AS3911 Datasheet, PDF (122/149 Pages) ams AG – NFC Initiator / HF Reader IC

Application Information
7. The reply form PICC for the REQA, WUPA and replies
within ANTICOLLISION sequence before till before SAK
do not contain CRC. In this case the no_CRC_rx bit in the
Auxiliary Definition Register must be set to 1 (receive
without CRC) before sending these commands.
REQA and WUPA
Sending of these two commands is simple since they are
implemented as the AS3911 direct commands (Transmit REQA
and Transmit WUPA). The end of transmission of these
commands is signaled to microcontroller by an interrupt - IRQ
due to end of transmission). After the transmission is executed,
the AS3911 receiver automatically starts to observe the RFI
inputs to detect a transponder after the expiration of the Mask
Receive Timer.
As a response to REQA (or WUPA) all the PICC in the field respond
simultaneously with an ATQA. A collision can occur in this state
if there are PICC with different UID size or has the Bit frame
anticollision bits set differently. Hence it is important to set the
antcl bit to 1. If there is any IRQ (except I_nre) that AS3911
signals, the microcontroller must consider as a valid presence
of tag and must proceed with the ANTICOLLISION procedure.
If more than one PICC are expected in the field, following
algorithm must be used to select multiple tags:
1. Send REQA, if there was any answer continue
2. Perform anticollision, and singulate one PICC
3. Select the found Tag and send HLTA to move it to HALT
state
4. Go to 1 and repeat this procedure till all the PICC are in
HALT state and all the UIDs have been extracted.
AS3911 – 122
ams Datasheet, Confidential: 2013-Oct [2-03]