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AS3932

8.7.3 Single Pattern Detection (Manchester decoder enabled)

The Figure shows the wakeup protocol in case both the pattern correlation and the Manchester decoder are enabled (R1<1>=1 and R1<3>=1)

for a 125 kHz carrier frequency. The initial carrier burst has to be at least 42 Manchester symbols long and has to be followed by a separation bit

(one bit of no-carrier). The carrier burst must be followed by a minimum 4 Manchester symbol preamble (10101010) and the pattern stored in the

R5<7:0> and R6<7:0>. The preamble can only be made up by integer Manchester symbol and the preamble duration together with the pattern

should not be longer than 40 bits. If the pattern is correct the signal on the WAKE pin is set to high, the data are internally decoded and the

Manchester clock is recovered. To set the AS3932 back to listening mode the direct command clear_false or the time out option (R7<7:5>) can

be used.

In case the On/OFF mode is enabled the Manchester decoder can not be used.

Figure 20. Wakeup Protocol Overview with Single Pattern Detection (Manchester decoder enabled)

8.8 False Wakeup Register

The wakeup strategy in the AS3932 is based on 2 steps:

1. Frequency Detection: in this phase the frequency of the received signal is checked.

2. Pattern Correlation: here the pattern is demodulated and checked whether it corresponds to the valid one.

If there is a disturber or noise capable to overcome the first step (frequency detection) without producing a valid pattern, then a false wakeup call

happens.Each time this event is recognized a counter is incremented by one and the respective counter value is stored in a memory cell (false

wakeup register). Thus, the microcontroller can periodically look at the false wakeup register, to get a feeling how noisy the surrounding

environment is and can then react accordingly (e.g. reducing the gain of the LNA during frequency detection, set the AS3932 temporarily to

power down etc.), as shown in the Figure 21. The false wakeup counter is a useful tool to quickly adapt the system to any changes in the noise

environment and thus avoid false wakeup events.

1212

13.20

13.20

1170

13.67

13.67

1128

14.18

14.18

1092

14.65

14.65

1056

15.15

15.15

1024

15.62

15.62

Table 21. Preamble Requirements in Standard Mode, Scanning Mode and ON/OFF Mode

Bit rate (bit/s)

Maximum duration of the carrier burst in Standard

Mode and Scanning Mode (ms)

Maximum duration of the carrier burst in ON/OFF Mode

(ms)