AS3909 Datasheet, PDF (19/77 Pages) ams AG – 13.56 MHz RFID Reader IC, ISO-14443 A/B

Application Information
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
ams Datasheet, Confidential: 2013-Oct [3-02]
AS3909/AS3910 – 19