ST25R3920 datasheet(32/152 Pages) STMICROELECTRONICS

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ST25R3920

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DS13261 Rev 9

the controller. The following two registers are display registers: the first one stores the

auto-averaging reference, the second one stores the result of the last measurement.

Wake-up mode configuration registers must be configured before Wake-up mode is actually

entered. Any modification of Wake-up mode configuration while it is active can 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 entered for the first time

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

the measurement reference register (for example Amplitude measurement reference

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).

4.2.6

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 that 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 the oscillator frequency is stable.

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

feedback loop controls the bias current to regulate amplitude on XTI pin to 1 VPP. To enable

a fast reader start-up, an interrupt is sent when the oscillator amplitude exceeds 750 mVPP.

Division by two assures that the 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

4.2.7

Timers

Several timers are integrated in the device, to eliminate the need to run counters in the

controller, thus reducing the effort of code implementation and improve its portability 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 can result in unpredictable behavior.

ST25R3920 Datasheet(PDF) 32 Page - STMicroelectronics

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