.
• If MIFARE read/write operations fail after card detection: Check authentication sequence. MIFARE Classic cards require executing PCD_Authenticate with Key A or Key B (default 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF) before accessing data blocks.

Notes

• Silicon Revisions: Revision 1.0 (MFRC52201HN1, returns 0x91 in VersionReg) vs Revision 2.0 (MFRC52202HN1, returns 0x92 in VersionReg). Revision 2.0 improves RF receiver stability and fixes CRC calculation when RxMultiple is set.
• NRND Notice: NXP lists MFRC522 as Not Recommended for New Designs. For new commercial designs, NXP recommends the CLRC663 Plus family (CLRC66303HN).