www.ams.com/LF-Receiver/AS3930

Revision 1.5

20 - 32

AS3930

Datasheet - Detailed Descr i p ti on

The comparator of the data slicer can work only with positive or with symmetrical threshold R3<6>. In addition the threshold can be 20 or 40 mV

R3<7>

. In case the length of the preamble is an issue the data slicer can also work with an absolute threshold R1<7>. In this case 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>.

8.5 Correlator

After frequency detection, the data correlation is only performed if the correlator is enabled (R1<1>=1).

The data correlation consists of checking the presence of a preamble (ON/OFF modulated carrier) followed by a certain pattern.

After the frequency detection the correlator waits 16 bits (see bit rate definition in Table 19) and if no preamble is detected the chip is set back to

listening mode and the false-wake-up register (R13<7:0>) is incremented by one.

To get started with the pattern correlation the correlator needs to detect at least 4 bits of the preamble (ON/OFF modulated carrier).

The bit duration is defined in the register R7<4:0> (see Table 19) as function of the Real Time Clock (RTC) periods.

Table 19. Bit Rate Setup

R7<4>

R7<3>

R7<2>

R7<1>

R7<0>

Bit Duration in RTC

Clock Periods

Bit Rate (bits/s)

Symbol Rate

(Manchester symbols/s)

0

0

0

1

1

4

8192

4096

0

0

1

0

0

5

6552

3276

0

0

1

0

1

6

5460

2730

0

0

1

1

0

7

4680

2340

0

0

1

1

1

8

4096

2048

0

1

0

0

0

9

3640

1820

0

1

0

0

1

10

3276

1638

0

1

0

1

0

11

2978

1489

0

1

0

1

1

12

2730

1365

0

1

1

0

0

13

2520

1260

0

110

1

14

2340

1170

0

1

1

1

0

15

2184

1092

0

1

1

1

1

16

2048

1024

1

000

0

17

1926

963

1

000

1

18

1820

910

1

001

0

19

1724

862

1

001

1

20

1638

819

1

010

0

21

1560

780

1

010

1

22

1488

744

1

011

0

23

1424

712

1

011

1

24

1364

682

1

100

0

25

1310

655

1

100

1

26

1260

630

1

101

0

27

1212

606

1

1

0

1

1

28

1170

585

1

1

1

0

0

29

1128

564

1

110

1

30

1092

546