Sending and receiving beacons on demand.

Question

Our device using nRF52840 is running an external (non-BT) radio most of the time. We need to send and receive data In between operations of this other radio. At any given time, there will be one sender and multiple receivers. Since there will be no BLE connection or interaction, this is essentially a beacon operation except for the scheduling. We are trying to understand if it is possible to manually send and receive without running a BLE schedule and without any interaction between sender and receiver. 
 We are assuming/hoping this will be possible using L2CAP functions instead of GAP/GATT functions and never starting any GATT scheduling. However the L2CAP message sequence charts in SD140 v7.0.1 all show two-way communication. 
 Please clarify how the scheme described here can be accomplished. 
 
 The overall schedule will be something like 
 Receive-only devices: 
 
 Receive other radio 
 Receive BLE (20-40ms interval) 
 (repeat steps 1 & 2 indefinitely)

Receive/Transmit devices: 
 
 Receive other radio 
 Receive BLE (20-40ms interval) 
 (repeat steps 1 & 2 N times) 
 Transmit other radio 
 Transmit BLE 
 (repeat steps 1 - 5 indefinitely)

Karl Ylvisaker · Accepted Answer

Hi again, Thank you for your patience. 
 ilnadi said: we have now run some experiments and not quite sure whether we are seeing an issue or not. 
 What do you mean by this, are you no longer seeing the original issue? 
 ilnadi said: Have you been able to confirm how soon after sd_ble_gap_adv_start() the first beacon will be transmitted? 
 I have not looked further into how long it takes the hardware to respond to the call, but However, please keep in mind the random 3 - 13 ms delay added before advertising packets, as written about in the S140 documentation - advertising scheduling and suggested intervals and windows in particular. The delay is part of the BLE specifications, and can not be omitted if you are to use BLE. You may check it by using a Sniffer tool for monitoring the BLE traffic, complete with timestamps and contents of each advertised packet. If this does not fit your application requirements, then I suggest making use of the Timeslot API to schedule proprietary radio communication windows. Using a proprietary protocol could significantly reduce your latency, which sounds to be your main concern. Timeslots will give you complete control(no SD interruption) over the radio for their duration. If you would like to take a look at this, I can recommend this tutorial on setting up the Timeslot API for starters. This example code could also be useful to have a look at, when looking for demonstrations of Timeslot API usage for the purpose you describe. Unfortunetely, said example is now a bit old - written for the nRF51 DK - but it is still relevant and the principles still hold. 
 ilnadi said: We are detecting the beacons using nRF Connect for Android v.4.24.1 and see a 30-50ms difference in beacon-to-beacon timing even though we are calling sd_ble_gap_adv_start() with fixed (3.2sec) intervals. We are pretty sure about our timing because this timing also controls the other radio which has been tested in detail. How dependable is the timing from nRF Connect for Android? 
 The timing from the nRF Connect app is dependable, but as I mentioned earlier, I suggest that you use a Sniffer tool to analyze BLE traffic, since it gives you a much higher level of detail. With the Sniffer tool running, you are also able to capture a sniffer trace - a log of the BLE traffic - so that I may better see what you are experiencing on your end. Looking forward to resolving this issue together, Best regards, Karl

Sending and receiving beacons on demand.

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