AS3911 datasheet(137/159 Pages) AMSCO

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ams Datasheet

Page 137

[v2-11] 2016-Jun-13

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AS3911 − Application Information

The tx_en bit and therefore switching on of the transmitter is

controlled by NFC Field ON commands. Switching OFF the field

is performed automatically after a message has been sent. The

General Purpose and No-Response Timer Control Register is

used to define the time during which the field stays switched

on after a message has been transmitted.

In order to receive the NFCIP-1 active reply only the AM

demodulation channel is used. Due to this the Receiver AM

channel has to be enabled. The preset done by Analog Preset

command enables only the AM demodulation channel, while

PM channel is disabled to save current.

In NFCIP-1 active communication the NFCIP-1Transport Frame

format (See Figure 123) is always used. Due to this the

ISO14443A and NFC 106kbit/s Settings Register bit nfc_f0 is set

by Analog Preset command (see Support of NFCIP-1Transport

Frame Format).

NFCIP-1 active communication sequence when bit nfc_ar in the

Mode Definition Register is set (automatic Response RF

Collision Avoidance sequence). During this sequence bits

nfc_n1 and nfc_n0 of the Auxiliary Definition Register have to

be 0 to produce Response Collision Avoidance sequence with

n=0:

First the direct command NFC Initial Field ON is sent. In

case no collision was detected during RF collision

avoidance the field is switched on and an IRQ with I_cat

flag set is sent to controller after TIRFG.

The message, which was prepared as in case of reader

to tag communication, is transmitted using Transmit

command.

After the message is sent the field is switched off. The

time between the end of the message and switching off

the field is defined by the General Purpose Timer. (The

General Purpose Timer IRQ may be masked since

controller does not need this information).

After switching off its field the AS3911 starts the

No-response Timer and observes the External Field

Detector output to detect the switching on of the target

field. In case the target field is not detected before

No-response Timer timeout, an IRQ due No-response

Timer expire is sent.

When Target field is detected an IRQ with I_eon flag set

is sent to controller and Mask-receive Timer is started.

After the Mask Receive Timer expires the receiver output

starts to be observed to detect start of the target

response. The reception process goes on as in case of

reader to tag communication.

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