ST25R3920 datasheet(30/152 Pages) STMICROELECTRONICS

30 / 152 page

Application information

ST25R3920

30/152

DS13261 Rev 9

Figure 6. Capacitive sensor block diagram

Any conductive object (human hand or tag) approaching the two electrodes changes the

capacitance between the excitation and sensing electrode as it 'shortens' the distance

between the two by providing conductance on the part of the path between the two

electrodes.

The capacitance measurement is started by sending the direct command Measure

capacitance. The ST25R3920 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 if the difference is greater than a predefined value

an IRQ is triggered to wake up the controller (see also Section 4.2.5).

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).

Capacitive 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 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 this

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

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.

MS42418V1

Oscillator

A/D converter

Synchronous

rectifier

CSO

CSI

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