> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/wokwi/wokwi-docs/llms.txt
> Use this file to discover all available pages before exploring further.

# MFRC522 RFID Reader

> RFID/NFC reader module with SPI interface for reading 13.56 MHz MIFARE cards and tags

MFRC522 RFID/NFC reader module with SPI interface for reading 13.56 MHz MIFARE cards and tags.

## Pins

<ParamField path="3.3V" type="power">
  Voltage supply (3.3V)
</ParamField>

<ParamField path="RST" type="input">
  Reset (active low)
</ParamField>

<ParamField path="GND" type="ground">
  Ground
</ParamField>

<ParamField path="IRQ" type="output">
  Interrupt request (active low)
</ParamField>

<ParamField path="MISO" type="spi">
  SPI data out
</ParamField>

<ParamField path="MOSI" type="spi">
  SPI data in
</ParamField>

<ParamField path="SCK" type="spi">
  SPI clock
</ParamField>

<ParamField path="SDA" type="spi">
  SPI chip select (active low)
</ParamField>

## Attributes

<ParamField path="uid" type="string" default="">
  Custom UID (for Blue Card only)
</ParamField>

### UID format

The UID can be specified in the following lengths:

* 4 bytes: `01:02:03:04` (MIFARE Classic)
* 7 bytes: `04:11:22:33:44:55:66` (MIFARE Ultralight)

Each byte must be two hexadecimal characters (00-FF), separated by colons.

## Card presets

The simulator provides built-in card presets for quick testing. Click on the MFRC522 to open the control panel where you can select a card:

| Index | Preset      | Color  | UID                  | Card Type         |
| ----- | ----------- | ------ | -------------------- | ----------------- |
| 0     | Blue Card   | Blue   | 01:02:03:04          | MIFARE Classic 1K |
| 1     | Green Card  | Green  | 11:22:33:44          | MIFARE Classic 1K |
| 2     | Yellow Card | Yellow | 55:66:77:88          | MIFARE Classic 1K |
| 3     | Red Card    | Red    | AA:BB:CC:DD          | MIFARE Classic 1K |
| 4     | NFC Tag     | Gray   | 04:11:22:33:44:55:66 | MIFARE Ultralight |
| 5     | Key Fob     | Orange | C0:FF:EE:99          | MIFARE Mini       |

The `uid` attribute only affects the **Blue Card** (index 0). Other cards always use their preset UIDs. This lets you customize your main card while keeping the others for multi-user testing.

## Controlling the reader

The control panel provides two ways to present a card:

* **Tap button** - Presents the card briefly (500ms), simulating a quick tap
* **Hold switch** - Keeps the card presented until you toggle it off

This allows testing both tap-and-go scenarios and scenarios where the card needs to stay on the reader.

### Keyboard shortcuts

Click on the control panel to focus it, then use these shortcuts:

| Key | Action                     |
| --- | -------------------------- |
| `b` | Select Blue Card           |
| `g` | Select Green Card          |
| `y` | Select Yellow Card         |
| `r` | Select Red Card            |
| `n` | Select NFC Tag             |
| `k` | Select Key Fob             |
| `t` | Tap (present card briefly) |

## Using the MFRC522

The MFRC522 communicates via SPI (Mode 0) and is commonly used with the [MFRC522 library](https://github.com/miguelbalboa/rfid) by Miguel Balboa or the [Arduino\_MFRC522v2 library](https://github.com/OSSLibraries/Arduino_MFRC522v2).

Common use cases:

* Access control systems
* Attendance tracking (multi-user)
* Smart home door locks
* NFC tag reading projects

## Arduino code example

The example below uses the MFRC522 library to detect and read card UIDs. On ESP32, the default SPI pins are used with GPIO 5 for chip select and GPIO 21 for reset.

```cpp theme={null}
#include <SPI.h>
#include <MFRC522.h>

#define SS_PIN  5
#define RST_PIN 21

MFRC522 rfid(SS_PIN, RST_PIN);

void setup() {
  Serial.begin(115200);
  SPI.begin();
  rfid.PCD_Init();
  Serial.println("MFRC522 Ready");
  Serial.println("Scan a card...");
}

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

  if (!rfid.PICC_ReadCardSerial()) {
    return;
  }

  Serial.print("Card 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();

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

### Arduino Uno wiring

| MFRC522 | Arduino Uno |
| ------- | ----------- |
| VCC     | 3.3V        |
| RST     | 9           |
| GND     | GND         |
| MISO    | 12          |
| MOSI    | 11          |
| SCK     | 13          |
| SDA     | 10          |

### ESP32 wiring

| MFRC522 | ESP32   |
| ------- | ------- |
| VCC     | 3.3V    |
| RST     | GPIO 21 |
| GND     | GND     |
| MISO    | GPIO 19 |
| MOSI    | GPIO 23 |
| SCK     | GPIO 18 |
| SDA     | GPIO 5  |

## Automation controls

The MFRC522 reader can be controlled using [Automation Scenarios](/wokwi-ci/automation-scenarios). It exposes the following controls:

| Control    | Type | Description                        |
| ---------- | ---- | ---------------------------------- |
| card       | int  | Card preset index (0-5, see table) |
| tagPresent | int  | 1 to present card, 0 to remove     |

### Example scenario: Multi-user attendance

The following scenario tests an attendance system with multiple cards:

```yaml theme={null}
name: Attendance System Test
version: 1

steps:
  - wait-serial: 'System Ready'

  # Select Blue Card and present it
  - set-control:
      part-id: rfid1
      control: card
      value: 0

  - set-control:
      part-id: rfid1
      control: tagPresent
      value: 1

  - wait-serial: 'Welcome, Alice'

  # Remove card
  - set-control:
      part-id: rfid1
      control: tagPresent
      value: 0

  - delay: 500ms

  # Select Red Card (unauthorized)
  - set-control:
      part-id: rfid1
      control: card
      value: 3

  - set-control:
      part-id: rfid1
      control: tagPresent
      value: 1

  - wait-serial: 'Access Denied'
```

## Simulator examples

* [MFRC522 Basic Reading](https://wokwi.com/projects/454873133691473921) - Basic RFID card detection with ESP32