ST25R3911B datasheet(39/116 Pages) STMICROELECTRONICS

39 / 116 page

According to ISO 14443A the FDT must be 1236/fc if last transmitted bit is 1, or 1172/fc if last transmitted bit

is 0. Figure 22 shows an example of how MRT and NRT timers are set for a given FDT.

Figure 22. Selection of MRT and NRT for a given FDT

PCD to PICC

FDT

MRT < FDT – 64/fc

NRE > FDT + 64/fc

PICC to PCD

The receiver and transmitter operation options related to operation mode must be defined. This is done

automatically by sending the direct command Analog Preset. If different options are required apart from

those defined by Analog Preset, they must be additionally set by writing the appropriate registers.

Set rx_en and tx_en in the Operation control register. RFID protocols usually require that the reader field is

turned on for a while before sending the first command (5 ms for ISO14443). General purpose timer can be

used to count this time.

The reply from PICC for the REQA, WUPA, and replies within anticollision sequence 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 these two commands is simple since they are implemented as 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 ST25R3911B 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 ST25R3911B signals, the

microcontroller must consider as a valid presence of tag and must proceed with the anticollision procedure.

If more than one PICC is expected in the field, the following algorithm must be used to select multiple tags:

Send REQA, if there is any answer continue

Perform anticollision, and select one PICC

Send HLTA to move the selected PICC to the HALT state

Go to step 1, and repeat this procedure until all the PICCs are in HALT state and all the UIDs have been

extracted.

Anticollision procedure

After receiving the ATQA from the tags in the field, the next step is to execute the anticollision procedure to

resolve the IDs of the tags.

The procedure mainly uses the ANTICOLLISION and SELECT commands, which consist of:

•

Select code SEL (1 byte)

•

Number of valid bits NVB (1 byte)

•

0 to 40 data bits of UID CLn according to the value of NVB

ST25R3911B

Application information

DS11793 - Rev 6

page 39/116

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