ST25R3916B datasheet(31/165 Pages) STMICROELECTRONICS

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DS13541 Rev 7

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ST25R3916B ST25R3917B

Application information

164

4.2.4

Wake-up mode

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

ST25R3916B and ST25R3917B into the Wake-up mode, used to perform low power

detection of card presence. The ST25R3916B and ST25R3917B feature two possibilities,

namely 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 performs a regular polling loop.

In Wake-up mode the ST25R3916B and ST25R3917B periodically perform 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.

Registers from 32h to 3Bh 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 which are going 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 3ADh configure the two 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 ST25R3916B and ST25R3917B 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

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.

ST25R3916B Datasheet(PDF) 31 Page - STMicroelectronics

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