Understanding RFIDs and Integrating them in your projects

#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

//this is the block number we will write into and then read.
int block = 13;

byte blockcontent[16] = {"This is Aaenics"};  //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.

//This array is used for reading out a block.
byte readbackblock[18];

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("Bring the card to be written to...");

  // Prepare the security key for the read and write functions.
  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()
{
  // Look for new cards
  if ( ! mfrc522.PICC_IsNewCardPresent()) {
    return;
  }

  // Select one of the cards
  if ( ! mfrc522.PICC_ReadCardSerial())
  {
    return;
  }
  Serial.println("card selected");

  //the blockcontent array is written into the card block
  writeBlock(block, blockcontent);

  //read the block back
  readBlock(block, readbackblock);

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

  //print the block contents
  Serial.print("read block: ");
  for (int j = 0 ; j < 16 ; j++)
  {
    Serial.write (readbackblock[j]);
  }
  Serial.println("");
}



//Write specific block
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("This is a trailer block:");
    return 2;
  }
  Serial.print(blockNumber);
  Serial.println(" This 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));
  if (status != MFRC522::STATUS_OK) {
    Serial.print("PCD_Authenticate() failed: ");
    Serial.println(mfrc522.GetStatusCodeName(status));
    return 3;//return "3" as error message
  }

  //writing the block
  status = mfrc522.MIFARE_Write(blockNumber, arrayAddress, 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 written");
}



//Read specific block
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));

  if (status != MFRC522::STATUS_OK) {
    Serial.print("PCD_Authenticate() failed (read): ");
    Serial.println(mfrc522.GetStatusCodeName(status));
    return 3;//return "3" as error message
  }

  //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");
}