ST25R3916B-AQWT datasheet(32/165 Pages) STMICROELECTRONICS

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Application information

ST25R3916B ST25R3917B

32/165

DS13541 Rev 7

Wake-up mode configuration registers have to be configured before Wake-up mode is

actually entered. Any modification of Wake-up mode configuration while it is active may

result in unpredictable behavior.

Auto-averaging

In case of auto-averaging the reference value is recalculated after every measurement. The

last measurement value, the old reference value and the weight are used in this calculation.

The following formula is used to calculate the new reference value:

new_reference = old_reference - (old_reference - measured_value) / weight

The calculation is done on 10 bits to have sufficient precision.

The auto-averaging process is initialized when Wake-up mode is first time entered after

initialization (power-up or using Set default command). The initial value is taken from the

measurement reference register (for example Amplitude measurement reference register) if

the content of this register is not 0. If content of this register is 0, the result of first

measurement is taken as initial value.

Every measurement configuration register contains a bit defining whether the measurement

that causes an interrupt is taken in account for the average value calculation (for example

bit am_aam of Amplitude measurement reference register).

4.2.5

Quartz crystal oscillator

The quartz crystal oscillator operates with 27.12 MHz crystals; its operation is enabled when

the Operation control register bit en is set to 1. An interrupt is sent to inform the

microcontroller when the oscillator amplitude is sufficiently high, meaning the frequency is

stable (see Main interrupt register).

The status of oscillator can be observed by checking the Auxiliary display register bit

osc_ok. This bit is set to 1 when oscillator frequency is stable.

The oscillator is based on an inverter stage supplied by a controlled current source. A

feedback loop is controlling the bias current in order to regulate amplitude on XTI pin to

1VPP. To enable a fast reader start-up an interrupt is sent when oscillator amplitude

exceeds 750 mVPP.

Division by two assures that 13.56 MHz signal has a duty cycle of 50%, which is better for

the transmitter performance (no PW distortion).

The oscillator output is also used to drive a clock signal output pin MCU_CLK, which can be

used by the external microcontroller. The MCU_CLK pin is configured in the IO configuration

4.2.6

Timers

The ST25R3916B and ST25R3917B contain several timers, which eliminate the need to run

counters in the controller, thus reducing the effort of the controller code implementation and

improve portability of code to different controllers.

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.

ST25R3916B-AQWT データシート(PDF) 32 Page - STMicroelectronics

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