ST25R3920 datasheet(16/152 Pages) STMICROELECTRONICS

16 / 152 page

Description

ST25R3920

16/152

DS13261 Rev 9

2.2.2

Receiver

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

receiver consists of two chains, 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

A built-in A/D converter, whose input can be multiplexed from different sources, 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

periodically measures the capacitance and compares the measured value to the stored

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

more than a register defined threshold then an interrupt is sent to the external controller.

ST25R3920 Datasheet(PDF) 16 Page - STMicroelectronics

ST25R3920 Datasheet(HTML) 16 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL