AS3911 datasheet(14/149 Pages) AMSCO

14 / 149 page

AS3911 – 14

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

De tail ed De sc rip t ion

Capacitive Sensor

The Capacitive sensor is used to implement low power

detection of transponder presence. Capacitive sensor performs

measurement of capacitance between its two electrodes.

Presence of an object (card, hand) changes the capacitance.

During calibration the reference capacitance, which represents

parasitic capacitance of environment is stored. In normal

operation capacitance is periodically measured and compared

to stored reference value. When the measured capacitance is

larger than stored reference value (threshold value can be

defined in a register) an interrupt is sent to external controller.

External Field Detector

The External Field Detector is a low power block which is used

in NFC mode to detect presence of external RF field. It supports

two different detection thresholds, Peer Detection Threshold

and Collision Avoidance Threshold. Peer Detection Threshold is

used in the NFCIP-1 target mode to detect presence of initiator

field. It is also used in active communication initiator mode to

detect activation of target field. Collision Avoidance Threshold

is used to detect a presence of RF field during NFCIP-1 RF

Collision Avoidance procedure.

Quartz Crystal Oscillator

The quartz crystal oscillator can operate with 13.56 MHz and

27.12 MHz crystals. At start-up the transconductance of the

oscillator is increased to achieve fast start-up. Since the start-up

time varies depending on crystal type, temperature and other

parameters, the oscillator amplitude is observed and an

interrupt is sent when stable operation is reached to inform the

controller that the clock signal is stable and reader field can be

switched on. The use of 27.12 MHz crystal is mandatory in case

VHBR framing is used.

It also provides a clock signal to the external microcontroller

(MCU_CLK) according to setting in the control register.

Power Supply Regulators

Integrated power supply regulators ensure high power supply

rejection of a complete reader system. In case PSRR of the reader

system has to be improved, the command Adjust Regulators is

sent. As result of this command, the power supply level of VDD

is measured in maximum load conditions and the regulated

voltage reference is set 250 mV below this measured level to

assure a stable regulated supply. The resulting regulated

voltage is stored in a register. It is also possible to define

regulated voltage by writing a configuration register. In order

to decouple any noise sources from different parts of IC there

are three regulators integrated with separated external

blocking capacitors (regulated voltage of all is the same in 3.3V

supply mode). One regulator is for the analog blocks, one for

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