ST25R3920B datasheet(23/158 Pages) STMICROELECTRONICS

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DS13890 Rev 4

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ST25R3920B

Application information

157

The driver load modulation is selected by bit lm_dri and the external MOS modulation is

selected by lm_ext option bits.

Normally, the internal driver or the external load modulation should be used exclusively, but

the device also allows simultaneous modulation.

The driver load modulation is based on the change of driver impedance. Typically, a high

impedance during non-modulated state and a lower impedance for the modulated state is

used. This yields modulation phase equal to passive tag modulation. It is also possible to

reverse the polarity of the driver load modulation by using low impedance during

non-modulated state and higher impedance for the modulated state.

During the non-modulated state the output impedance is defined by pt_res3:0 option bits.

During modulation the output impedance is defined by ptm_res3:0 option bits.

Load modulation through an external MOS transistor and a diode is selected by the lm_ext

option bit. In this case the EXT_LM pin is driven by the digital representation of the load

modulation signal (848 kHz subcarrier or 424 / 212 kHz modulation signal). The EXT_LM is

used to drive a gate of the external modulation MOS. The bit lm_ext_pol sets inverse

polarity for the external load modulation.

The pt_res3:0 and ptm_res3:0 bits must be set prior entering passive target mode (reg 03h),

because in passive target mode the resistance value propagates through the TX driver only

when the extracted clock is available.

Driver load modulation is based on change of the driver impedance. Typically high

impedance is used during non-modulated state, and decreased for modulated state,

resulting in modulation phase equal to Passive tag modulation.

It is also possible to set inverse polarity driver load modulation by using low impedance

during non-modulated state and higher impedance for the modulated state.

During non-modulated / modulated state the output impedance is defined, respectively, by

pt_res3:0 / ptm_res3:0 option bits.

An external MOS transistor and a diode modulation is selected by lm_ext option bit. In this

case the EXT_LM pin is driven by digital representation of the load modulation signal

(848 kHz subcarrier or 424 / 212 kHz modulation signal). The EXT_LM is used to drive a

gate of the external modulation MOS.

Bit lm_ext_pol sets inverse polarity for the External load modulation.

Bits pt_res<3:0> and ptm_res<3:0> must be set before entering Passive target mode (reg

03h), as in Passive target mode the resistance value propagates through the TX driver only

when extracted clock is available (during PT data transmission, including FDT).

4.2.2

Receiver

The receiver performs demodulation of the tag subcarrier modulation that is superimposed

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

band-pass filtering and digitalization of subcarrier signals. It also performs RSSI

measurement, automatic gain control (AGC) and Squelch function.

The reception chain has two separate channels for AM and PM demodulation. When both

channels are active the selection for reception framing is done automatically by the receiver

logic. The receiver is switched on when Operation control register bit rx_en is set to 1.

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