AS3909 datasheet(19/77 Pages) AMSCO

19 / 77 page

ams Datasheet, Confidential: 2013-Oct [3-02]

AS3909/AS3910 – 19

Appl icatio n I n f o rm a t i o n

PM Demodulation

The phase detector has low pass characteristics in case of PM

demodulation. This is to allow phase demodulation of the 848

kHz subcarrier signal. The output is then fed to the Receiver.

Amplitude Detector

Signals from pads RFI1 and RFI2 are used as inputs to the self

mixing stage. Output of this stage is DC voltage proportional

to amplitude of signal on RFI1 and RFI2 pads. This signal is fed

to A/D converter when amplitude of signal on RFI inputs has to

be measured (direct commands Measure RF and Calibrate

Modulation Depth).

External Field Detector

The External Field Detector is used in NFC mode to detect the

presence of an external 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 (#14). The outputs of both detectors are fed

to a logic or gate, output of which goes 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 (#17). 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

AS3909 数据表(PDF) 19 Page - ams AG

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