ST25R3918 datasheet(29/141 Pages) STMICROELECTRONICS

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DS13490 Rev 2

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ST25R3918

Application information

140

Clock extractor

The clock extractor observes the RFI1 and RFI2 differential signal and provides a clock

signal synchronous with the incoming RF field. The extracted clock is used for synchronous

demodulation, for correct frame delay time and for correct data timing during passive

transmission. The clock extractor is active down to 60 mVPP input signal.

4.2.3

Wake-up mode

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

ST25R3918 into the Wake-up mode, used to perform low power detection of card presence.

The ST25R3918 features phase and amplitude measurement for the Wake-up mode. 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 ST25R3918 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.

Registers from 32h to 3Ah 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 3Ah 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 ST25R3918 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

ST25R3918 Datasheet(PDF) 29 Page - STMicroelectronics

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