RE: Accurate time synchronization across devices

24thFrame — Wed, 10 Jun 2020 16:35:07 GMT

Hi Edvin,

Thanks so much for the quick and thorough response!

For #2, that makes sense. I've found this guide based on your suggestion that looks like it covers what you mentioned.

For #3, that's good to know!

Thanks again!

RE: Accurate time synchronization across devices

Edvin — Wed, 10 Jun 2020 13:58:16 GMT

Hello,

The accuracy will be given by the external XTAL if you use an external XTAL.

This wouldn't work with BLE because the devices only communicate on certain timeslots, every connection interval. If you want this accurate timing you would need something extra. One alternative is to use a proprietary protocol (SDK\examples\proprietary_rf) in a timeslot next to BLE, or to do some fancy logic stuff with BLE.

The challenge with BLE is not only that you can only talk on the connection interval timeout, but that if a packet isn't ACKed, then the transmitter has to send the identical packet the next connection interval. With the softdevice there is no way of seeing if the received has been retransmitted or not, so you can't know whether it was actually sent this connection interval, or if it is from a previous.

3. The softdevice supports up to 20 connections, but depending on the memory usage and radio usage of the application it may be less. It is possible to cycle through connections. In fact, if you already have the maximum number of connections, you will not be able to advertise, so you can't connect to the peripheral at that point in time, because you can't see the advertisements that you want to connect to.

That being said, it is a reasonable approach. But in that case, the peripheral should maybe actively disconnect from some devices once it reaches 15-16 devices, if you expect there to always be 20 centrals in the area. Just to reduce the time the new centrals have to wait.

Another approach is to look into using one of the Mesh technologies. Bluetooth Mesh, ZigBee or OpenThread, for example. But you would need your "centrals" to also support this protocol of course.

Best regards,

Edvin