AS3911 datasheet(27/149 Pages) AMSCO

27 / 149 page

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

AS3911 – 27

Appl icatio n I n f o rm a t i o n

activation time needed to send a protocol activation command.

Additionally the power level during the measurement can be

lower than the power level during normal operation since the

card does not have to be powered to produce the coupling

effect. The emitted power can be reduced by increasing the RFO

Normal Level Definition Register.

Capacitance Sensor detects a change of the parasite

capacitance between the two excitation electrodes which is

caused by a card antenna and a hand holding it. See “Capacitive

Sensor” on page 14 for a detailed information on the capacitive

sensor.

The registries on locations from 31h to 3Dh are dedicated to

Wake-up configuration and display. The Wake-up Timer Control

timeout period between the successive detections and the

measurements which are going to be used are selected in this

available, 100 ms is the default value. Any combination of

available measurements can be selected (one, two or all of

them).

The following twelve registers (32h to 3Dh) are configuring the

three possible detection measurements and storing the results,

four registers are used for each measurement.

An IRQ is sent when the difference between a measured value

and reference value is larger than configured threshold value.

There are two possibilities how to define the reference value:

• The AS3911 can calculate the reference based on previous

measurements (auto-averaging)

• The controller determines the reference and stores it in a

The first register in the series of four is the Measurement

Configuration Register (see for e.g. Amplitude Measurement

Configuration Register). The difference to reference which

triggers the IRQ, the method of reference value definition and

the weight of last measurement result in case of auto-averaging

are defined in this register. The next register is storing the

reference value in case the reference is defined by the

controller. The following two registers are display registers. The

first one stores the auto-averaging reference; the second one

stores the result of the last measurement.

Wake-up mode configuration registers have to be configured

before wake-up mode is actually entered. Any modification of

Wake-up mode configuration while it is active may result in

unpredictable behavior.

AS3911 Datasheet(PDF) 27 Page - ams AG

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