Sending and receiving beacons on demand.

[ilnadi]

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:

1. Receive other radio
2. Receive BLE (20-40ms interval)
3. (repeat steps 1 & 2 indefinitely)

Receive/Transmit devices:

1. Receive other radio
2. Receive BLE (20-40ms interval)
3. (repeat steps 1 & 2 N times)
4. Transmit other radio
5. Transmit BLE
6. (repeat steps 1 - 5 indefinitely)

[ilnadi]

Karl Ylvisaker said:

What size is the data that you are looking to broadcast on the BLE transmit?

small, likely 2-3 bytes.

Karl Ylvisaker said:

Since I do not know what other radio protocol you are using, I assume that you know you have sufficient time to switch between BLE and the other radio

We have a 20ms window to switch, transmit, switch back.  The receivers will be listening for as much of that 20ms as they can.  My calculations say this is enough.

Karl Ylvisaker said:

Have you seen the BLE Beacon example? If you are not familiar with advertising in BLE, I suggest reading this short tutorial.

I have not, I have used nRF parts & SD in classical peripheral devices but learning about beacons now.  I will read both of these and reply here if I have any questions.

Karl Ylvisaker said:

Is this only because you want to have a non-connectable, broadcasting device?

Correct.

thank you

[Karl Ylvisaker]

Based on your replies I think you will find the BLE Beacon example very helpful.
Please also see the short tutorial I linked, as it details how you can easily set up the broadcasting/beacon in the manufacturer specific data field of the advertisement packets.

ilnadi said:

thank you

No problem at all, I am happy to help!
Let me know if you should encounter any issues or questions moving forward.

Best of luck with you development!

Best regards,
Karl

[ilnadi]

Karl Ylvisaker said:

Please also see the short tutorial I linked, as it details how you can easily set up the broadcasting/beacon in the manufacturer specific data field of the advertisement packets.

I have read the tutorial and run the example and they are both based on GAP/GATT-based scheduling (i.e. they use sd_ble_gap_adv_start() ). 

I need to be able to send the beacon on my own schedule

• fortunately the main schedule is divisible by 625usec. 
• unfortunately the beacon must fall in a specific 20ms window.  Is there a way to make the advertising scheduled synchronized or start i at a particular time?
• unfortunately the customer has additional requirements for repeating the information asynchronously based on criteria outside BLE considerations.

All of this leads me to think we need to go back to L2CAP so we can

• on the sender "send this packet now" (see message flow)
• on the receiver "turn on receiver" then after the window is over, "turn off receiver" (see message flow)

Are there any examples of L2CAP usage?  The only thing I can fidn related to L2CAP in the SDK (15.3.0) is related to serialization.

[Karl Ylvisaker]

Hello,

I understand.
This is demonstrated in the Solar Beacon example. Note how the sensor_handler is called at an interval, every time a reading is to be done and thereafter transferred, the rest of the time the device consumes very little power.
To make sure that the intervals are synchronized with your other device, could you for example use a GPIO interrupt? Would that be feasible?

ilnadi said:

unfortunately the beacon must fall in a specific 20ms window.  Is there a way to make the advertising scheduled synchronized or start i at a particular time?

Yes, this is possible. See the Solar Beacon example I linked above for an example on how to do this.

ilnadi said:

unfortunately the customer has additional requirements for repeating the information asynchronously based on criteria outside BLE considerations.

I am not sure I understood what you meant here. Is the intervals for transmission changing? If so, the GPIO interrupt might solve this as well.

I would recommend using the Beacon examples as a basis for your application, instead of going the route with L2CAP.

Looking forward to solving this issue together!

Best regards,
Karl

[ilnadi]

Karl Ylvisaker said:

ilnadi said:

unfortunately the customer has additional requirements for repeating the information asynchronously based on criteria outside BLE considerations.

I am not sure I understood what you meant here. Is the intervals for transmission changing? If so, the GPIO interrupt might solve this as well.

first,both radios are controlled by the 52840 so we can easily schedule both.  the question was more about sending BLE packets on demand.  

What I meant above is: although we have a periodic schedule and a specific 20ms window allocated to each device within that schedule, we cannot simply do a periodic beacon (even if we could synchronize).  This is because in certain circumstances the customer wants us to re-broadcast in unused slots to increase the chance of reception. 

Having said all that, being able to transmit and receive on demand will meet all our needs.

I will check the solar beacon example.

thank you

[ilnadi]

Karl Ylvisaker said:

This is demonstrated in the Solar Beacon example. Note how the sensor_handler is called at an interval, every time a reading is to be done and thereafter transferred, the rest of the time the device consumes very little power.

I wanted to confirm a few things:

• this example is 5yrs old and does not use softdevice or nRF SDK. Instead it writes directly to a handful of RADIO registers.  I want to make sure it is still OK to use.
• Is there any equivalent SDK functionality to use (without SD)?  I found ble_radio_notification, radio_config, and nrf_radio.h but none of these have send/receive functions.

[Karl Ylvisaker]

Hello,

ilnadi said:

first,both radios are controlled by the 52840 so we can easily schedule both.  the question was more about sending BLE packets on demand.  

I see. Well, here I would have to refer to the BLE specification with regards to how transfers between connected devices must be done - at the agreed upon connection intervals. However, with non-connectable devices such as beacons(broadcasters) in your case, the advertisements can happen at any given interval within the specific advertising range - by setting the advertisement interval accordingly. You could potentially also start the advertisement following an interrupt, thus ensuring that the advertisements sync up with your other devices.

Alternatively, you can ensure that you are broadcasting in the allotted time-slot by having an advertising interval which is bound to occur within the time-slot(preferably also accounting for potential packet loss).

ilnadi said:

This is because in certain circumstances the customer wants us to re-broadcast in unused slots to increase the chance of reception. 

Could you possibly provide an overview of the entire radio period, which indications of how long each time-slot is, and how many there are? I am having a hard time understanding why setting up a broadcaster, such as in the beacon example, is not feasible for your application.

ilnadi said:

this example is 5yrs old and does not use softdevice or nRF SDK. Instead it writes directly to a handful of RADIO registers.  I want to make sure it is still OK to use.

ilnadi said:

Is there any equivalent SDK functionality to use (without SD)?  I found ble_radio_notification, radio_config, and nrf_radio.h but none of these have send/receive functions.

The example demonstrates use of the Radio HAL directly, which you could read more about here. You are free to do this as well, but I would highly recommend that you rather make use of the drivers and examples from the SDK - to make things a lot easier.

Looking forward to solving this issue together,

Best regards,
Karl