AS3911 datasheet(32/159 Pages) AMSCO

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AS3911 − Application Information

General Purpose Timer

The triggering of the General Purpose Timer is configured by

setting the General Purpose and No-Response Timer Control

process (triggering by start of reception, after SOF) or to time

out the PCD to PICC response time (triggered by end of

reception, after EOF). In the NFCIP-1 active communication

mode it is used to timeout the field switching off. In all cases an

IRQ is sent when it expires.

The General Purpose Timer can also be started by sending the

direct command Start General Purpose Timer. In case this

command is sent while the timer is running, it is reset and

restarted.

Wake-Up Timer

Wake timer is primarily used in the Wake-up mode (see

Wake-Up Mode). Additionally it can be used by sending a direct

command Start Wake-up Timer. This command is accepted in

any operation mode except Wake-up mode. When this

command is send the RC oscillator, which is used as clock source

for wake-up timer is started, timeout is defined by setting in the

Wake-Up Timer Control Register. When the timer expires, an IRQ

with the I_wt flag in the Error and Wake-Up Interrupt Register

is sent.

Wake-up timer is useful in the Power-down mode, in which

other timers cannot be used (in the Power-down mode the

crystal oscillator, which is clock source for the other timers, is

not running). Please note that the tolerance of wake-up timer

timeout is defined by tolerance of the RC oscillator.

A/D Converter

The AS3911 contains an 8-bit successive approximation A/D

converter. Input to A/D converter can be multiplexed from

different sources to be used in several direct commands and

adjustment procedures. The result of last A/D conversion is

stored in the A/D Converter Output Register. Typical conversion

time is 224/fc (16.5 μs).

The A/D converter has two operating modes, absolute and

relative.

• In absolute mode the low reference is 0V and the high

reference is 2V. This means that A/D converter input range

is from 0 to 2V, 00h code means input is 0V or lower, FFh

means that input is 2V - 1LSB or higher, LSB is 7.8125 mV.

• In relative mode low reference is 1/6 of VSP_A and high

reference is 5/6 of VSP_A, so the input range is from 1/6

VSP_A to 5/6 VSP_A.

Relative mode is only used in phase measurement (phase

detector output is proportional to power supply). In all other

cases absolute mode is used.

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