AS3909 Просмотр технического описания (PDF) - austriamicrosystems AG
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AS3909 Datasheet PDF : 77 Pages
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Application Information
correct threshold when detection is made. The threshold of the
RF Collision Avoidance Detector can be adjusted in the range
from 50 to 1080mVpp.
Quartz Crystal Oscillator
The quartz crystal oscillator can operate with 13.56MHz and
27.12MHz crystals. The oscillator is based on an inverter stage
supplied by controlled current source. A feedback loop is
controlling the bias current in order to regulate amplitude on
XTI pin to 1Vpp. This feedback assures reliable operation even
in case of low quality crystals with Rs up to 50Ω. In order to
enable a fast reader start-up an interrupt is sent when oscillator
amplitude exceeds 750mVpp.
The oscillator block always provides 13.56MHz clock signal to
the rest of the IC. In case of 27.12MHz crystal clock signal is
internally divided by two. Divider is controlled by Configuration
Register 2 (#02) bit osc.
Division by two assures that 13.56 MHz signal has a duty cycle
of 50% which is better for the Transmitter performance (no PW
distortion). Use of 27.12MHz crystal is therefore recommended
for better performance.
In case of 13.56MHz crystal, the bias current of stage which is
digitizing oscillator signal is increased to assure as low PW
distortion as possible.
The oscillator output is also used to drive a clock signal output
pin which can be used by the external microcontroller
(MCU_CLK). By setting bits in Configuration Register 2 MCU_CLK
a frequency can chosen between 13.56MHz, 6.78MHz and
3.39MHz. Any microcontroller processing generates noise
which may be captured by the AS3909/10 receiver. Using
MCU_CLK as the microcontroller clock source generates noise
which is synchronous to the reader carrier frequency and is
therefore filtered out by the receiver while using some other
incoherent clock source produces noise which may generate
some sideband signals captured by Receiver. Due to this fact it
is recommended to use MCU_CLK as microcontroller clock
source.
AS3909/AS3910 – 20
ams Datasheet, Confidential: 2013-Oct [3-02]
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