ST25R3911B datasheet(12/116 Pages) STMICROELECTRONICS

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The capacitance measurement is started by sending the direct command Measure Capacitance. The

ST25R3911B can also be configured to automatically wake-up and perform periodic capacitance measurements.

The result is compared to a stored reference or to an average of previous measurements and in case the

difference is greater than a predefined value an IRQ is triggered to wake-up the controller (see also Section

1.2.5 Wake-up mode).

The capacitive sensor gain can be adjusted in Capacitive sensor control register. The default gain is 2.8 V/pF

(typical value), the maximum gain is 6.5 V/pF (typical value). Since the LSB of the A/D converter corresponds to

approximately 7.8 mV, the default gain results in a sensitivity of 2.8 fF/LSB (1.2 fF/LSB in case of maximum gain).

The duration of the capacitance measurement is 200 μs, and the current consumption during the measurement is

1.1 mA (typical value). As an example, if the capacitive measurement is performed every 100 ms in Wake-Up

mode, then the resulting average current consumption is 5.8 μA (3.6 μA is the standby consumption in Wake-Up

mode).

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

Capacitive Sensor command and the resulting calibration value are stored in the Capacitive sensor display

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 register.

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 timer control register. The timeout period between the successive

detections and the measurements are selected in this register. Timeouts in the range from 10 to 800 ms are

available, 100 ms is the default value. Any combination of available measurements can be selected (one, two or

all of them).

The next twelve registers (32h to 3Dh) are configuring the three possible detection measurements and storing the

results, four registers are used for each measurement.

ST25R3911B

Application information

DS11793 - Rev 6

page 12/116

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