MFRC522.cpp

/*
* MFRC522.cpp - Library to use ARDUINO RFID MODULE KIT 13.56 MHZ WITH TAGS SPI W AND R BY COOQROBOT.
* _Please_ see the comments in MFRC522.h - they give useful hints and background.
* Released into the public domain.
*/

#include "MFRC522.h"

static const char* const _TypeNamePICC[] =
{
  "Unknown type",
  "PICC compliant with ISO/IEC 14443-4",
  "PICC compliant with ISO/IEC 18092 (NFC)",
  "MIFARE Mini, 320 bytes",
  "MIFARE 1KB",
  "MIFARE 4KB",
  "MIFARE Ultralight or Ultralight C",
  "MIFARE Plus",
  "MIFARE TNP3XXX",

  /* not complete UID */
  "SAK indicates UID is not complete"
};

static const char* const _ErrorMessage[] =
{
  "Unknown error",
  "Success",
  "Error in communication",
  "Collision detected",
  "Timeout in communication",
  "A buffer is not big enough",
  "Internal error in the code, should not happen",
  "Invalid argument",
  "The CRC_A does not match",
  "A MIFARE PICC responded with NAK"
};

#define MFRC522_MaxPICCs (sizeof(_TypeNamePICC)/sizeof(_TypeNamePICC[0]))
#define MFRC522_MaxError (sizeof(_ErrorMessage)/sizeof(_ErrorMessage[0]))

/////////////////////////////////////////////////////////////////////////////////////
// Functions for setting up the driver
/////////////////////////////////////////////////////////////////////////////////////

/**
 * Constructor.
 * Prepares the output pins.
 */
MFRC522::MFRC522(PinName mosi,
                 PinName miso,
                 PinName sclk,
                 PinName cs,
                 PinName reset) : m_SPI(mosi, miso, sclk), m_CS(cs), m_RESET(reset)
{
  /* Configure SPI bus */
  m_SPI.format(8, 0);
  m_SPI.frequency(8000000);

  /* Release SPI-CS pin */
  m_CS       = 1;

  /* Release RESET pin */
  m_RESET    = 1;
} // End constructor


/**
 * Destructor.
 */
MFRC522::~MFRC522()
{

}


/////////////////////////////////////////////////////////////////////////////////////
// Basic interface functions for communicating with the MFRC522
/////////////////////////////////////////////////////////////////////////////////////

/**
 * Writes a byte to the specified register in the MFRC522 chip.
 * The interface is described in the datasheet section 8.1.2.
 */
void MFRC522::PCD_WriteRegister(uint8_t reg, uint8_t value)
{
  m_CS = 0; /* Select SPI Chip MFRC522 */

  // MSB == 0 is for writing. LSB is not used in address. Datasheet section 8.1.2.3.
  (void) m_SPI.write(reg & 0x7E);
  (void) m_SPI.write(value);

  m_CS = 1; /* Release SPI Chip MFRC522 */
} // End PCD_WriteRegister()

/**
 * Writes a number of bytes to the specified register in the MFRC522 chip.
 * The interface is described in the datasheet section 8.1.2.
 */
void MFRC522::PCD_WriteRegister(uint8_t reg, uint8_t count, uint8_t *values)
{
  m_CS = 0; /* Select SPI Chip MFRC522 */

  // MSB == 0 is for writing. LSB is not used in address. Datasheet section 8.1.2.3.
  (void) m_SPI.write(reg & 0x7E);
  for (uint8_t index = 0; index < count; index++)
  {
    (void) m_SPI.write(values[index]);
  }

  m_CS = 1; /* Release SPI Chip MFRC522 */
} // End PCD_WriteRegister()

/**
 * Reads a byte from the specified register in the MFRC522 chip.
 * The interface is described in the datasheet section 8.1.2.
 */
uint8_t MFRC522::PCD_ReadRegister(uint8_t reg)
{
  uint8_t value;
  m_CS = 0; /* Select SPI Chip MFRC522 */

  // MSB == 1 is for reading. LSB is not used in address. Datasheet section 8.1.2.3.
  (void) m_SPI.write(0x80 | reg);

  // Read the value back. Send 0 to stop reading.
  value = m_SPI.write(0);

  m_CS = 1; /* Release SPI Chip MFRC522 */

  return value;
} // End PCD_ReadRegister()

/**
 * Reads a number of bytes from the specified register in the MFRC522 chip.
 * The interface is described in the datasheet section 8.1.2.
 */
void MFRC522::PCD_ReadRegister(uint8_t reg, uint8_t count, uint8_t *values, uint8_t rxAlign)
{
  if (count == 0) { return; }

  uint8_t address = 0x80 | reg;  // MSB == 1 is for reading. LSB is not used in address. Datasheet section 8.1.2.3.
  uint8_t index = 0;             // Index in values array.

  m_CS = 0;                      /* Select SPI Chip MFRC522 */
  count--;                       // One read is performed outside of the loop
  (void) m_SPI.write(address);   // Tell MFRC522 which address we want to read

  while (index < count)
  {
    if ((index == 0) && rxAlign) // Only update bit positions rxAlign..7 in values[0]
    {
      // Create bit mask for bit positions rxAlign..7
      uint8_t mask = 0;
      for (uint8_t i = rxAlign; i <= 7; i++)
      {
        mask |= (1 << i);
      }

      // Read value and tell that we want to read the same address again.
      uint8_t value = m_SPI.write(address);

      // Apply mask to both current value of values[0] and the new data in value.
      values[0] = (values[index] & ~mask) | (value & mask);
    }
    else
    {
      // Read value and tell that we want to read the same address again.
      values[index] = m_SPI.write(address);
    }

    index++;
  }

  values[index] = m_SPI.write(0); // Read the final byte. Send 0 to stop reading.

  m_CS = 1;                       /* Release SPI Chip MFRC522 */
} // End PCD_ReadRegister()

/**
 * Sets the bits given in mask in register reg.
 */
void MFRC522::PCD_SetRegisterBits(uint8_t reg, uint8_t mask)
{
  uint8_t tmp = PCD_ReadRegister(reg);
  PCD_WriteRegister(reg, tmp | mask);     // set bit mask
} // End PCD_SetRegisterBitMask()

/**
 * Clears the bits given in mask from register reg.
 */
void MFRC522::PCD_ClrRegisterBits(uint8_t reg, uint8_t mask)
{
  uint8_t tmp = PCD_ReadRegister(reg);
  PCD_WriteRegister(reg, tmp & (~mask));    // clear bit mask
} // End PCD_ClearRegisterBitMask()


/**
 * Use the CRC coprocessor in the MFRC522 to calculate a CRC_A.
 */
uint8_t MFRC522::PCD_CalculateCRC(uint8_t *data, uint8_t length, uint8_t *result)
{
  PCD_WriteRegister(CommandReg, PCD_Idle);      // Stop any active command.
  PCD_WriteRegister(DivIrqReg, 0x04);           // Clear the CRCIRq interrupt request bit
  PCD_SetRegisterBits(FIFOLevelReg, 0x80);      // FlushBuffer = 1, FIFO initialization
  PCD_WriteRegister(FIFODataReg, length, data); // Write data to the FIFO
  PCD_WriteRegister(CommandReg, PCD_CalcCRC);   // Start the calculation

  // Wait for the CRC calculation to complete. Each iteration of the while-loop takes 17.73us.
  uint16_t i = 5000;
  uint8_t n;
  while (1)
  {
    n = PCD_ReadRegister(DivIrqReg);  // DivIrqReg[7..0] bits are: Set2 reserved reserved MfinActIRq   reserved CRCIRq reserved reserved
    if (n & 0x04)
    {
      // CRCIRq bit set - calculation done
      break;
    }
    
    if (--i == 0)
    {
      // The emergency break. We will eventually terminate on this one after 89ms.
      // Communication with the MFRC522 might be down.
      return STATUS_TIMEOUT;
    }
  }

  // Stop calculating CRC for new content in the FIFO.
  PCD_WriteRegister(CommandReg, PCD_Idle);

  // Transfer the result from the registers to the result buffer
  result[0] = PCD_ReadRegister(CRCResultRegL);
  result[1] = PCD_ReadRegister(CRCResultRegH);
  return STATUS_OK;
} // End PCD_CalculateCRC()


/////////////////////////////////////////////////////////////////////////////////////
// Functions for manipulating the MFRC522
/////////////////////////////////////////////////////////////////////////////////////

/**
 * Initializes the MFRC522 chip.
 */
void MFRC522::PCD_Init()
{
  /* Reset MFRC522 */
  m_RESET = 0;
  thread_sleep_for(10);
  m_RESET = 1;
  
  // Section 8.8.2 in the datasheet says the oscillator start-up time is the start up time of the crystal + 37,74us. Let us be generous: 50ms.
  thread_sleep_for(50);

  // When communicating with a PICC we need a timeout if something goes wrong.
  // f_timer = 13.56 MHz / (2*TPreScaler+1) where TPreScaler = [TPrescaler_Hi:TPrescaler_Lo].
  // TPrescaler_Hi are the four low bits in TModeReg. TPrescaler_Lo is TPrescalerReg.
  PCD_WriteRegister(TModeReg, 0x80);      // TAuto=1; timer starts automatically at the end of the transmission in all communication modes at all speeds
  PCD_WriteRegister(TPrescalerReg, 0xA9); // TPreScaler = TModeReg[3..0]:TPrescalerReg, ie 0x0A9 = 169 => f_timer=40kHz, ie a timer period of 25us.
  PCD_WriteRegister(TReloadRegH, 0x03);   // Reload timer with 0x3E8 = 1000, ie 25ms before timeout.
  PCD_WriteRegister(TReloadRegL, 0xE8);

  PCD_WriteRegister(TxASKReg, 0x40);      // Default 0x00. Force a 100 % ASK modulation independent of the ModGsPReg register setting
  PCD_WriteRegister(ModeReg, 0x3D);       // Default 0x3F. Set the preset value for the CRC coprocessor for the CalcCRC command to 0x6363 (ISO 14443-3 part 6.2.4)

  PCD_WriteRegister(RFCfgReg, (0x07<<4)); // Set Rx Gain to max

  PCD_AntennaOn();                        // Enable the antenna driver pins TX1 and TX2 (they were disabled by the reset)
} // End PCD_Init()

/**
 * Performs a soft reset on the MFRC522 chip and waits for it to be ready again.
 */
void MFRC522::PCD_Reset()
{
  PCD_WriteRegister(CommandReg, PCD_SoftReset); // Issue the SoftReset command.
  // The datasheet does not mention how long the SoftRest command takes to complete.
  // But the MFRC522 might have been in soft power-down mode (triggered by bit 4 of CommandReg)
  // Section 8.8.2 in the datasheet says the oscillator start-up time is the start up time of the crystal + 37,74us. Let us be generous: 50ms.
  thread_sleep_for(50);

  // Wait for the PowerDown bit in CommandReg to be cleared
  while (PCD_ReadRegister(CommandReg) & (1<<4))
  {
    // PCD still restarting - unlikely after waiting 50ms, but better safe than sorry.
  }
} // End PCD_Reset()

/**
 * Turns the antenna on by enabling pins TX1 and TX2.
 * After a reset these pins disabled.
 */
void MFRC522::PCD_AntennaOn()
{
  uint8_t value = PCD_ReadRegister(TxControlReg);
  if ((value & 0x03) != 0x03)
  {
    PCD_WriteRegister(TxControlReg, value | 0x03);
  }
} // End PCD_AntennaOn()

/////////////////////////////////////////////////////////////////////////////////////
// Functions for communicating with PICCs
/////////////////////////////////////////////////////////////////////////////////////

/**
 * Executes the Transceive command.
 * CRC validation can only be done if backData and backLen are specified.
 */
uint8_t MFRC522::PCD_TransceiveData(uint8_t *sendData,
                                    uint8_t sendLen,
                                    uint8_t *backData,
                                    uint8_t *backLen,
                                    uint8_t *validBits,
                                    uint8_t rxAlign,
                                    bool    checkCRC)
{
  uint8_t waitIRq = 0x30;    // RxIRq and IdleIRq
  return PCD_CommunicateWithPICC(PCD_Transceive, waitIRq, sendData, sendLen, backData, backLen, validBits, rxAlign, checkCRC);
} // End PCD_TransceiveData()

/**
 * Transfers data to the MFRC522 FIFO, executes a commend, waits for completion and transfers data back from the FIFO.
 * CRC validation can only be done if backData and backLen are specified.
 */
uint8_t MFRC522::PCD_CommunicateWithPICC(uint8_t command,
                                         uint8_t waitIRq,
                                         uint8_t *sendData,
                                         uint8_t sendLen,
                                         uint8_t *backData,
                                         uint8_t *backLen,
                                         uint8_t *validBits,
                                         uint8_t rxAlign,
                                         bool    checkCRC)
{
  uint8_t n, _validBits = 0;
  uint32_t i;

  // Prepare values for BitFramingReg
  uint8_t txLastBits = validBits ? *validBits : 0;
  uint8_t bitFraming = (rxAlign << 4) + txLastBits;   // RxAlign = BitFramingReg[6..4]. TxLastBits = BitFramingReg[2..0]

  PCD_WriteRegister(CommandReg, PCD_Idle);            // Stop any active command.
  PCD_WriteRegister(ComIrqReg, 0x7F);                 // Clear all seven interrupt request bits
  PCD_SetRegisterBits(FIFOLevelReg, 0x80);            // FlushBuffer = 1, FIFO initialization
  PCD_WriteRegister(FIFODataReg, sendLen, sendData);  // Write sendData to the FIFO
  PCD_WriteRegister(BitFramingReg, bitFraming);       // Bit adjustments
  PCD_WriteRegister(CommandReg, command);             // Execute the command
  if (command == PCD_Transceive)
  {
    PCD_SetRegisterBits(BitFramingReg, 0x80);      // StartSend=1, transmission of data starts
  }

  // Wait for the command to complete.
  // In PCD_Init() we set the TAuto flag in TModeReg. This means the timer automatically starts when the PCD stops transmitting.
  // Each iteration of the do-while-loop takes 17.86us.
  i = 2000;
  while (1)
  {
    n = PCD_ReadRegister(ComIrqReg);  // ComIrqReg[7..0] bits are: Set1 TxIRq RxIRq IdleIRq   HiAlertIRq LoAlertIRq ErrIRq TimerIRq
    if (n & waitIRq)
    {          // One of the interrupts that signal success has been set.
      break;
    }

    if (n & 0x01)
    {           // Timer interrupt - nothing received in 25ms
      return STATUS_TIMEOUT;
    }

    if (--i == 0)
    {           // The emergency break. If all other condions fail we will eventually terminate on this one after 35.7ms. Communication with the MFRC522 might be down.
      return STATUS_TIMEOUT;
    }
  }

  // Stop now if any errors except collisions were detected.
  uint8_t errorRegValue = PCD_ReadRegister(ErrorReg); // ErrorReg[7..0] bits are: WrErr TempErr reserved BufferOvfl   CollErr CRCErr ParityErr ProtocolErr
  if (errorRegValue & 0x13)
  {  // BufferOvfl ParityErr ProtocolErr
    return STATUS_ERROR;
  }

  // If the caller wants data back, get it from the MFRC522.
  if (backData && backLen)
  {
    n = PCD_ReadRegister(FIFOLevelReg);           // Number of bytes in the FIFO
    if (n > *backLen)
    {
      return STATUS_NO_ROOM;
    }

    *backLen = n;                       // Number of bytes returned
    PCD_ReadRegister(FIFODataReg, n, backData, rxAlign);    // Get received data from FIFO
    _validBits = PCD_ReadRegister(ControlReg) & 0x07; // RxLastBits[2:0] indicates the number of valid bits in the last received byte. If this value is 000b, the whole byte is valid.
    if (validBits)
    {
      *validBits = _validBits;
    }
  }

  // Tell about collisions
  if (errorRegValue & 0x08)
  { // CollErr
    return STATUS_COLLISION;
  }

  // Perform CRC_A validation if requested.
  if (backData && backLen && checkCRC)
  {
    // In this case a MIFARE Classic NAK is not OK.
    if ((*backLen == 1) && (_validBits == 4))
    {
      return STATUS_MIFARE_NACK;
    }

    // We need at least the CRC_A value and all 8 bits of the last byte must be received.
    if ((*backLen < 2) || (_validBits != 0))
    {
      return STATUS_CRC_WRONG;
    }

    // Verify CRC_A - do our own calculation and store the control in controlBuffer.
    uint8_t controlBuffer[2];
    n = PCD_CalculateCRC(&backData[0], *backLen - 2, &controlBuffer[0]);
    if (n != STATUS_OK)
    {
      return n;
    }

    if ((backData[*backLen - 2] != controlBuffer[0]) || (backData[*backLen - 1] != controlBuffer[1]))
    {
      return STATUS_CRC_WRONG;
    }
  }

  return STATUS_OK;
} // End PCD_CommunicateWithPICC()

/*
 * Transmits a REQuest command, Type A. Invites PICCs in state IDLE to go to READY and prepare for anticollision or selection. 7 bit frame.
 * Beware: When two PICCs are in the field at the same time I often get STATUS_TIMEOUT - probably due do bad antenna design.
 */
uint8_t MFRC522::PICC_RequestA(uint8_t *bufferATQA, uint8_t *bufferSize)
{
  return PICC_REQA_or_WUPA(PICC_CMD_REQA, bufferATQA, bufferSize);
} // End PICC_RequestA()

/**
 * Transmits a Wake-UP command, Type A. Invites PICCs in state IDLE and HALT to go to READY(*) and prepare for anticollision or selection. 7 bit frame.
 * Beware: When two PICCs are in the field at the same time I often get STATUS_TIMEOUT - probably due do bad antenna design.
 */
uint8_t MFRC522::PICC_WakeupA(uint8_t *bufferATQA, uint8_t *bufferSize)
{
  return PICC_REQA_or_WUPA(PICC_CMD_WUPA, bufferATQA, bufferSize);
} // End PICC_WakeupA()

/*
 * Transmits REQA or WUPA commands.
 * Beware: When two PICCs are in the field at the same time I often get STATUS_TIMEOUT - probably due do bad antenna design.
 */
uint8_t MFRC522::PICC_REQA_or_WUPA(uint8_t command, uint8_t *bufferATQA, uint8_t *bufferSize)
{
  uint8_t validBits;
  uint8_t status;

  if (bufferATQA == NULL || *bufferSize < 2)
  {  // The ATQA response is 2 bytes long.
    return STATUS_NO_ROOM;
  }

  // ValuesAfterColl=1 => Bits received after collision are cleared.
  PCD_ClrRegisterBits(CollReg, 0x80);

  // For REQA and WUPA we need the short frame format
  // - transmit only 7 bits of the last (and only) byte. TxLastBits = BitFramingReg[2..0]
  validBits = 7;

  status = PCD_TransceiveData(&command, 1, bufferATQA, bufferSize, &validBits);
  if (status != STATUS_OK)
  {
    return status;
  }

  if ((*bufferSize != 2) || (validBits != 0))
  {   // ATQA must be exactly 16 bits.
    return STATUS_ERROR;
  }

  return STATUS_OK;
} // End PICC_REQA_or_WUPA()

/*
 * Transmits SELECT/ANTICOLLISION commands to select a single PICC.
 */
uint8_t MFRC522::PICC_Select(Uid *uid, uint8_t validBits)
{
  bool uidComplete;
  bool selectDone;
  bool useCascadeTag;
  uint8_t cascadeLevel = 1;
  uint8_t result;
  uint8_t count;
  uint8_t index;
  uint8_t uidIndex;          // The first index in uid->uidByte[] that is used in the current Cascade Level.
  uint8_t currentLevelKnownBits;   // The number of known UID bits in the current Cascade Level.
  uint8_t buffer[9];         // The SELECT/ANTICOLLISION commands uses a 7 byte standard frame + 2 bytes CRC_A
  uint8_t bufferUsed;        // The number of bytes used in the buffer, ie the number of bytes to transfer to the FIFO.
  uint8_t rxAlign;           // Used in BitFramingReg. Defines the bit position for the first bit received.
  uint8_t txLastBits;        // Used in BitFramingReg. The number of valid bits in the last transmitted byte.
  uint8_t *responseBuffer;
  uint8_t responseLength;

  // Description of buffer structure:
  //    Byte 0: SEL         Indicates the Cascade Level: PICC_CMD_SEL_CL1, PICC_CMD_SEL_CL2 or PICC_CMD_SEL_CL3
  //    Byte 1: NVB         Number of Valid Bits (in complete command, not just the UID): High nibble: complete bytes, Low nibble: Extra bits.
  //    Byte 2: UID-data or CT    See explanation below. CT means Cascade Tag.
  //    Byte 3: UID-data
  //    Byte 4: UID-data
  //    Byte 5: UID-data
  //    Byte 6: BCC         Block Check Character - XOR of bytes 2-5
  //    Byte 7: CRC_A
  //    Byte 8: CRC_A
  // The BCC and CRC_A is only transmitted if we know all the UID bits of the current Cascade Level.
  //
  // Description of bytes 2-5: (Section 6.5.4 of the ISO/IEC 14443-3 draft: UID contents and cascade levels)
  //    UID size  Cascade level Byte2 Byte3 Byte4 Byte5
  //    ========  ============= ===== ===== ===== =====
  //     4 bytes    1     uid0  uid1  uid2  uid3
  //     7 bytes    1     CT    uid0  uid1  uid2
  //                2     uid3  uid4  uid5  uid6
  //    10 bytes    1     CT    uid0  uid1  uid2
  //                2     CT    uid3  uid4  uid5
  //                3     uid6  uid7  uid8  uid9

  // Sanity checks
  if (validBits > 80)
  {
    return STATUS_INVALID;
  }

  // Prepare MFRC522
  // ValuesAfterColl=1 => Bits received after collision are cleared.
  PCD_ClrRegisterBits(CollReg, 0x80);

  // Repeat Cascade Level loop until we have a complete UID.
  uidComplete = false;
  while ( ! uidComplete)
  {
    // Set the Cascade Level in the SEL byte, find out if we need to use the Cascade Tag in byte 2.
    switch (cascadeLevel)
    {
      case 1:
        buffer[0] = PICC_CMD_SEL_CL1;
        uidIndex = 0;
        useCascadeTag = validBits && (uid->size > 4); // When we know that the UID has more than 4 bytes
        break;

      case 2:
        buffer[0] = PICC_CMD_SEL_CL2;
        uidIndex = 3;
        useCascadeTag = validBits && (uid->size > 7); // When we know that the UID has more than 7 bytes
        break;

      case 3:
        buffer[0] = PICC_CMD_SEL_CL3;
        uidIndex = 6;
        useCascadeTag = false;            // Never used in CL3.
        break;

      default:
        return STATUS_INTERNAL_ERROR;
        //break;
    }

    // How many UID bits are known in this Cascade Level?
    if(validBits > (8 * uidIndex))
    {
      currentLevelKnownBits = validBits - (8 * uidIndex);
    }
    else
    {
      currentLevelKnownBits = 0;
    }

    // Copy the known bits from uid->uidByte[] to buffer[]
    index = 2; // destination index in buffer[]
    if (useCascadeTag)
    {
      buffer[index++] = PICC_CMD_CT;
    }

    uint8_t bytesToCopy = currentLevelKnownBits / 8 + (currentLevelKnownBits % 8 ? 1 : 0); // The number of bytes needed to represent the known bits for this level.
    if (bytesToCopy)
    {
      // Max 4 bytes in each Cascade Level. Only 3 left if we use the Cascade Tag
      uint8_t maxBytes = useCascadeTag ? 3 : 4;
      if (bytesToCopy > maxBytes)
      {
        bytesToCopy = maxBytes;
      }

      for (count = 0; count < bytesToCopy; count++)
      {
        buffer[index++] = uid->uidByte[uidIndex + count];
      }
    }

    // Now that the data has been copied we need to include the 8 bits in CT in currentLevelKnownBits
    if (useCascadeTag)
    {
      currentLevelKnownBits += 8;
    }

    // Repeat anti collision loop until we can transmit all UID bits + BCC and receive a SAK - max 32 iterations.
    selectDone = false;
    while ( ! selectDone)
    {
      // Find out how many bits and bytes to send and receive.
      if (currentLevelKnownBits >= 32)
      { // All UID bits in this Cascade Level are known. This is a SELECT.
        //Serial.print("SELECT: currentLevelKnownBits="); Serial.println(currentLevelKnownBits, DEC);
        buffer[1] = 0x70; // NVB - Number of Valid Bits: Seven whole bytes

        // Calulate BCC - Block Check Character
        buffer[6] = buffer[2] ^ buffer[3] ^ buffer[4] ^ buffer[5];

        // Calculate CRC_A
        result = PCD_CalculateCRC(buffer, 7, &buffer[7]);
        if (result != STATUS_OK)
        {
          return result;
        }

        txLastBits      = 0; // 0 => All 8 bits are valid.
        bufferUsed      = 9;

        // Store response in the last 3 bytes of buffer (BCC and CRC_A - not needed after tx)
        responseBuffer  = &buffer[6];
        responseLength  = 3;
      }
      else
      { // This is an ANTICOLLISION.
        //Serial.print("ANTICOLLISION: currentLevelKnownBits="); Serial.println(currentLevelKnownBits, DEC);
        txLastBits     = currentLevelKnownBits % 8;
        count          = currentLevelKnownBits / 8;  // Number of whole bytes in the UID part.
        index          = 2 + count;                  // Number of whole bytes: SEL + NVB + UIDs
        buffer[1]      = (index << 4) + txLastBits;  // NVB - Number of Valid Bits
        bufferUsed     = index + (txLastBits ? 1 : 0);

        // Store response in the unused part of buffer
        responseBuffer = &buffer[index];
        responseLength = sizeof(buffer) - index;
      }

      // Set bit adjustments
      rxAlign = txLastBits;                     // Having a seperate variable is overkill. But it makes the next line easier to read.
      PCD_WriteRegister(BitFramingReg, (rxAlign << 4) + txLastBits);  // RxAlign = BitFramingReg[6..4]. TxLastBits = BitFramingReg[2..0]

      // Transmit the buffer and receive the response.
      result = PCD_TransceiveData(buffer, bufferUsed, responseBuffer, &responseLength, &txLastBits, rxAlign);
      if (result == STATUS_COLLISION)
      { // More than one PICC in the field => collision.
        result = PCD_ReadRegister(CollReg);     // CollReg[7..0] bits are: ValuesAfterColl reserved CollPosNotValid CollPos[4:0]
        if (result & 0x20)
        { // CollPosNotValid
          return STATUS_COLLISION; // Without a valid collision position we cannot continue
        }

        uint8_t collisionPos = result & 0x1F; // Values 0-31, 0 means bit 32.
        if (collisionPos == 0)
        {
          collisionPos = 32;
        }

        if (collisionPos <= currentLevelKnownBits)
        { // No progress - should not happen
          return STATUS_INTERNAL_ERROR;
        }

        // Choose the PICC with the bit set.
        currentLevelKnownBits = collisionPos;
        count          = (currentLevelKnownBits - 1) % 8; // The bit to modify
        index          = 1 + (currentLevelKnownBits / 8) + (count ? 1 : 0); // First byte is index 0.
        buffer[index] |= (1 << count);
      }
      else if (result != STATUS_OK)
      {
        return result;
      }
      else
      { // STATUS_OK
        if (currentLevelKnownBits >= 32)
        { // This was a SELECT.
          selectDone = true; // No more anticollision
          // We continue below outside the while.
        }
        else
        { // This was an ANTICOLLISION.
          // We now have all 32 bits of the UID in this Cascade Level
          currentLevelKnownBits = 32;
          // Run loop again to do the SELECT.
        }
      }
    } // End of while ( ! selectDone)

    // We do not check the CBB - it was constructed by us above.

    // Copy the found UID bytes from buffer[] to uid->uidByte[]
    index       = (buffer[2] == PICC_CMD_CT) ? 3 : 2; // source index in buffer[]
    bytesToCopy = (buffer[2] == PICC_CMD_CT) ? 3 : 4;
    for (count = 0; count < bytesToCopy; count++)
    {
      uid->uidByte[uidIndex + count] = buffer[index++];
    }

    // Check response SAK (Select Acknowledge)
    if (responseLength != 3 || txLastBits != 0)
    {   // SAK must be exactly 24 bits (1 byte + CRC_A).
      return STATUS_ERROR;
    }

    // Verify CRC_A - do our own calculation and store the control in buffer[2..3] - those bytes are not needed anymore.
    result = PCD_CalculateCRC(responseBuffer, 1, &buffer[2]);
    if (result != STATUS_OK)
    {
      return result;
    }

    if ((buffer[2] != responseBuffer[1]) || (buffer[3] != responseBuffer[2]))
    {
      return STATUS_CRC_WRONG;
    }

    if (responseBuffer[0] & 0x04)
    { // Cascade bit set - UID not complete yes
      cascadeLevel++;
    }
    else
    {
      uidComplete = true;
      uid->sak = responseBuffer[0];
    }
  } // End of while ( ! uidComplete)

  // Set correct uid->size
  uid->size = 3 * cascadeLevel + 1;

  return STATUS_OK;
} // End PICC_Select()

/*
 * Instructs a PICC in state ACTIVE(*) to go to state HALT.
 */
uint8_t MFRC522::PICC_HaltA()
{
  uint8_t result;
  uint8_t buffer[4];

  // Build command buffer
  buffer[0] = PICC_CMD_HLTA;
  buffer[1] = 0;

  // Calculate CRC_A
  result = PCD_CalculateCRC(buffer, 2, &buffer[2]);
  if (result == STATUS_OK)
  {
    // Send the command.
    // The standard says:
    //    If the PICC responds with any modulation during a period of 1 ms after the end of the frame containing the
    //    HLTA command, this response shall be interpreted as 'not acknowledge'.
    // We interpret that this way: Only STATUS_TIMEOUT is an success.
    result = PCD_TransceiveData(buffer, sizeof(buffer), NULL, 0);
    if (result == STATUS_TIMEOUT)
    {
      result = STATUS_OK;
    }
    else if (result == STATUS_OK)
    { // That is ironically NOT ok in this case ;-)
      result = STATUS_ERROR;
    }
  }

  return result;
} // End PICC_HaltA()


/////////////////////////////////////////////////////////////////////////////////////
// Functions for communicating with MIFARE PICCs
/////////////////////////////////////////////////////////////////////////////////////

/*
 * Executes the MFRC522 MFAuthent command.
 */
uint8_t MFRC522::PCD_Authenticate(uint8_t command, uint8_t blockAddr, MIFARE_Key *key, Uid *uid)
{
  uint8_t i, waitIRq = 0x10;    // IdleIRq

  // Build command buffer
  uint8_t sendData[12];
  sendData[0] = command;
  sendData[1] = blockAddr;

  for (i = 0; i < MF_KEY_SIZE; i++)
  {  // 6 key bytes
    sendData[2+i] = key->keyByte[i];
  }

  for (i = 0; i < 4; i++)
  { // The first 4 bytes of the UID
    sendData[8+i] = uid->uidByte[i];
  }

  // Start the authentication.
  return PCD_CommunicateWithPICC(PCD_MFAuthent, waitIRq, &sendData[0], sizeof(sendData));
} // End PCD_Authenticate()

/*
 * Used to exit the PCD from its authenticated state.
 * Remember to call this function after communicating with an authenticated PICC - otherwise no new communications can start.
 */
void MFRC522::PCD_StopCrypto1()
{
  // Clear MFCrypto1On bit
  PCD_ClrRegisterBits(Status2Reg, 0x08); // Status2Reg[7..0] bits are: TempSensClear I2CForceHS reserved reserved   MFCrypto1On ModemState[2:0]
} // End PCD_StopCrypto1()

/*
 * Reads 16 bytes (+ 2 bytes CRC_A) from the active PICC.
 */
uint8_t MFRC522::MIFARE_Read(uint8_t blockAddr, uint8_t *buffer, uint8_t *bufferSize)
{
  uint8_t result = STATUS_NO_ROOM;

  // Sanity check
  if ((buffer == NULL) || (*bufferSize < 18))
  {
    return result;
  }

  // Build command buffer
  buffer[0] = PICC_CMD_MF_READ;
  buffer[1] = blockAddr;

  // Calculate CRC_A
  result = PCD_CalculateCRC(buffer, 2, &buffer[2]);
  if (result != STATUS_OK)
  {
    return result;
  }

  // Transmit the buffer and receive the response, validate CRC_A.
  return PCD_TransceiveData(buffer, 4, buffer, bufferSize, NULL, 0, true);
} // End MIFARE_Read()

/*
 * Writes 16 bytes to the active PICC.
 */
uint8_t MFRC522::MIFARE_Write(uint8_t blockAddr, uint8_t *buffer, uint8_t bufferSize)
{
  uint8_t result;

  // Sanity check
  if (buffer == NULL || bufferSize < 16)
  {
    return STATUS_INVALID;
  }

  // Mifare Classic protocol requires two communications to perform a write.
  // Step 1: Tell the PICC we want to write to block blockAddr.
  uint8_t cmdBuffer[2];
  cmdBuffer[0] = PICC_CMD_MF_WRITE;
  cmdBuffer[1] = blockAddr;
  // Adds CRC_A and checks that the response is MF_ACK.
  result = PCD_MIFARE_Transceive(cmdBuffer, 2);
  if (result != STATUS_OK)
  {
    return result;
  }

  // Step 2: Transfer the data
  // Adds CRC_A and checks that the response is MF_ACK.
  result = PCD_MIFARE_Transceive(buffer, bufferSize);
  if (result != STATUS_OK)
  {
    return result;
  }

  return STATUS_OK;
} // End MIFARE_Write()

/*
 * Writes a 4 byte page to the active MIFARE Ultralight PICC.
 */
uint8_t MFRC522::MIFARE_UltralightWrite(uint8_t page, uint8_t *buffer, uint8_t bufferSize)
{
  uint8_t result;

  // Sanity check
  if (buffer == NULL || bufferSize < 4)
  {
    return STATUS_INVALID;
  }

  // Build commmand buffer
  uint8_t cmdBuffer[6];
  cmdBuffer[0] = PICC_CMD_UL_WRITE;
  cmdBuffer[1] = page;
  memcpy(&cmdBuffer[2], buffer, 4);

  // Perform the write
  result = PCD_MIFARE_Transceive(cmdBuffer, 6); // Adds CRC_A and checks that the response is MF_ACK.
  if (result != STATUS_OK)
  {
    return result;
  }

  return STATUS_OK;
} // End MIFARE_Ultralight_Write()

/*
 * MIFARE Decrement subtracts the delta from the value of the addressed block, and stores the result in a volatile memory.
 */
uint8_t MFRC522::MIFARE_Decrement(uint8_t blockAddr, uint32_t delta)
{
  return MIFARE_TwoStepHelper(PICC_CMD_MF_DECREMENT, blockAddr, delta);
} // End MIFARE_Decrement()

/*
 * MIFARE Increment adds the delta to the value of the addressed block, and stores the result in a volatile memory.
 */
uint8_t MFRC522::MIFARE_Increment(uint8_t blockAddr, uint32_t delta)
{
  return MIFARE_TwoStepHelper(PICC_CMD_MF_INCREMENT, blockAddr, delta);
} // End MIFARE_Increment()

/**
 * MIFARE Restore copies the value of the addressed block into a volatile memory.
 */
uint8_t MFRC522::MIFARE_Restore(uint8_t blockAddr)
{
  // The datasheet describes Restore as a two step operation, but does not explain what data to transfer in step 2.
  // Doing only a single step does not work, so I chose to transfer 0L in step two.
  return MIFARE_TwoStepHelper(PICC_CMD_MF_RESTORE, blockAddr, 0L);
} // End MIFARE_Restore()

/*
 * Helper function for the two-step MIFARE Classic protocol operations Decrement, Increment and Restore.
 */
uint8_t MFRC522::MIFARE_TwoStepHelper(uint8_t command, uint8_t blockAddr, uint32_t data)
{
  uint8_t result;
  uint8_t cmdBuffer[2]; // We only need room for 2 bytes.

  // Step 1: Tell the PICC the command and block address
  cmdBuffer[0] = command;
  cmdBuffer[1] = blockAddr;

  // Adds CRC_A and checks that the response is MF_ACK.
  result = PCD_MIFARE_Transceive(cmdBuffer, 2);
  if (result != STATUS_OK)
  {
    return result;
  }

  // Step 2: Transfer the data
  // Adds CRC_A and accept timeout as success.
  result = PCD_MIFARE_Transceive((uint8_t *) &data, 4, true);
  if (result != STATUS_OK)
  {
    return result;
  }

  return STATUS_OK;
} // End MIFARE_TwoStepHelper()

/*
 * MIFARE Transfer writes the value stored in the volatile memory into one MIFARE Classic block.
 */
uint8_t MFRC522::MIFARE_Transfer(uint8_t blockAddr)
{
  uint8_t cmdBuffer[2]; // We only need room for 2 bytes.

  // Tell the PICC we want to transfer the result into block blockAddr.
  cmdBuffer[0] = PICC_CMD_MF_TRANSFER;
  cmdBuffer[1] = blockAddr;

  // Adds CRC_A and checks that the response is MF_ACK.
  return PCD_MIFARE_Transceive(cmdBuffer, 2);
} // End MIFARE_Transfer()


/////////////////////////////////////////////////////////////////////////////////////
// Support functions
/////////////////////////////////////////////////////////////////////////////////////

/*
 * Wrapper for MIFARE protocol communication.
 * Adds CRC_A, executes the Transceive command and checks that the response is MF_ACK or a timeout.
 */
uint8_t MFRC522::PCD_MIFARE_Transceive(uint8_t *sendData, uint8_t sendLen, bool acceptTimeout)
{
  uint8_t result;
  uint8_t cmdBuffer[18]; // We need room for 16 bytes data and 2 bytes CRC_A.

  // Sanity check
  if (sendData == NULL || sendLen > 16)
  {
    return STATUS_INVALID;
  }

  // Copy sendData[] to cmdBuffer[] and add CRC_A
  memcpy(cmdBuffer, sendData, sendLen);
  result = PCD_CalculateCRC(cmdBuffer, sendLen, &cmdBuffer[sendLen]);
  if (result != STATUS_OK)
  {
    return result;
  }

  sendLen += 2;

  // Transceive the data, store the reply in cmdBuffer[]
  uint8_t waitIRq = 0x30;    // RxIRq and IdleIRq
  uint8_t cmdBufferSize = sizeof(cmdBuffer);
  uint8_t validBits = 0;
  result = PCD_CommunicateWithPICC(PCD_Transceive, waitIRq, cmdBuffer, sendLen, cmdBuffer, &cmdBufferSize, &validBits);
  if (acceptTimeout && result == STATUS_TIMEOUT)
  {
    return STATUS_OK;
  }

  if (result != STATUS_OK)
  {
    return result;
  }

  // The PICC must reply with a 4 bit ACK
  if (cmdBufferSize != 1 || validBits != 4)
  {
    return STATUS_ERROR;
  }

  if (cmdBuffer[0] != MF_ACK)
  {
    return STATUS_MIFARE_NACK;
  }

  return STATUS_OK;
} // End PCD_MIFARE_Transceive()


/*
 * Translates the SAK (Select Acknowledge) to a PICC type.
 */
uint8_t MFRC522::PICC_GetType(uint8_t sak)
{
  uint8_t retType = PICC_TYPE_UNKNOWN;

  if (sak & 0x04)
  { // UID not complete
    retType = PICC_TYPE_NOT_COMPLETE;
  }
  else
  {
    switch (sak)
    {
      case 0x09: retType = PICC_TYPE_MIFARE_MINI; break;
      case 0x08: retType = PICC_TYPE_MIFARE_1K;   break;
      case 0x18: retType = PICC_TYPE_MIFARE_4K;   break;
      case 0x00: retType = PICC_TYPE_MIFARE_UL;   break;
      case 0x10:
      case 0x11: retType = PICC_TYPE_MIFARE_PLUS; break;
      case 0x01: retType = PICC_TYPE_TNP3XXX;     break;
      default:
        if (sak & 0x20)
        {
          retType = PICC_TYPE_ISO_14443_4;
        }
        else if (sak & 0x40)
        {
          retType = PICC_TYPE_ISO_18092;
        }
        break;
    }
  }

  return (retType);
} // End PICC_GetType()

/*
 * Returns a string pointer to the PICC type name.
 */
char* MFRC522::PICC_GetTypeName(uint8_t piccType)
{
  if(piccType == PICC_TYPE_NOT_COMPLETE)
  {
    piccType = MFRC522_MaxPICCs - 1;
  }

  return((char *) _TypeNamePICC[piccType]);
} // End PICC_GetTypeName()

/*
 * Returns a string pointer to a status code name.
 */
char* MFRC522::GetStatusCodeName(uint8_t code)
{
  return((char *) _ErrorMessage[code]);
} // End GetStatusCodeName()

/*
 * Calculates the bit pattern needed for the specified access bits. In the [C1 C2 C3] tupples C1 is MSB (=4) and C3 is LSB (=1).
 */
void MFRC522::MIFARE_SetAccessBits(uint8_t *accessBitBuffer,  
                                   uint8_t g0,                
                                   uint8_t g1,                
                                   uint8_t g2,                
                                   uint8_t g3)
{
  uint8_t c1 = ((g3 & 4) << 1) | ((g2 & 4) << 0) | ((g1 & 4) >> 1) | ((g0 & 4) >> 2);
  uint8_t c2 = ((g3 & 2) << 2) | ((g2 & 2) << 1) | ((g1 & 2) << 0) | ((g0 & 2) >> 1);
  uint8_t c3 = ((g3 & 1) << 3) | ((g2 & 1) << 2) | ((g1 & 1) << 1) | ((g0 & 1) << 0);

  accessBitBuffer[0] = (~c2 & 0xF) << 4 | (~c1 & 0xF);
  accessBitBuffer[1] =          c1 << 4 | (~c3 & 0xF);
  accessBitBuffer[2] =          c3 << 4 | c2;
} // End MIFARE_SetAccessBits()

/////////////////////////////////////////////////////////////////////////////////////
// Convenience functions - does not add extra functionality
/////////////////////////////////////////////////////////////////////////////////////

/*
 * Returns true if a PICC responds to PICC_CMD_REQA.
 * Only "new" cards in state IDLE are invited. Sleeping cards in state HALT are ignored.
 */
bool MFRC522::PICC_IsNewCardPresent(void)
{
  uint8_t bufferATQA[2];
  uint8_t bufferSize = sizeof(bufferATQA);
  uint8_t result = PICC_RequestA(bufferATQA, &bufferSize);
  return ((result == STATUS_OK) || (result == STATUS_COLLISION));
} // End PICC_IsNewCardPresent()

/*
 * Simple wrapper around PICC_Select.
 */
bool MFRC522::PICC_ReadCardSerial(void)
{
  uint8_t result = PICC_Select(&uid);
  return (result == STATUS_OK);
} // End PICC_ReadCardSerial()