ST25R3911B datasheet(16/116 Pages) STMICROELECTRONICS

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Figure 6. Phase detector inputs and output in case of 90º phase shift

VSP_A

VSP_A/2

Input 1

Input 2

Output

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

VSP_A

VSP_A/2

Input 1

Input 2

Output

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 amplitude of signal on pins RFI1 and RFI2. During execution of direct command Measure

Amplitude this output is multiplexed to A/D converter input.

1.2.10

External field detector

The External Field Detector is used to detect the presence of an external device generating an RF field. It is

automatically switched on in NFCIP-1 active communication modes; it can also be used in other modes. The

External field detector supports two different detection thresholds, Peer Detection Threshold and Collision

Avoidance Threshold. The two thresholds can be independently set by writing the External field detector threshold

Peer detection threshold

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

(initiator field in case the ST25R3911B is a target and the opposite, target field in case the ST25R3911B is an

initiator). It can be selected in the range from 75 to 800 mVpp. When this threshold is enabled the External Field

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 is useful to detect the moment when the peer NFC device (it can be either an initiator or a target)

has stopped emitting an RF field.

ST25R3911B

Application information

DS11793 - Rev 6

page 16/116

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