AS3911 datasheet(26/149 Pages) AMSCO

26 / 149 page

AS3911 – 26

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

Capacitor Sensor Calibration

Capacitor sensor comprises calibration unit internally

compensates the parasitic capacitances between CSI and CSO,

thus leaving full measurement range for information about

capacitance variation. 5 bits are used to control the calibration,

minimum calibration step is 0.1pF, calibration range is 3.1pF.

Calibration can be done manually by writing Capacitive Sensor

Control Register or automatically by sending direct command

Calibrate Capacitive Sensor. The status of Calibrate Capacitive

Sensor command and resulting calibration value are stored in

the Capacitive Sensor Display Register.

In order to avoid interference of Capacitive Sensor with Xtal

oscillator and reader magnetic field and to assure repetitive

results it is strongly recommended to perform capacitance

measurement and calibration in Power-down mode only.

Wake-up Mode

Asserting the Operation Control Register bit wu while the other

bits are set to 0 puts the AS3911 into the Wake-up mode which

is used to perform low power detection of card presence. The

AS3911 includes several possibilities of low power detection of

a card presence (capacitive sensor, phase measurement,

amplitude measurement). Low power 32kHz RC oscillator and

periodic detection.

Usually the presence of a card is detected by so called polling.

In this process the reader is periodically turned on and the

controller activates the protocol to check whether a card is

present. Such procedure consumes a lot of energy since reader

field has to be turned on for 5ms before a command can be

issued.

Low power detection of card presence is performed by

detecting a change in reader environment, which is produced

by presence of a card. When a change is detected, an interrupt

is sent to the controller. As a result, the controller can activate

the protocol for tag detection.

In the Wake-up mode the AS3911 periodically performs the

configured measurements and sends an IRQ to the controller,

which is in deep sleep to minimize the current consumption,

only when a difference to the build in reference is detected.

Detection of card presence can be done by performing phase,

amplitude and capacitive sensor measurements.

Presence of a card close to the reader antenna coil produces

due to the magnetic coupling of the two coils a change of the

antenna LC tank signal phase and amplitude. The reader field

activation time needed to perform the phase or the amplitude

measurement is extremely short (~20μs) comparing to the

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