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Edit:

based on the below picture as you can see ,in ISO 14443-3 when we send REQA command to the PCD,all PICC in the reading field will answer with their ATQR. If PCD receives more than ATQR ,this will interpreted to There are more than 1 PICC in the field and PCD must perform anti-collision loop to select right PICC.

I test this with mifare classic PICC and there are no differences when there are a PICC in the field or there are multi,all times ATRQ= 0x0004 will recieved. (I send REQA command in 100 ms periods)

My question is how PCD received multi ATQR? Are they received in turn and continuously? If yes then how they synchronous to determine which one must send ATRQ first? and why I only received one ATRQ when there are multi PICC in the field? 2- Also ISO 14443-3 described coding format of the ATRQ that b1~b5 bits are used for bit frame anti-collision ,is this a random value or its depend on each PICC family or ?

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  • Are you asking specifically about collisions in the ATQA (in that case you would certainly need to tell us what ATQA both cards send) or about collisions during enumeration based on the UID (hence, during the next phase after the REQA/ATQA)?
    – Michael Roland
    Commented Oct 31, 2016 at 20:09

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See 6.2.1.1 of ISO14443-3:

All PICCs in the field shall respond in a synchronous way to the commands REQA, WUPA, ANTICOLLISION and SELECT. This is needed for anticollision.

In your case, all your cards are replying synchronously to REQA, and this makes no collision as they transmit the very same data stream. No collision while receiving ATQA does not mean there is only one PICC in the field, it only means they are all of the same type (roughly).

Whatever the response to ATQA (with or without collisions), you must perform a SELECT, which will eliminate collisions for sure.

After selection, only one card will reply to REQA, and you'll get an ATQA with no collisions.

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  • Thank you for reply,But if there are different type of PICC in the field(e.g. MF classic and MF Desfire) I receive same answer.And what about coding of ATQA?
    – Dzzz
    Commented Nov 12, 2016 at 4:46
  • See AN10833 from NXP. There si an intersting side note in it: "Never use ATQA to identify a chip or to extract UID size."
    – Nipo
    Commented Nov 12, 2016 at 17:35

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