AS3911 datasheet(28/149 Pages) AMSCO

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AS3911 – 28

ams Datasheet, Confidential: 2013-Oct [2-03]

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

Auto-averaging

In case of auto-averaging the reference value is recalculated

after every measurement. The last measurement value, the old

reference value and the weight are used in this calculation. The

following formula is used to calculate the new reference value.

The calculation is done on 10 bits to have sufficient precision.

The auto-averaging process is initialized when Wake-up mode

is first time entered after initialization (power-up or using Set

Default command). The initial value is taken from the

Measurement Reference Register (for example Amplitude

Measurement Reference Register) in case content of this

result of first measurement is taken as initial value.

Every Measurement Configuration register contains a bit which

defines whether the measurement which causes an interrupt is

taken in account for the average value calculation (for example

bit am_aam of the Amplitude Measurement Reference Regis-

ter).

Quartz Crystal Oscillator

The quartz crystal oscillator can operate with 13.56 MHz and

27.12 MHz crystals. The operation of quartz crystal oscillator is

enabled when the Operation Control Register bit en is set to

one. An interrupt is sent to inform the microcontroller when the

oscillator frequency is stable (see Main Interrupt Register).

The status of oscillator can be observed by observing the

Auxiliary Display Register bit osc_ok. This bit is set to ‘1’ when

oscillator frequency is stable.

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 50 Ω.In order to enable a fast reader

start-up an interrupt is sent when oscillator amplitude exceeds

750 mVpp.

Division by two assures that 13.56MHz 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 13.56 MHz crystal, the bias current of stage which is

digitizing oscillator signal is increased to assure as low PW

distortion as possible.

Please note that in case of VHBR reception (bit rates fc/8 and

above) it is mandatory to use the 27.12MHz crystal since high

frequency clock is needed for receive framing.

NewReference

OldReference

MeasuredValue

–

Weight

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