ST25R3918 datasheet(31/142 Pages) STMICROELECTRONICS

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DS13490 Rev 1

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ST25R3918

Application information

141

Every timer has one or more associated configuration registers in which the timeout

duration and different operating modes are defined. These configuration registers have to

be set while the corresponding timer is not running. Any modification of timer configuration

while the timer is active may result in unpredictable behavior.

All timers are stopped by the direct command Stop all activities.

Mask receive timer (MRT)

In Reader mode this 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 Mask Receive timer is running the Squelch is automatically turned on when

enabled. The MRT does not produce an IRQ.

The MRT timeout is configured in the Mask receive timer register and is automatically

started at the end of data transmission (at the end of EOF).

The MRT can be triggered by direct command Start Mask-receive timer. In this case the

squelch is enabled, according to the Squelch timer register.

In the NFCIP-1 Passive Target communication mode the MRT is started when the other

device turns on its field and the external field detector signals I_eon.

MRT supports a longer timing needed for NFCIP1 by setting option bit mrt_step. The bit

switches between fc/64 and fc/512 step size.

The MRT starts also in the low power Initial NFC target mode. After the initiator field has

been detected the controller turns on the 27 kHz RC oscillator, regulator, crystal oscillator,

receiver and MRT. After the MRT expires the receiver output starts to be observed to detect

start of the initiator message.

For correct operation in the low power Initial NFC target mode the mrt_step = 1 must be

used. The 27 kHz RC oscillator is used as a MRT clock source for the time before the crystal

oscillator stabilises. This enables that the actual MRT time is a good approximation to the

targeted time, also in case the crystal oscillator is not running yet.

No-response timer (NRT)

The purpose of this timer is intended to observe whether a response is detected during 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 NRT is configured by writing No-response timer register 1 and No-response timer

and EMV control register.

The NRT is automatically started at the end of transmission.

Bit nrt_step configures the time step of the No-response timer. Two steps are available,

64/fc (4.72 μs) and 4096/fc, covering, respectively, the range up to 309 ms and 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 if the NRT expires before a

start of a response is detected. The rx_on is set low to disable the receiver. In the

opposite case, when the 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 Stop all activities. This means that the IRQ is

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