How to write/read GATT attribute values?

RE: How to write/read GATT attribute values?

Simon — Fri, 14 May 2021 13:26:40 GMT

It's a long time since I've worked with the nRF5 SDK (I mostly work with the nRF Connect SDK), would you be able to open a new ticket and ask your question there? Then it will get assigned to an engineer with knowledge about this and you will get help.

RE: How to write/read GATT attribute values?

Shivek — Thu, 13 May 2021 17:12:48 GMT

Hi Simon,

Thanks for your efforts. I am using Softdevice S140 now rather than using Zephyr APIs. I am wondering how can I implement my custom service as stated earlier. I am following this tutorial. The problem is I have UUIDs for Service and Characteristic unrelated (Characteristic UUIDs are not related to the Primary service UUID).

I tried using sd_ble_gatts_service_add() API, to add custom service without using sd_ble_uuid_vs_add() as I was trying to avoid supplying BASE UUID and 16 bit UUID separately. But I guess we need to use sd_ble_uuid_vs_add() to add vendor specific UUID to ATT table. In my case, SERVICE_UUID (refer to my first post), DEV_ID_UUID, DEV_TYPE_UUID,.. all are different.

Can you please tell me how to implement custom services using proprietary UUIDs for service and cahracteristics?

In a nutshell, I want to implement what I am doing in the second part of my earlier post using Softdevice S140 APIs. FYI this would be the structure for my service:

Primary Service with UUID = f3800767-c8a6-4857-9049-896521c7379e

DEV_ID Characteristic with UUID = 22d7b64e-9bd3-4f30-b1ea-8eb3445c9877

DEV_TYPE Characteristic with UUID = 89d7c21e-5be3-6d60-c3ea-7be1236b1410

RE: How to write/read GATT attribute values?

Simon — Wed, 12 May 2021 23:53:07 GMT

I recommend you to study the samples peripheral_cts_client and central_uart and their associated service libraries cts_client.c and nus_client.c, where you can see examples of how to use the functions bt_gatt_write() and bt_gatt_read().

Regarding the offset field, It was hard to find information about it. All the other bluetooth samples are just setting it to 0. I looked at zephyr\subsys\bluetooth\host\gatt.c and saw that it would trigger bt_gatt_prepare_write() if the offset was set to a nonzero value, while gatt_send() would get triggered if the offset is set to zero (and the length is less than the MTU), you may look into these function to understand the offset field better. I think the prepare write function is used for long write situations, but I need to investigate this more to give you a certain answer.

However, I think you should be fine just setting it to 0.

I will investigate this further on Friday or next Tuesday (There are national holidays on Thursday and Monday in Norway).

Best regards,

Simon