ST25R3911B datasheet(14/116 Pages) STMICROELECTRONICS

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Mask receive timer

The mask receive timer is blocking the receiver and reception process in framing logic by keeping the rx_on

signal low after the end of Tx during the time the tag reply is not expected.

While the mask receive timer is running, the Squelch is automatically turned on (if enabled). mask receive timer

does not produce an IRQ.

The mask receive timer timeout is configured in the Mask receive timer register.

In the NFCIP-1 active communication mode the mask receive timer is started when the peer NFC device (a

device with whom communication is going on) switches on its field.

The mask receive timer has a special use in the low power Initial NFC Target Mode. After the initiator field has

been detected the controller turns on the oscillator, regulator and receiver. mask receive timer is started by

sending direct command Start mask receive Timer. After the mask receive Timer expires the receiver output starts

to be observed to detect start of the initiator message. In this mode the mask receive timer clock is additionally

divided by eight it (one count is 512/fc) to cover range up to about 9.6 ms.

No-response timer

As its name indicates, this timer is intended to observe whether a tag response is detected in a configured time

started by end of transmission. The I_nre flag in the Timer and NFC interrupt register is signaling interrupt events

resulting from this timer timeout.

The no-response timer is configured by writing the two registers No-response timer register 1 and No-response

timer register 2. Operation options of the no-response timer are defined by setting bits nrt_emv and nrt_step in the

General purpose and no-response timer control register.

Bit nrt_step configures the time step of the no-response timer. Two steps are available, 64/fc (4.72 μs) to cover

range up to 309 ms, and 4096/fc, covering the range up to 19.8 s.

Bit nrt_emv controls the timer operation mode:

•

When this bit is set to 0 (default mode) the IRQ is produced in case the no-response timer expires before a

start of a tag reply is detected and rx_on is forced to low to stop receiver process. In the opposite case,

when start of a tag reply is detected before timeout, the timer is stopped, and no IRQ is produced.

•

When this bit is set to 1 the timer unconditionally produces an IRQ when it expires, it is also not stopped by

direct command Clear. This means that IRQ is independent of the fact whether or not a tag reply was

detected. In case at the moment of timeout a tag reply is being processed no other action is taken, in the

opposite case, when no tag response is being processed additionally the signal rx_on is forced to low to stop

receive process.

The no-response timer can also be started using direct command Start no-response timer. The intention of this

command is to extend the no-response timer timeout beyond the range defined in the no-response timer control

registers. In case this command is sent while the timer is running, it is reset and restarted. In NFCIP-1 active

communication mode the no-response timer cannot be started using the direct command.

In case this timer expires before the peer NFC device (a device with whom communication is going on) switches

on its field an interrupt is sent.

In all modes, where timer is set to nonzero value, it is a must that M_txe is not set and interrupt I_txe is read via

SPI for synchronization between transmitter and timer.

General purpose timer

The triggering of the general purpose timer is configured by setting the General purpose and no-response timer

control register. It can be used to survey the duration of the reception process (triggering by start of reception,

after SOF) or to time out the PCD to PICC response time (triggered by end of reception, after EOF). In the

NFCIP-1 active communication mode it is used to timeout the field switching off. In all cases an IRQ is sent when

it expires.

The general purpose timer can also be started by sending the direct command Start general purpose timer. In

case this command is sent while the timer is running, it is reset and restarted.

ST25R3911B

Application information

DS11793 - Rev 6

page 14/116

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