I am writing a program that:

  1. Reads string (one line) from com port
  2. Waits for an RFID card to be present
  3. Reads a block from the card

Here's the code (it uses this MFRC522 library, also the code is based on makecourse.com RFID tutorial):

#include <SPI.h> //include the SPI bus library
#include <MFRC522.h> //include the RFID reader library

#define SS_PIN 10  //slave select pin
#define RST_PIN 9  //reset pin
MFRC522 mfrc522(SS_PIN, RST_PIN);        // instatiate a MFRC522 reader object.
MFRC522::MIFARE_Key key; //create a MIFARE_Key struct named 'key', which will hold the card information

byte readbackblock[18]; //This array is used for reading out a block. The MIFARE_Read method requires a buffer that is at least 18 bytes to hold the 16 bytes of a block.

String inString = ""; // COM port incoming data buffer

void setup() {
  Serial.begin(9600);        // Initialize serial communications with the PC
  SPI.begin();               // Init SPI bus
  mfrc522.PCD_Init();        // Init MFRC522 card (in case you wonder what PCD means: proximity coupling device)
  Serial.println("Scan a MIFARE Classic card");

  // Prepare the security key for the read and write functions - all six key bytes are set to 0xFF at chip delivery from the factory.
  // Since the cards in the kit are new and the keys were never defined, they are 0xFF
  // if we had a card that was programmed by someone else, we would need to know the key to be able to access it. This key would then need to be stored in 'key' instead.

  for (byte i = 0; i < 6; i++) {
    key.keyByte[i] = 0xFF;//keyByte is defined in the "MIFARE_Key" 'struct' definition in the .h file of the library


void loop() {
  while (Serial.available() > 0) {
    int inChar = Serial.read();

    if (inChar != '\n') {
      inString += (char)inChar;
    } else {
      // New line

      while (!initcard()); // Detecting new card
      readBlockToCom(2); // Reading a block
      inString = "";

bool initcard()
  // Look for new cards (in case you wonder what PICC means: proximity integrated circuit card)
  if ( ! mfrc522.PICC_IsNewCardPresent()) {//if PICC_IsNewCardPresent returns 1, a new card has been found and we continue
    return false; //if it did not find a new card is returns a '0' and we return to the start of the loop

  // Select one of the cards
  if ( ! mfrc522.PICC_ReadCardSerial()) {//if PICC_ReadCardSerial returns 1, the "uid" struct (see MFRC522.h lines 238-45)) contains the ID of the read card.
    return false; //if it returns a '0' something went wrong and we return to the start of the loop
  return true;

void readBlockToCom(int number)
  readBlock(number, readbackblock);//read the block back
  Serial.print("read block: ");
  for (int j = 0 ; j < 16 ; j++) //print the block contents
    Serial.write (readbackblock[j]);//Serial.write() transmits the ASCII numbers as human readable characters to serial monitor

int readBlock(int blockNumber, byte arrayAddress[]) 
  int largestModulo4Number=blockNumber/4*4;
  int trailerBlock=largestModulo4Number+3;//determine trailer block for the sector

  /*****************************************authentication of the desired block for access***********************************************************/
  byte status = mfrc522.PCD_Authenticate(MFRC522::PICC_CMD_MF_AUTH_KEY_A, trailerBlock, &key, &(mfrc522.uid));
  //byte PCD_Authenticate(byte command, byte blockAddr, MIFARE_Key *key, Uid *uid);
  //this method is used to authenticate a certain block for writing or reading
  //command: See enumerations above -> PICC_CMD_MF_AUTH_KEY_A   = 0x60 (=1100000),      // this command performs authentication with Key A
  //blockAddr is the number of the block from 0 to 15.
  //MIFARE_Key *key is a pointer to the MIFARE_Key struct defined above, this struct needs to be defined for each block. New cards have all A/B= FF FF FF FF FF FF
  //Uid *uid is a pointer to the UID struct that contains the user ID of the card.
  if (status != MFRC522::STATUS_OK) {
         Serial.print("PCD_Authenticate() failed (read): ");
         return 3;//return "3" as error message
  //it appears the authentication needs to be made before every block read/write within a specific sector.
  //If a different sector is being authenticated access to the previous one is lost.

  /*****************************************reading a block***********************************************************/

  byte buffersize = 18;//we need to define a variable with the read buffer size, since the MIFARE_Read method below needs a pointer to the variable that contains the size... 
  status = mfrc522.MIFARE_Read(blockNumber, arrayAddress, &buffersize);//&buffersize is a pointer to the buffersize variable; MIFARE_Read requires a pointer instead of just a number
  if (status != MFRC522::STATUS_OK) {
          Serial.print("MIFARE_read() failed: ");
          return 4;//return "4" as error message
  Serial.println("block was read");

The problem I'm having is that

  1. I send first line via com port
  2. Apply a card to RFID-RC522
  3. The program reads the block alright
  4. But then I send the second line
  5. Arduino hangs on card detect

Here's the output I receive:

Scan a MIFARE Classic card
block was read
read block: makecourse_____

I tried removing card and applying it again or applying a different card, but it still freezes on card detect, sometimes after some time it works, though.

1 Answer 1


You should try adding this, at the end of readBlock():

// Halt PICC
// Stop encryption on PCD
  • Added this code after readBlockToCom(2); and it worked. Thank you.
    – rfg
    Commented Mar 18, 2016 at 13:53

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