Scanning time during dual-role operation (not dual-mode).

[Brian]

  I am in the process of evaluating different BLE solutions for a project.  The project consists of a pair of BLE-enabled devices which need to be able to go into dual-role mode (simultaneous advertising and scanning) and connect to the other device.  I would like to understand how this works in the Nordic BLE MCU's (nRF51, nRF52, nRF5340, etc).  I've prototyped this on another vendor's BLE MCU, and I see that there is a dead time of about 10 ms on each side of the advertising events.  This causes a problem getting a connection established reliably between the 2 devices.  Using a BLE sniffer, I can see that if the advertising events from the 2 devices are aligned with each other (within about 5 msec), then I'm unable to establish a connection.  See the attached scope probe picture.

 

  This shows the device advertising at a rate of 45 ms, and scanning at 100% of the remaining time.  You can see the ~10 ms of dead-time on each side of the advertising event.

  Does the Nordic MCU's also have this dead-time when the radio goes from advertising to scanning and then back to advertising.  If so, what is the duration of the dead-time?  If there is dead-time, is there a way to reduce it?

Thanks!

Brian

[Einar Thorsrud]

Hi Brian,

The delays on the nRF5 devices with SoftDevice depend on several factors, as you can see under Scheduling. I do not have an exact number for this though.

Regarding what you see (on the non-Nordic product), I can only speculate that the 10 ms could be related to the 0-10 ms random delay added to the advertising interval. This is a delay that you cannot escape since that is enforced by the BLE specification.

If you want to increase the likelihood of the two devices being able to connect (not ending up in a situation where they both scan or advertise at the same time for a long time), you could perhaps regularly change the scan/advertising duty cycle randomly?