RE: Help with the nrf52 dev board, tag scan and channel frequency

kaplacat — Thu, 29 Aug 2019 13:55:58 GMT

Thanks for the explanation, I understand how it works.
I change the scan window (as you said) to equal the scan interval and it works perfectly. I can continue my tests thanks to you.

Regards

RE: Help with the nrf52 dev board, tag scan and channel frequency

tesc — Thu, 29 Aug 2019 13:25:17 GMT

Hi,

The "radio break" happens because scan windows and advertising events are almost synchronized but slightly drifting relative to each other. Let me explain.

The default scan parameters in that example, is SCAN_INTERVAL of 0xA0 and SCAN_WINDOW of 0x50, which means scan interval of 100 ms and scan window of 50 ms. That means the scan will happen like this:

50 ms scanning on channel 37 (one scan window)
50 ms no scanning (waiting for next scan event, new scan event each scan interval)
50 ms scanning on channel 38
50 ms no scanning
50 ms scanning on channel 39
50 ms no scanning
repeat from top

For advertising, in addition to the advertising interval, there is a random extra delay of 0 to 10 ms between advertising events. That means, on average, with advertising interval of 1 s, you get 1005 ms from one advertisement to the next. Each advertising event sends the advertising packet on all three advertising channels in rapid succession.

The scan interval (100 ms) perfectly divides the advertising interval (1000 s), but then the 5 - 10 ms random delay comes on top of that. This means, relative to the scan window, an advertisement is sent 5 ms later each time, on average. After a few advertising intervals, it has drifted out of the scan window (and now advertisements are sent during the pauses between scan windows.) After having drifted 50 ms more, i.e. around ten advertisements (ten seconds) later, you should expect it to have drifted into a scan window again, and will receive advertisements until drifting out of the scan window.

If you use half the advertising interval, you still get 5 ms average random addition to each interval, which means the drifting relative to the scan windows happens at double the speed.

In order to detect most advertisements, you can change the scan window to equal the scan interval. Then you will scan at 100 % duty cycle (instead of 50 % duty cycle as you do now).

Regards,
Terje