random delay

[Marius Heil]

Hello,

I'm trying to do a synchronization of two devices over BLE. I want to be able to open a scan window only for as little time as possible to receive a timed advertising packet. This is a little hard to achieve because of the random advertising delay according to the BLE SPEC Vol 6 4.4.2.2. If I read correctly, there is a delay between the start of two consecutive advertising packets which must be <= 10ms. Furthermore, there is another delay of 0-10ms after each advertising Interval. Are these two delays deterministic values based on a pseudo random number generator in the Softdevice (I am using SD130)? If yes, could I have these calculations so that I could implement a proper synchronization? According to the sniffer, the advDelay seems to be in the range of 10ms like in the spec and the start of two different advertisement packets is mostly within 4-7ms.

Could I maybe achieve synchronization by switching advertising on and off in specific intervals? How long will it take until the softdevice starts advertising after I call sd_ble_gap_adv_start?

[David Edwin]

The softdevice generated advertisements are randomized with a 0 to 10ms using a hardware based random number generator. You can use the Timeslot API for directly generating the advertisements and then scanning for them. An example to do so is located in the NordicSemiconductor github repository.

[Marius Heil]

Aw, thanks. I was hoping a PRNG was used so this would have been deterministic. I know about the timeslot API, but I need to understand the synchronization problematics at first, thanks :-)