Arduino Void loop() does not loop

Question

I am new to Arduino. I'm looking at Makecourse tutorial on RC522 RFID reader/writer

I pasted the script below. Basically, it detects a card then writes some data into block 2.

After compiling and uploading the code, I put up one card to the RFID reader, it works - but just once. When i put up a second card, nothing happens. Why?

I've compared this script with other example scripts from the mfrc522 library, they are pretty similar - in the void loop() section, it checks if NewCardPresent... ReadCardSerial... then proceeds to run the intended action. I've tried those example scripts and I can keep presenting a new card to the reader and the script will run again.

However this sketch, the loops only ran once. Is it the structure? How can i edit it to make it run continuously? I apologise if the answer is very simple :/

#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 5  //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


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
        }

}

int block=2;//this is the block number we will write into and then read. Do not write into 'sector trailer' block, since this can make the block unusable.

byte blockcontent[16] = {"makecourse_____"};//an array with 16 bytes to be written into one of the 64 card blocks is defined
//byte blockcontent[16] = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};//all zeros. This can be used to delete a block.
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.

void loop()
{

        /*****************************************establishing contact with a tag/card**********************************************************************/

    // 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;//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;//if it returns a '0' something went wrong and we return to the start of the loop
    }

        // Among other things, the PICC_ReadCardSerial() method reads the UID and the SAK (Select acknowledge) into the mfrc522.uid struct, which is also instantiated
        // during this process.
        // The UID is needed during the authentication process
            //The Uid struct:
            //typedef struct {
        //byte      size;           // Number of bytes in the UID. 4, 7 or 10.
        //byte      uidByte[10];            //the user ID in 10 bytes.
        //byte      sak;            // The SAK (Select acknowledge) byte returned from the PICC after successful selection.
            //} Uid;

         Serial.println("card selected");

         /*****************************************writing and reading a block on the card**********************************************************************/

         writeBlock(block, blockcontent);//the blockcontent array is written into the card block
         //mfrc522.PICC_DumpToSerial(&(mfrc522.uid));

         //The 'PICC_DumpToSerial' method 'dumps' the entire MIFARE data block into the serial monitor. Very useful while programming a sketch with the RFID reader...
         //Notes:
         //(1) MIFARE cards conceal key A in all trailer blocks, and shows 0x00 instead of 0xFF. This is a secutiry feature. Key B appears to be public by default.
         //(2) The card needs to be on the reader for the entire duration of the dump. If it is removed prematurely, the dump interrupts and an error message will appear.
         //(3) The dump takes longer than the time alloted for interaction per pairing between reader and card, i.e. the readBlock function below will produce a timeout if
         //    the dump is used.

     //mfrc522.PICC_DumpToSerial(&(mfrc522.uid));//uncomment this if you want to see the entire 1k memory with the block written into it.

         readBlock(block, 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
         }
         Serial.println("");


}

and this is the function that came along with the sketch:

int writeBlock(int blockNumber, byte arrayAddress[]) 
{
  //this makes sure that we only write into data blocks. Every 4th block is a trailer block for the access/security info.
  int largestModulo4Number=blockNumber/4*4;
  int trailerBlock=largestModulo4Number+3;//determine trailer block for the sector
  if (blockNumber > 2 && (blockNumber+1)%4 == 0){Serial.print(blockNumber);Serial.println(" is a trailer block:");return 2;}//block number is a trailer block (modulo 4); quit and send error code 2
  Serial.print(blockNumber);
  Serial.println(" is a data block:");

  /*****************************************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: ");
         Serial.println(mfrc522.GetStatusCodeName(status));
         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.


  /*****************************************writing the block***********************************************************/

  status = mfrc522.MIFARE_Write(blockNumber, arrayAddress, 16);//valueBlockA is the block number, MIFARE_Write(block number (0-15), byte array containing 16 values, number of bytes in block (=16))
  //status = mfrc522.MIFARE_Write(9, value1Block, 16);
  if (status != MFRC522::STATUS_OK) {
           Serial.print("MIFARE_Write() failed: ");
           Serial.println(mfrc522.GetStatusCodeName(status));
           return 4;//return "4" as error message
  }
  Serial.println("block was written");
}


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): ");
         Serial.println(mfrc522.GetStatusCodeName(status));
         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: ");
          Serial.println(mfrc522.GetStatusCodeName(status));
          return 4;//return "4" as error message
  }
  Serial.println("block was read");
}

Answer 1

I think the problem is not with the loop. It is ruining continuously. You can check it by using some serial print in the beginning of the loop. Problem is after a card is detected the first time this program doesn't identify a card the second time.So focus on that. The loop is fine.

Answer 2

Turns out adding this few lines at the end of the script resolved the issue:

delay(1000);

mfrc522.PICC_HaltA();
mfrc522.PCD_StopCrypto1();

:)

Answer 3

Arduino Loop() is supposed to be called infinitely unlike you observed. Therefore you need to check two possibilities as below. 1) any return is happening at the beginning of the loop(). I saw two return statements. You'd better insert debug messages among them so that you could know how far reached before returning this function. 2) any blocking is happing in the loop(). I don't know but you'd better check this as well.