ST25R3911B datasheet(3/116 Pages) STMICROELECTRONICS

3 / 116 page

1.1.1

Transmitter

The transmitter incorporates drivers that drive external antenna through pins RFO1 and RFO2. Single sided and

differential driving is possible. The transmitter block additionally contains a sub-block that modulates transmitted

signal (OOK or configurable AM modulation).

The ST25R3911B transmitter is intended to directly drive antennas (without 50 Ω cable, usually antenna is on the

same PCB). Operation with 50 Ω cable is also possible, but in that case some of the advanced features are not

available.

1.1.2

Receiver

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

contains two receive chains (one for AM and another for PM demodulation) composed of a peak detector followed

by two gain and filtering stages and a final digitizer stage. The filter characteristics are adjusted to optimize

performance for each mode and bit rate (sub-carrier frequencies from 212 kHz to 6.8 MHz are supported). The

receiver chain inputs are the RFI1 and RFI2 pins. The receiver chain incorporates several features that enable

reliable operation in challenging phase and noise conditions.

1.1.3

Phase and amplitude detector

The phase detector is observing the phase difference between the transmitter output signals (RFO1 and RFO2)

and the receiver input signals (RFI1 and RFI2). The amplitude detector is observing the amplitude of the receiver

input signals (RFI1 and RFI2) via self-mixing. The amplitude of the receiver input signals (RFI1 and RFI2) is

directly proportional to the amplitude of the antenna LC tank signal.

The phase detector and the amplitude detector can be used for the following purposes:

•

PM demodulation, by observing RFI1 and RFI2 phase variation

•

Average phase difference between RFOx pins and RFIx pins is used to check and optimize antenna tuning

•

Amplitude of signal present on RFI1 and RFI2 pins is used to check and optimize antenna tuning

1.1.4

A/D converter

The ST25R3911B contains a built-in analog to digital (A/D) converter. Its input can be multiplexed from different

sources, and it is used in several applications (such as measurement of RF amplitude and phase, calibration of

modulation depth). The result of the A/D conversion is stored in the A/D converter Output register and can be

read via SPI.

1.1.5

Capacitive sensor

The capacitive sensor is used to implement low power detection of transponder presence, it measures the

capacitance between two copper patches connected to the CSI and CSO pins. The capacitance changes with the

presence of an object (card, hand). During calibration the reference capacitance (representing parasitic

capacitance of the environment) is stored. In normal operation the capacitance is periodically measured and

compared 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.

1.1.6

External field detector

The External field detector is a low power block used in NFC mode to detect the presence of an external RF field.

It supports two different detection thresholds, Peer Detection Threshold and Collision Avoidance Threshold. The

Peer Detection Threshold is used in the NFCIP-1 target mode to detect the presence of an initiator field, and is

also used in active communication initiator mode to detect the activation of the target field. The Collision

Avoidance Threshold is used to detect the presence of an RF field during the NFCIP-1 RF Collision Avoidance

procedure.

1.1.7

Quartz crystal oscillator

The quartz crystal oscillator can operate with 13.56 MHz and 27.12 MHz crystals. At start-up the

transconductance of the oscillator is increased to achieve a fast start-up. The start-up time varies with crystal

type, temperature and other parameters, hence the oscillator amplitude is observed and an interrupt is sent when

stable oscillator operation is reached. The use of a 27.12 MHz crystal is mandatory for VHBR operation.

The oscillator block also provides a clock signal to the external microcontroller (MCU_CLK), according to the

settings in the IO configuration register 1.

ST25R3911B

Block diagram

DS11793 - Rev 6

page 3/116

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