Interfacing RC522 NFC / RFID with STM32 Blue Pill

RC522 practical reference

A low-cost SPI RFID/NFC reader operating at 13.56MHz, the go-to choice for DIY access-control and attendance-tracking builds. Built around NXP's MFRC522 chip, it reads (and writes) common Mifare-family tags and cards over SPI, letting a microcontroller check a scanned card's unique ID against a stored list to grant or deny access — the default entry point for anyone building an RFID door lock, attendance system, or simple tag-based authentication project.

Key specifications

• Chipset: NXP MFRC522 13.56MHz RFID/NFC reader IC
• Operating voltage: 3.3V DC (NOT 5V tolerant on SPI or logic pins)
• Operating current: ~13-26 mA typical during read operations
• Frequency: 13.56MHz
• Supported tags: Mifare Classic 1K/4K, Mifare Ultralight, and other ISO/IEC 14443A compliant cards/tags
• Read range: ~2.5-5cm typical, antenna and card dependent
• Interface: SPI (also supports I2C/UART on the bare IC, but most breakout boards hardwire SPI only)
• Included accessories: Modules commonly ship with a keychain tag and a card, both pre-programmed with unique IDs for testing

Pinout and connection notes

The pin labeled SDA on most RC522 breakout boards is actually the SPI chip-select line, not an I2C data pin — a frequent point of confusion for newcomers expecting I2C wiring. All logic pins are 3.3V only, so when wiring to a 5V Arduino Uno, use a logic-level shifter on MOSI, SCK, and RST (MISO can be safely read directly since the RC522 drives it).

• 3.3V: Power, 3.3V DC only — do not supply 5V
• RST: Active-LOW reset pin
• GND: Ground
• MISO: SPI data out
• MOSI: SPI data in
• SCK: SPI clock
• SDA (SS/CS): SPI chip select, active LOW (labeled SDA on the module despite being an SPI pin, a common source of confusion)
• IRQ: Interrupt output, optional — signals when a card enters the field

Integration checklist

1. Confirm the exact part number and breakout-board revision.
2. Verify supply voltage, current demand, logic levels, and signal direction against the datasheet.
3. Check controller pin assignments and bus address or interface mode against the example code.
4. Power up with external loads disconnected, then test signals before full integration.
5. If readings or communication fail, check power and ground first, then pin mapping, interface settings, and library initialization.

Troubleshooting and reproducibility

Record the board used, wiring, library/version, supply arrangement, and measured behavior. Use the selected board-specific wiring example as the source of truth. Never assume visually similar modules share identical voltage tolerance or pin order.