NFC - multiple tags reading, PN532 library modifications

Products & Technology Shields
#1

Hello,

I’ve got NFC Shield 1.0v connected to Arduino. I would like to read multiple tags ID (one by one, not at once). Correct me if I wrong, but from what I have read - if I put mifare tag into a NFC field and after activation I send HALT (HLTA) it should stay inactive until I take it away from the field and put it again. So, until it is inactive (halt) I should be able to read another tag that comes into the field. My problem is, I don’t know how to send HALT to the tag.

I made little change in the PN532 library in function ‘readPassiveTargetID’. In order to PN532 datasheet I can add UID of specified tag I want to find and activate to InListPassiveTarget command. So I modified function in PN532.cpp to do that:

uint32_t PN532::readPassiveTargetID(uint8_t UIDx[4])  // Pass 4-bytes UID 
{
  uint32_t cid;

  pn532_packetbuffer[0] = PN532_INLISTPASSIVETARGET;
  pn532_packetbuffer[1] = 1;  // max 1 cards at once (we can set this to 2)
  pn532_packetbuffer[2] = PN532_MIFARE_ISO14443A;
  
  pn532_packetbuffer[3] = UIDx[0];
  pn532_packetbuffer[4] = UIDx[1];
  pn532_packetbuffer[5] = UIDx[2];
  pn532_packetbuffer[6] = UIDx[3];    
  
  if (! sendCommandCheckAck(pn532_packetbuffer, 7))
    return 0x0;  // no cards read

  ...

}

It works well. I thought I could use this function in a loop passing UIDs I want to find, so I did something like this (arduino code):

  for(int t = 0; t < 6; t++)
  {
    delay(1000);
    boolean found = nfc.readPassiveTargetID(tags[t]);
 
    if(found)
    {
       Serial.print("Found tag nr: ");
       Serial.println(t+1);         
    }
  }

‘tags[]’ array contains known UIDs of six tags. It works good for 3-4 tags. If I add more then communication hangs. I would like to try send HLTA (InDeselect) to the found tag and then put ‘new’ tags one by one in order to get their IDs. I was trying to use InDeselect command in that way:

boolean releaseTarget()
{
  pn532_packetbuffer[0] = 0x52;
  pn532_packetbuffer[1] = 0x00; // realese all targets
  
   if (sendCommandCheckAck(pn532_packetbuffer, 2) )
    return true;

   return false;
}

But it did not work I suppose… Could you please help with this?

Thanks,
Adrian

#2

First of all, your function releaseTarget uses the opcode for InRelease and not the one for InDeselect. The good news is that using InRelease is the right way to go.

The PN532 requires you to release the targets in order to inlist new ones.

for(int t = 0; t < 6; t++)
  {
    delay(1000);
    boolean found = nfc.readPassiveTargetID(tags[t]);

    if(found)
    {
       Serial.print("Found tag nr: ");
       Serial.println(t+1);         
    }
    releaseTarget();
  }

I guess using InDeselect could also work. You could then use InSelect to operate on the target without inlisting it again. I never tried that way. But afaik the PN532 can keep more than 2 targets in the internal memory, but can only operate on 2 selected at the same time.

You should extend readPassiveTargetID in order to read two tags simultaneously.