ST25R3918 datasheet(34/141 Pages) STMICROELECTRONICS

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Application information

ST25R3918

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DS13490 Rev 2

Figure 7. Phase detector inputs and output in case of 135º phase shift

Amplitude detector

Signals from pins RFI1 and RFI2 are used as inputs to the self-mixing stage. The output of

this stage is a DC voltage proportional to the amplitude of signals on pins RFI1 and RFI2.

During execution of direct command Measure amplitude this output is multiplexed to the A/D

converter input.

4.2.8

External field detector

This block is used to detect the presence of an external device generating an RF field. It is

used in Passive target modes. It is enabled by en_fd_c<1:0> option bits. The external field

detector supports two different detection thresholds, namely Peer detection and Collision

avoidance. The two thresholds can be independently set by writing the External field

detector activation threshold register. The actual state of the detector output can be

checked by reading the Auxiliary display register. Input to this block is the signal from the

RFI1 pin.

For both thresholds there is a possibility to separately set the activation and deactivation

levels.

If the External field level is not detected yet, the Activation threshold is used. If the External

field level is detected, the Deactivation threshold is used.

The Activation threshold must be set higher than or equal to the Deactivation threshold.

If the Activation is higher than the Deactivation, the hysteresis is given by the difference

between the two levels.

If the Activation and Deactivation levels are equal, there is no the hysteresis in the system

and multiple field-on/off events can verify if the actual field level persists in proximity of the

selected threshold.

Peer detection threshold

This threshold is used to detect the field emitted by peer NFC device with whom NFC

communication is going on. It can be selected in the range from 75 to 800 mVPP. When this

threshold is enabled the detector is in low power mode. An interrupt is generated when an

external field is detected and also when it is switched off. With such implementation it can

also be used to detect the moment when the external field disappears. This can be used to

MS42427V1

VSP_A

VSP_A/2

Input 1

Input 2

Output

ST25R3918 Datasheet(PDF) 34 Page - STMicroelectronics