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AS3930

Data Sheet - D e t a i l e d D e s c r i p t i o n

If the bit rate gets higher, the time constant in the envelop detector must be set to a smaller value, this means that higher noise is injected

because of the wider band. The next table is a rough indication of how the envelop detector looks like for different bit rates. By using proper data

slicer settings it is possible to improve the noise immunity paying the penalty of a longer preamble. In fact if the data slicer has a bigger time

constant it is possible to reject more noise, but every time a transmission occurs, the data slicer need time to settle. This settling time will

influence the length of the preamble. Table 17 gives a correlation between data slicer setup and minimum required preamble length.

Note:

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

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>).

8.6 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 18) and if no preamble is detected the chip is set back to

listening mode and the false-wakeup 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> according to the Table 18 as function of the Real Time Clock (RTC) periods.

Table 17. Bit setup for Data Slicer for Different Preamble Length

R3<5>

R3<4>

R3<3>

Minimum preamble length [ms]

00

0

0.8

00

1

1.15

01

0

1.55

01

1

1.9

10

0

2.3

10

1

2.65

11

0

3

11

1

3.5

Table 18. 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

1

1

0

1

14

2340

1170