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[v1-08] 2015-Sep-02

AS3933 − Detailed Description

Figure 46:

Envelop Detector Signals - Absolute Threshold

Correlator

In order to prevent that the AS3933 wakes up the host system

(MCU) from noise or disturbers (LF transmitter within the field)

the internal correlator checks that the bit sequence delivered

from the data slicer corresponds to stored pattern. The wanted

pattern can be stored in the registers R5<7:0> and R6<7:0>. The

data correlation is performed only if the correlator is enabled

(R1<1>=1) and can start only after frequency detection.

The pattern correlation is successful (Wake goes high) only if

the bits sequence (pattern) and its timing (duration of the single

bit) matches.

Pattern: Bit and Symbol Definition in Manchester

Code

The AS3933 can correlate the incoming pattern without the

help of an external unit (MCU). The chosen pattern must be

Manchester encoded. In the Manchester code each “Symbol” is

defined by a transition (high-to-low for 1 and low-to-high for

0), therefore consists of two “bits”. In the Figure 47 it is shown,

as an example, how the encoding technique works. In this

sequence a simple message made up by 3 symbols (1 0 1) is

Manchester encoded. In the Manchester encoded bit stream

there can not be three consecutive zeros or ones (in each

symbol there is always a transition). This helps the receiver to

recover the clock.

Absolute

Threshold (blue)

Fast Envelop

(red)

Data Slicer Output

AS3933 데이터시트(PDF) 41 Page - ams AG