ST25R3910 datasheet(12/65 Pages) STMICROELECTRONICS

12 / 65 page

Functional overview

ST25R3910

12/65

DocID029768 Rev 2

this low power mode, only the Target Activation Detector, which will detect a presence of

external RF field, is active. Once the presence of external RF field is detected, an interrupt is

sent to the microcontroller, which will in turn switch on the oscillator and the receiver.

1.2.2

Transmitter

The transmitter contains two identical push-pull driver blocks connected to the pins RFO1

and RFO2. Each driver is composed of eight segments having binary weighted output

resistance. The MSB segment typical ON resistance is 3 Ω, when all segments are turned

on; the output resistance is typically 1.5 Ω. All segments are turned on to define the normal

transmission (non-modulated) level. It is also possible to switch off certain segments when

driving the non-modulated level to reduce the amplitude of the signal on the antenna and/or

to reduce the antenna Q factor without making any hardware changes.

AM modulation and operation of the driver segments is controlled by writing AM modulation

depth and antenna driver registers. Antenna Driver Non-Modulated Level Definition Register

defines which segments will be used to define normal, non-modulated level. Modulation

Depth Definition Register and Antenna Driver AM Modulated Level Definition Register are

used to define how the AM modulated level is set-up. It can be set-up automatically by

definition of modulation depth and the direct command Calibrate Modulation Depth or by a

direct definition of segments which are turned off during AM modulation.

1.2.3

Receiver

The receiver performs demodulation of the tag sub-carrier modulation that is superimposed

on the 13.56 MHz carrier frequency. It performs AM and/or PM demodulation, amplification,

band-pass filtering and digitalization of sub-carrier signals (848, 424 and 212 kHz subcarrier

frequencies are supported). Additionally it performs RSSI measurement, automatic gain

control (AGC) and Squelch.

The receiver is switched on when Operation Control Register bit rx_en is set. The operation

of the receiver is additionally controlled by the signal rx_on, set high when modulated signal

is expected on the receiver input. This is automatically done after every Transmit command.

Signal rx_on can be also forced high by sending direct command Unmask Receive Data.

Signal rx_on is used to control features like RSSI and AGC.

AM demodulation is performed using a peak follower. Both the positive and negative peaks

are tracked to suppress common mode signal. In case external demodulation is carried out

the peak follower stage can be bypassed by setting bit envi in Configuration Register 2. In

case of PM demodulation signal coming from the phase detector is replacing the output of

peak follower.

Next stage in signal processing is the buffer amplifier followed by second order low pass

filter with adjustable corner frequency. Final stage is a first order high pass filter with

adjustable corner frequency. The digital signal representing tag subcarrier modulation is

produced by a window comparator.

Filter setting is done automatically when ISO mode and data rate are chosen by writing ISO

Mode Definition Register. Setting is displayed in the Receiver Configuration Register and

can be changed by rewriting this register. In Transparent mode the ISO Mode Definition

mode. In this mode, power consumption is lower but receiver sensitivity is reduced (see

Section 3: Electrical characteristics on page 58).

ST25R3910 Datasheet(PDF) 12 Page - STMicroelectronics

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