ST25R3916 datasheet(25/157 Pages) STMICROELECTRONICS

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DS12484 Rev 4

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ST25R3916/7

Application information

156

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).

Slow transmitter ramping

When the transmitter is enabled it starts to drive the antenna LC tank with full power, the

ramping of field emitted by antenna is defined by antenna LC tank Q factor.

However there are some reader systems where the reader field has to ramp up with a

longer transition time when it is enabled. The STIF (Syndicat des transports d'Ile de France)

specification requires a transition time from 10 to 90% of field longer than or equal to 10 μs.

The ST25R3916/7 supports slow transmitter ramping by collapsing VDD_RF regulated

voltage when transmitter is disabled and ramping it when transmitter is enabled. Typical

transition time is 15 μs at 3 V supply and 20 μs at 5 V supply.

Procedure to implement the slow transition:

When transmitter is disabled set IO configuration register 2 bit slow_up to 1. Keep this

state at least 2ms to allow discharge of VDD_RF.

Enable transmitter, its output will ramp slowly

Before sending any command set the bit slow_up back to 0.

ST25R3916 Datasheet(PDF) 25 Page - STMicroelectronics

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