How to enable CCCD automatically in NCS(zephry)

[stars]

Hi

Develop peripherals.nrf devices do slave machines.

Due to project requirements. The standard (notification) operation process needs to be changed (In the absence of binding).The following blogs have confirmed that the project needs can be achieved in the SDK：

https://devzone.nordicsemi.com/f/nordic-q-a/15571/automatically-start-notification-upon-connection-event-manually-write-cccd---short-tutorial-on-notifications#post-id-90507

https://devzone.nordicsemi.com/f/nordic-q-a/63606/enabling-notification-by-default/259329

SDK added as follows：

The SDK test meets the requirements.

But eventually I need to implement this functionality in NCS, so I want to know how to implement it in NCS. Currently, I can't find an API like sd_ble_gatts_sys_attr_set in the NCS list of GATT layer apis:

I need your help. Thank you very much!

Best regards

Stars

[Einar Thorsrud]

Hi,

In the nRF5 SDK bonding was handled by the application, and for bonded devices, the CCCD value is kept across connections. So there was a need for support for this in the SoftDevice APIs. With Zephyr it is different, as bonds are handled in the Bluetooth stack, and therefor there was no need for such an API, and it is not present. Note it is not allowed in Bluetooth to send notifications to a GATT client that has not enabled that in the CCCD.

[stars]

Hi,

One thing I do know is that the two bond devices can indeed communicate without cccd being enabled. But in the previous implementation method. Without binding, communication can be carried out without enabling CCCD (although this method is not standard, but it can meet the application scenario and is an acceptable way).

Is there a way to use the protocol stack stack in the SDK in zephyr? The functionality is then implemented through the sd_ble_gatts_sys_attr_set function.

Best regards

Stars

[Einar Thorsrud]

stars said:

One thing I do know is that the two bond devices can indeed communicate without cccd being enabled.

Yes, certainly (by reads and writes), but not by notifications or indications. Or rather, with the SoftDevice it would be possible, but it is not allowed per Bluetooth specification.

stars said:

Is there a way to use the protocol stack stack in the SDK in zephyr?

No, the SoftDevice is not compatible with Zephyr.

[stars]

Hi,

Thank you very much for your reply.

After the above communication, can I understand that using zephyr's ble stack, the two bound devices still cannot be set up to transfer data in notifications or indications when the host does not enable cccd?

If I understand it wrong, it can be done. Can you give me some reference on how to set up auto-enable cccd when there is binding? Thank you very much!

Best regards

Stars

[Einar Thorsrud]

Hi Stars,

I do not have any pointers for that, unfortunately. You would have to modify the Bluetooth stack or modify the bonding data in flash after is has been stored by the stack and before it is read back (a dirty hack). 

[stars]

Hi

Thank you very much for your reply.

Best regards

Stars

[Einar Thorsrud]

Hi Stars,

It turns out there are more to this than what I wrote before. It is in fact possible to send notifications or indications to clients that have not enabled it in the CCCD by adding CONFIG_BT_GATT_ENFORCE_SUBSCRIPTION=n to your configuration (this is enabled by default). By disabling this, the check for if notifications are enabled or not in the CCCD is skipped, and it is sent regardless.

Einar

[stars]

Hi,

As you suggested, I have added CONFIG_BT_GATT_ENFORCE_SUBSCRIPTION=n to the prj.conf file in NUS's example for testing.Unfortunately, when I disabled the CCCD, I still couldn't receive data from the nrf device.

Is there anything I missed? After all the new. The macro CONFIG_BT_GATT_ENFORCE_SUBSCRIPTION cannot be checked in both autoconf.h files.

Best regards

Stars

[Einar Thorsrud]

Hi Stars,

That is expected. It is set by default, but when you set CONFIG_BT_GATT_ENFORCE_SUBSCRIPTION=n nothing gets defined, so the only related thing you will see in your build folder then is a line with

# CONFIG_BT_GATT_ENFORCE_SUBSCRIPTION is not set

in build/zephyr/.config

In what way does it not work now? Perhaps you check the CCCD earlier and don't try to send a notification if it is not set? The mentioned config only removes the restriction that prevents it in zephyr/subsys/bluetooth/host/gatt.c. Also, which SDK version are you using?

Einar

[stars]

Hi Einar Thorsrud，

I used NCS_2.3.The SDK I use is SDK_17.1.

This is determined directly when CCCD is not enabled on nrf_connect, as in SDK_17.1. The nrf device sends the data directly to see if nrf_connect can receive it.

Best regards

Stars

[Einar Thorsrud]

Hi Stars,

It is not entirely clear to me what the issue is now (if any). Perhaps you can elaborate?

Einar

[stars]

Hi,

Using the soifdevice in the SDK to disable CCCD, the nrf connect terminal can still receive data. But how do you change it in NCS.Just like in SDK_17.1

Best regards

Stars