ST25R3916 datasheet(16/157 Pages) STMICROELECTRONICS

16 / 157 page

Description

ST25R3916/7

16/157

DS12484 Rev 4

2.2.2

Receiver

The receiver detects card modulation superimposed on the 13.56 MHz carrier signal. The

receiver consists of two receive chains that are built from a set of demodulators, followed by

two gain and filtering stages and a final digitizer stage. The demodulators can operate as

AM/PM demodulator or as I/Q demodulator. The filter characteristics can be adjusted to

match the selected RF mode and bit rate to optimize performance (subcarrier frequencies

from 212 to 848 kHz are supported). Apart from the filter stage the receiver incorporates

several other features (AGC, squelch) that enable reliable operation in noisy conditions.

The receiver is connected to the antenna via the pins RFI1 and RFI2. The output of the

receiver is connected to the framing block that decodes the demodulated and digitized

subcarrier signal.

2.2.3

Phase and amplitude detector

The phase detector measures the phase difference between the transmitter output signals

(RFO1 and RFO2) and the receiver input signals (RFI1 and RFI2).

The amplitude detector measures the amplitude of the differential RF carrier signal between

the receiver inputs RFI1 and RFI2. This differential amplitude signal is directly proportional

to the amplitude of the RF signal on the antenna LC tank.

The phase- and amplitude detectors are used for several purposes:



PM demodulation, by observing RFI1 and RFI2 phase variations (LF signal is fed to the

receiver)



Average phase difference between RFOx pins and RFIx pins, to check antenna tuning



Measure amplitude of signal present on pins RFI1 and RFI2, proportional to the

antenna voltage

2.2.4

Automatic antenna tuning (AAT)

The AAT block consists of two independent 8-bit D/A converters. These converters generate

a programmable voltage (from 0 to 3.3 V) to control external variable capacitors.

Note:

Using hardware based wake-up in combination with the automatic antenna tuning feature is

not recommended. Contact your nearest ST office for further support.

2.2.5

A/D converter

The ST25R3916/7 features a built in A/D converter. Its input can be multiplexed from

different sources and it is used for the diagnostic functions and the low power card

detection. The result of the A/D conversion is stored in a register that can be read through

the host interface.

2.2.6

Capacitive sensor

The capacitive sensor is used to implement low power detection of card presence. It

measures the capacitance between two copper patches connected to the CSI and CSO

pins. This capacitance changes with the presence of an object like a card, or a hand.

During calibration the reference capacitance (representing parasitic capacitance of the

environment) is stored. In the capacitive low power card detection mode the device

(ST25R3916 only) periodically measures the capacitance and compares the measured

value to the stored reference value. If the measured capacitance differs from the stored

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