AS3933 datasheet(40/64 Pages) AMSCO

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ams Datasheet

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[v1-08] 2015-Sep-02

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AS3933 − Detailed Description

Figure 45:

Minimum Required Preamble Lengths as Function of Slow

Envelop Settings

Note(s) and/or Footnote(s):

1. These times are minimum required, but it is recommended to prolong the

preamble.

With the bits R3<6> and R3<7> it is possible to change the

hysteresis on the data slicer comparator (only positive, positive

negative, 20mV, 40mV).

The slow envelop signal (blue signal in Figure 43) represents

the average of the demodulated signal, therefore acts as a

reference signal for the data slicer. In case the chosen protocol

has a duty cycle far away from 50% (for example in the NRZ

protocol there can be several consecutive ones or zeros) the

slow envelop signal would not be a stable reference signal for

the data slicer. In this case the data slicer can also work with an

absolute threshold (R1<7>), as shown in the Figure 46. Should

the absolute threshold be enabled the bits R3<2:0> would not

influence the performance. It is even possible to reduce the

absolute threshold in case the environment is not particularly

noisy (R2<7>).

As the input signal may be damped due to physical influences

of the transmitter environment, the symbol rate needs to be

adapted (lowered) if absolute threshold is enabled to ensure a

proper detection of the wake-up signal. The peak level of the

signal should be reached within 1/3 of the symbol duration

which is defined as two times the bit duration. The bit duration

is defined in register R7 <4:0> as a function of the Clock

Generator periods.

R3<5>

R3<4>

R3<3>

Minimum Preamble

Length [ms]

0.8

1.15

1.55

1.9

2.3

2.65

3.5

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