Source: https://mokxi.com/libraries/mfrc522 Updated: 2026-11-02 Libraries / Communication Arduino library · Communication # MFRC522: RFID cards and key fobs on an RC522 The MFRC522 library by Miguel Balboa drives the NXP MFRC522 reader on the common RC522 boards that come with a white card and a blue key fob in starter kits. The reader works at 13.56 MHz and talks to the board over SPI; the library handles the radio exchange with MIFARE Classic, MIFARE Ultralight and NTAG cards. Open the example in the editor See the calls Built in on Mokxi: #include and it builds, with nothing to install. Free, and no account needed. ## What MFRC522 does Most projects only need the card’s UID, a 4 or 7-byte serial number. The loop pattern is fixed: PICC_IsNewCardPresent() checks whether a card has entered the field, PICC_ReadCardSerial() reads its UID into mfrc522.uid, and PICC_HaltA() puts it to sleep so it is not read again on every pass. Reading or writing the data blocks of a MIFARE Classic card needs authentication with a key first, the factory key being six bytes of 0xFF. The RC522 runs on 3.3 V: on a 5 V board power it from the 3.3 V pin, as every tutorial does. Library Manager name | MFRC522 Author | GithubCommunity (miguelbalboa) Include | #include Boards | every board Mokxi compiles for but ATtiny85 In the simulator | Simulated: it runs against the part the way it does on the bench. Parts it drives | RFID reader, MFRC522 ## Install it It builds on every board Mokxi compiles for but ATtiny85. In Mokxi it is built in: #include and the sketch compiles, with no download, no zip and no Library Manager. On a real board, install "MFRC522" by GithubCommunity (miguelbalboa) from the Arduino IDE’s Library Manager (Sketch, Include Library, Manage Libraries), or add MFRC522 to lib_deps in PlatformIO. Mokxi follows the calls of release 1.4.11, so a sketch written here builds there too. What to know in the simulator: The part’s cards answer READ and WRITE: increment, decrement, restore and transfer, the UID backdoor and NTAG passwords get the refusal a genuine card gives. ESP32-C3 · Arduino Uno · Arduino Nano · Arduino Mega 2560 · Arduino Leonardo · SparkFun Pro Micro · Arduino Micro · ESP32-C6 · Raspberry Pi Pico · Raspberry Pi Pico W · Seeed XIAO SAMD21 · STM32 · STM32 Nucleo-F411RE · STM32 Blue Pill · micro:bit V2 · Arduino Uno R4 · ESP32 DevKit · ESP32-S3-DevKitC-1 · ESP8266 NodeMCU · Arduino Pro Mini 3.3 V · Seeed XIAO ESP32-C3 · ESP32-C3 SuperMini · Seeed XIAO ESP32-C6 · ESP32-DevKitC V4 · LOLIN32 Lite · Wemos D1 R32 · Cheap Yellow Display · TTGO T-Display · Seeed XIAO ESP32-S3 · Arduino Nano ESP32 · Arduino Nano RP2040 Connect · Seeed XIAO RP2040 · Waveshare RP2040-Zero · Arduino Uno R4 WiFi ## The calls you will use In the order a sketch meets them. ### MFRC522 mfrc522(SS_PIN, RST_PIN) Creates the reader with its chip select and reset pins (10 and 9 in most Uno tutorials). ### mfrc522.PCD_Init() Resets the reader and turns the antenna on. Call SPI.begin() first. ### mfrc522.PICC_IsNewCardPresent() True when a new card is in the field. ### mfrc522.PICC_ReadCardSerial() Reads the card’s UID into mfrc522.uid. Returns false if the read failed. ### mfrc522.uid.uidByte[i] The UID bytes, with mfrc522.uid.size saying how many (4, 7 or 10). ### mfrc522.PICC_HaltA() Puts the card to sleep, so the same card is not read again until it leaves and returns. ### mfrc522.PCD_Authenticate(MFRC522::PICC_CMD_MF_AUTH_KEY_A, block, &key, &(mfrc522.uid)) Unlocks a MIFARE Classic sector with key A before reading or writing it. key is a MFRC522::MIFARE_Key. ### mfrc522.MIFARE_Read(block, buffer, &size) Reads a 16-byte block into an 18-byte buffer. MIFARE_Write(block, data, 16) writes one. ### mfrc522.PCD_StopCrypto1() Ends the authenticated session after reading or writing, so the next card can be read. ## Example: RFID door On the Arduino Uno R3. It runs in the browser exactly as printed. sketch.ino · Arduino Uno R3 Open it in the editor, wired and ready to run · RFID door: the project page ## Common errors and how to fix them ### Firmware Version: 0x0 = (unknown) PCD_DumpVersionToSerial() got no answer over SPI. Check the SPI wiring (on an Uno MOSI 11, MISO 12, SCK 13), the SS and RST pins in the constructor, and that the module is on 3.3 V. ### Nothing happens when a card is held to the reader The card must be within about 3 cm, flat to the antenna. Check SPI.begin() runs before PCD_Init(), and that the loop returns early only when PICC_IsNewCardPresent() is false. ### PCD_Authenticate() failed: Timeout in communication. The key or the block is wrong, or the card is not MIFARE Classic. Use the factory key (six 0xFF bytes), authenticate the sector the block is in, and call PCD_StopCrypto1() after each card. ## Projects that use MFRC522 Built-in projects that include it. Each page has the full sketch and opens in the editor, already wired. RC522 RFID reader (SS 10, RST 9) An RC522 RFID reader on the SPI pins with SDA (SS) on pin 10 and RST on pin 9: hold a card to it and the MFRC522 library prints the card’s ID. ESP32 traffic emergency system with RFID Three roads’ lights take turns on an ESP32. RFID reader with an I2C LCD An RC522 reader on the SPI pins with SS on 10 and RST on 9 reads a card’s UID and shows it on an I2C LCD on A4 and A5, as well as in the serial monitor. Access control: RFID card opens a stepper door An RC522 reader (SS 10, RST 9) checks each card against the one it knows; the right card turns a 28BYJ-48 stepper on pins 2 to 5 a quarter turn to open the door, and an I2C LCD says who may pass. RFID time clock with an LCD and a DS1307 Swipe a card on an RC522 (SS 10, RST A1) to clock in: the LCD names the person, a DS1307 stamps the time, and a line goes to the serial monitor. RFID attendance system RFID attendance system on an Arduino: tap a card on an RC522, the LCD shows IN, LATE or OUT with the DS3231 time, and a CSV on the SD card logs it. ## Keep going ### Learn - Arduino RC522 RFID Reader: Wiring and Code ### Parts - RFID reader, MFRC522 ### More communication libraries - OneWire - TinyGPS - TinyGPSPlus - SoftwareSerial - AltSoftSerial - i2cdetect - Keyboard - Mouse Every built-in library · Open the editor ## Try MFRC522 without installing anything Open the example, change the sketch, and keep your version in a free account. Start building free Open the editor