ST25R3918 datasheet(56/141 Pages) STMICROELECTRONICS

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Application information

ST25R3918

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DS13490 Rev 2

This command does not update any register apart from the FIFO status registers. Therefore

it does not disable the field detector in CE mode (if it was enabled), and it does not switch off

the field (if it was enabled).

4.4.3

Clear FIFO

This direct command clears the FIFO and the FIFO status registers. It does not clear the

IRQ line or IRQ status bits.

To prepare a transmission send this command first before writing data into the FIFO. If a

Clear FIFO command is sent during an ongoing data transmission, then the data

transmission is aborted and FIFO and FIFO status registers are cleared.

4.4.4

Transmit commands

The transmit direct commands are used to start a data transmission from the ST25R3918.

They switch the device to reception mode after the transmission is completed.

Before sending commands Transmit with CRC and Transmit without CRC, direct command

Clear FIFO has to be sent, followed by the definition of the number of transmitted bytes and

writing data to be transmitted in FIFO.

Use the direct commands Transmit REQA and Transmit WUPA to transmit ISO14443A

short frame commands REQA and WUPA respectively. It is not necessary to send the direct

command Clear FIFO before these two commands.

If the antcl bit is set, then the number of valid bits in the last byte must be set to 0 (nbtx<2:0>

in the Number of transmitted bytes register 2) prior to the direct command Transmit REQA

or Transmit WUPA.

The direct commands Transmit REQA and Transmit WUPA automatically disable the CRC

check of the response frame. The CRC check is enabled again after any of the below

conditions:



Transmit with CRC direct command



Mask receive data direct command



No Response timer expires

4.4.5

NFC field ON commands

The NFC field ON direct commands are used to perform RF Collision Avoidance. The

external field detector must be enabled for these commands to work correctly.

To determine whether an external field is present the ST25R3918 compares the RF voltage

level on the RFI1 pin with the collision avoidance threshold defined in the External field

detector activation threshold register.

If no external field is detected, then the ST25R3918 transmitter is switched on automatically

(bit tx_en in the Operation control register is set) and an I_apon IRQ is signaled. After the

RF guard time defined in the NFC field on guard timer register has passed an I_cat IRQ is

signaled. At this point the controller can initiate a data transmission using a transmit

command.

If an external field is detected a I_cac IRQ is signaled, and the ST25R3918 transmitter stays

off.

ST25R3918 Datasheet(PDF) 56 Page - STMicroelectronics

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