ST25R3916 datasheet(31/157 Pages) STMICROELECTRONICS

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DS12484 Rev 4

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ST25R3916/7

Application information

156

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

Antenna tuning

The ST25R3916/7 support antenna tuning through external variable capacitors. The

variable capacitor is connected on its position in the matching network, and the tuning

control voltage is connected to one of the control output pins (AAT_A, AAT_B).

The variable capacitors can be connected in series and parallel configurations in the

matching network. Further information on the various configuration options can be found in

the application note AN5322, available on www.st.com.

The phase and amplitude detector block is used for resonance frequency checking. The

algorithm in the MCU evaluates the result and adjusts the tuning voltages at AAT_A and

AAT_B output pins via the Antenna tuning control register 1 and Antenna tuning control

The AAT_A/B pin voltages are actively set according to Antenna tuning control register 1

and Antenna tuning control register 2 when en and aat_en option bits are both set.

If aat_en=1 and en=0, the AAT_A/B voltages are set to a fixed value between 1.5 and 2.2 V

(typically 1.9 V).

4.2.4

Capacitive sensor

The capacitive sensor enables measuring a capacitance between two pads, it can be used.

as an example, for Wake-up mode / Low power card detection.

The capacitive measurement system comprises two electrodes, one is the excitation

electrode emitting an electrical field of a fixed frequency in the range of a few hundred kHz

(CSO), and the second one is the sensing electrode (CSI). The amount of charge generated

in the sensing electrode represents the capacitance between the two electrodes. The

capacitive sensor electrodes are tolerant to parasitic capacitance to ground (up to 25 pF)

and to input leakage (up to 1 MΩ).

As the charge on the sensing electrode is generated with the frequency of the excitation

electrode, a synchronous rectifier is used to detect it. This ensures good rejection of

interference and high tolerance to parasitic capacitances (to all nodes except the excitation

electrode). The synchronous rectifier output is a DC voltage linearly proportional to the

capacitance between the excitation and sensing electrode. The output DC voltage is

converted by the A/D converter in absolute mode. The result is stored in the A/D converter

output register (see also Section 2.2.5: A/D converter).

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