ST25R3910 datasheet(15/65 Pages) STMICROELECTRONICS

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ST25R3910

Functional overview

If the antenna LC tank is detuned, phase shift changes, resulting in a different phase

detector output voltage. In case of command Check Antenna Resonance phase detector

output is applied to A/ D converter in relative mode. Output of phase detector is also

observed by comparator with reference signal VSP/2. Output of this comparator is used in

execution of direct command Calibrate Antenna.

The phase detector has low pass characteristics in case of PM demodulation. This is to

enable phase demodulation of the 848 kHz subcarrier signal. The output is then fed to the

Receiver.

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. This

signal is fed to the A/D converter when amplitude of signal on RFI inputs has to be

measured (direct commands Measure RF and Calibrate Modulation Depth).

1.2.6

External field detector

The External Field Detector is used in NFC mode to detect the presence of an RF field. It is

composed of two sub-blocks, Target Activation Detector and a RF Collision Avoidance

Detector. Input to both blocks is the signal from the RFI1 pad. The thresholds of the two

blocks can be independently set by writing the NFCIP Field Detection Threshold Register.

The outputs of both detectors are fed to a logic OR gate, whose output is fed to the Control

logic. A low to high transition of this logic or gate output triggers an interrupt (Interrupt due to

NFC event).

Target Activation Detector

This block is turned on in NFC target mode to detect the presence of an interrogator field. It

is enabled by setting the Operation Control Register bit nfc_t. It is a low power block with an

adjustable threshold in the range from 145mVpp and 590mVpp. This block generates an

interrupt when an external field is detected and also when it disappears. 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 external NFC device (it can either an interrogator

or a target) has stopped emitting an RF field since a response can only be sent afterwards.

Actual state of the Target Activation Detector can be checked by reading the bit rfp in the

Receiver State Display Register. When this bit is set to logic one, there is a signal higher

than the threshold present on the input of Target Activation Detector.

RF Collision Avoidance Detector

This block is activated during the RF Collision Avoidance sequence which is executed

before every request or response in NFC active communication (Initial or Response RF

Collision Avoidance). In case during the RF Collision Avoidance sequence the presence of

an external field is detected, request/response is not sent, an interrupt is generated to

inform the external controller about collision. During RF Collision Avoidance, the Target

Activation Detector is disabled in order to have the correct threshold when detection is

made. The threshold of the RF Collision Avoidance Detector can be adjusted in the range

from 50 to 1080 mVpp.

ST25R3910 Datasheet(PDF) 15 Page - STMicroelectronics

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