SCCD not visible in nrfToolbox

RE: SCCD not visible in nrfToolbox

jfDEVmoto — Wed, 10 Aug 2016 14:04:09 GMT

Thanks for checking, this is definitely one of those good days where we both learned something and improved our knowledge :-) You have been of great help !

RE: SCCD not visible in nrfToolbox

Hung Bui — Wed, 10 Aug 2016 13:39:27 GMT

Hi jfDev,

I was checking again with our developer, it seems that I was wrong.

You can create your proprietary descriptor as you already have done. The only limitation of a descriptor compare to a characteristic is that you can't do notification or indication from a descriptor. Other than that you can do pretty much the same (read/write).

As you may already know, GATT layer is on top of the ATT layer, so on the ATT layer, service, characteristic and descriptor all are simply attributes.

I was confused that if the spec limit the descriptor to only the ones defined in the spec or user can define their own. Sorry for the confusion.

RE: SCCD not visible in nrfToolbox

jfDEVmoto — Wed, 10 Aug 2016 12:50:54 GMT

Good to know, however, it seems to work, I am able to create a descriptor with a custom uuid....maybe it is not allowed and will not operate properly, as I have not played with it yet...

RE: SCCD not visible in nrfToolbox

Hung Bui — Wed, 10 Aug 2016 11:01:55 GMT

@JfDev: sd_ble_gatts_descriptor_add can be used to add descriptor to a charactersitic. But it's not possible to create proprietary descriptor, only the descriptor's UUIDs defined in the spec are allowed.

RE: SCCD not visible in nrfToolbox

jfDEVmoto — Tue, 09 Aug 2016 13:11:17 GMT

Your answer is surprising as I supposed the function sd_ble_gatts_descriptor_add actually allows to create descriptor attributes for one characteristic. I have not tried it yet.

RE: SCCD not visible in nrfToolbox

Hung Bui — Tue, 09 Aug 2016 13:06:57 GMT

I don't think there is an option to add custom proprietary descriptor with BLE. You would need to describe a new characteristic for your use. You can, for example add the Characteristic User Description, but the use of it is to define in text (UTF-8) information about the characteristic. Not for other purpose.

sd_ble_gatts_sys_attr_get() is only for the CCCD not all the descriptor. You should use sd_ble_gatts_value_get() to read the descriptor value, like any attribute /characteristic value.

I would suggest to add a characteristic to the same service to handle the hardware. Or you can add one extra byte in the current characteristic to handle that, it's up to your design.

RE: SCCD not visible in nrfToolbox

jfDEVmoto — Tue, 09 Aug 2016 11:08:24 GMT

Thanks for your answer. I agree with your advise, however, is it possible to add a custom descriptor instead of using a custom characteristic ? Then do you confirm the descriptor value can be read with sd_ble_gatts_sys_attr_get ?

I have currently different services, one of them is sensor service with one characteristic per sensor, than I wish to have one descriptor for each sensor char in order to activate the hardware sensor. I think this is easier for modularity than having a characteristic to activate the hardware sensor. However, having one characteristic to handle all sensors hardware status may be of lower memory overhead but I think it wil be hard to maintain when different versions of the product may have different set of sensor features.

RE: SCCD not visible in nrfToolbox

Hung Bui — Tue, 09 Aug 2016 10:33:09 GMT

Hi jfDEVmoto,

I don't think the SCCD would fit with what you planed to do with it. The only defined function that the SCCD would do is defined in the spec is "Broadcast" which means the client enable the value of the characteristic to be added to advertising packet if possible. I guess you already figured it out.

It's not a good idea to use the RFU bit for your purpose.

If you want to control a hardware on the server, you can simply create a new proprietary characteristic add to the server att table and use it for your purpose.

If you still want to use the SCCD, you would need to treat it as a normal descriptor, and use getDescriptors and BluetoothGattDescriptor (on Android) to read/write to it.

RE: SCCD not visible in nrfToolbox

jfDEVmoto — Mon, 08 Aug 2016 10:59:36 GMT

OK, I got it work as I realized it was linked to the broadcast property of the char metadata.

char_md.char_props.broadcast = 1;

I don't know about this broacast thing and if this is suitable to my project...

Then playing with SCCD I wish to use RFU bits for custom purpose (I understand using a custom descriptor is better approach and I will evaluate this later).

how can I read descriptor value from application ? do I hve to use sd_ble_gatts_sys_attr_get ?