ST25R3920 datasheet(31/152 Pages) STMICROELECTRONICS

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DS13261 Rev 9

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ST25R3920

Application information

151

4.2.5

Wake-up mode

Asserting the Operation control register bit wu while the other bits are set to 0 puts the

ST25R3920 into the Wake-up mode, used to perform low power detection of card presence.

The ST25R3920 features three possibilities, namely capacitive sensor, phase and

amplitude measurement. An integrated low power 32 kHz RC oscillator and register

configurable Wake-up timer are used to schedule periodic detection.

Usually the presence of a card is detected by a so-called polling. In this process the reader

field is periodically turned on and the controller checks whether a card is present using RF

commands. This procedure consumes a lot of energy since reader field has to be turned on

for 5 ms before a command can be issued.

Low power detection of card presence is performed by detecting a change in the reader

environment, produced by a card. When a change is detected, an interrupt is sent to the

controller. As a result, the controller can perform a regular polling loop.

In Wake-up mode the ST25R3920 periodically performs the configured reader environment

measurements, and sends an IRQ to the controller when a difference to the configured

reference value is detected.

Card detection

The presence of a card close to the reader antenna coil produces a change of the antenna

LC tank signal phase and amplitude. The reader field activation time needed to perform the

phase or the amplitude measurement is extremely short (~20 μs) compared to the activation

time needed to send a protocol activation command.

The power level during the measurement can be lower than that during normal operation as

the card does not have to be powered to produce a coupling effect. The emitted power can

be reduced by changing the RFO driver resistance.

The capacitive sensor detects a change of the parasitic capacitance between the two

excitation electrodes. This change in capacitance can be caused by a card antenna or a

hand holding the card (see Section 2.2.6: Capacitive sensor).

Registers from 32h to 3Eh are dedicated to Wake-up configuration and display. The Wake-

up timer control register is the main Wake-up mode configuration register. The timeout

period between the successive detections and the measurements to be used are selected in

this register. Timeouts in the range from 10 to 800 ms are available, 100 ms being the

default value.

Registers from 33h to 3Dh configure the possible detection measurements and store the

results, four registers are used for each method.

An IRQ is sent when the difference between a measured value and reference value is larger

than configured threshold value. There are two possibilities how to define the reference

value:

•

The ST25R3920 can calculate the reference based on previous measurements

(auto-averaging)

•

The controller determines the reference and stores it in a register.

The first register in the series of four is the Amplitude measurement configuration register.

The difference to reference, which triggers the IRQ, the method of reference value

definition, and the weight of last measurement result in case of auto-averaging are defined

in this register. The next register stores the reference value when the reference is defined by

ST25R3920 Datasheet(PDF) 31 Page - STMicroelectronics

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