ST25R3911B datasheet(23/129 Pages) STMICROELECTRONICS

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ST25R3911B

Functional overview

Capacitor sensor calibration

The capacitive sensor comprises a calibration unit that internally compensates the parasitic

capacitances between CSI and CSO, thus leaving full measurement range for information

about capacitance variation. Five bits are used to control the calibration. The minimum

calibration step and the available calibration range are, respectively, 0.1 pF and 3.1 pF. The

calibration can be done manually by writing to the Capacitive Sensor Control Register or

automatically by sending the direct command Calibrate Capacitive Sensor. The status of the

Calibrate Capacitive Sensor command and the resulting calibration value are stored in the

Capacitive Sensor Display Register.

In order to avoid interference of the capacitive sensor with the Xtal oscillator and the reader

magnetic field and to assure repetitive results it is strongly recommended to perform

capacitance measurement and calibration in Power-down mode only.

1.2.5

Wake-Up mode

Asserting the Operation Control Register bit wu while the other bits are set to 0 puts the

ST25R3911B in Wake-Up mode, used to perform low power detection of card presence.

The ST25R3911B includes several possibilities of low power detection of a card presence

(capacitive sensor, phase measurement, amplitude measurement). An integrated low power

32 kHz RC oscillator and a register configurable Wake-Up timer are used to schedule

periodic detection.

Usually the presence of a card is detected by a so-called polling loop. 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 the 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 the Wake-Up mode the ST25R3911B periodically performs the configured reader

environment measurements and sends an IRQ to the controller when a difference to the

configured reference value is detected.

Detection of card presence can be done by performing phase,amplitude and capacitive

sensor measurements.

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.

Additionally the power level during the measurement can be lower than the power level

during normal operation since the card does not have to be powered to produce a coupling

effect. The emitted power can be reduced by changing the RFO Normal Level Definition

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 1.1.5: Capacitive sensor for a detailed information on

the capacitive sensor.

The registers on locations from 31h to 3Dh are dedicated to Wake-Up timer configuration

and display. The Wake-Up Timer Control Register is the main Wake-Up mode configuration

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