AS3909 datasheet(20/77 Pages) AMSCO

20 / 77 page

AS3909/AS3910 – 20

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

A p pl i c at io n I n f o r m at i o n

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

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 датащи(PDF) 20 Page - ams AG