How to configure GATT server in NRF app to test CTS example

RE: How to configure GATT server in NRF app to test CTS example

nordev — Mon, 31 Aug 2020 18:53:56 GMT

Thank you again for your extremely helpful and detailed replies. I have one last question:

what does the "c" stand for in ble_cts_c? Is it "client"? I see this in various places in the nordic code and am not sure what the notation means... what is the purpose of this notation?

RE: How to configure GATT server in NRF app to test CTS example

Einar Thorsrud — Fri, 28 Aug 2020 11:57:44 GMT

Hi,

[quote user="nordev"]Hypothetically, if I develop a custom service for the central, should I put something equivalent to custom_service_on_db_disc_evt() into the db_disc_handler() as well, just like how it is for CTS?[/quote]

Yes, if you make a custom service client, then you need to be able to discover it. And it makes sense to use the same approach as in the SDK examples.

[quote user="nordev"]ble_cts_c_on_ble_evt() seems like a callback function too, but it doesn't seem to be registered anywhere.[/quote]

It is a bit difficult to see, but you should look at the definition of BLE_CTS_C_DEF in ble_cts_c.h. This includes NRF_SDH_BLE_OBSERVER which adds ble_cts_c_on_ble_evt to the list of BLE observers. This is a section variable holding function pointers that should get all BLE events, so this makes the SoftDevice handler call ble_cts_c_on_ble_evt. Section variables are a bit like black magic, but it works 🙂 .

[quote user="nordev"]What passes it the events? [/quote]

If you want to look into the details you can check nrf_sdh.c, which is where the handler is called from. If you have configured to use the scheduler for BLE events, this is done by nrf_sdh_evts_poll().

[quote user="nordev"]To clarify, the applications event handler is on_cts_c_evt(), and ble_cts_c_on_ble_evt() is the SoftDevice event handler?[/quote]

Yes, sort off. on_cts_c_evt() is the applications event handler that handles events from the CTS client (ble_cts_c), and ble_cts_c_on_ble_evt() is the CTS client module's event handler for handling events form the SoftDevice.

RE: How to configure GATT server in NRF app to test CTS example

nordev — Fri, 28 Aug 2020 04:15:24 GMT

[quote userid="7377" url="~/f/nordic-q-a/65209/how-to-configure-gatt-server-in-nrf-app-to-test-cts-example/266411"]Events from the discovery module is passed to the CTS implementation by the call to ble_cts_c_on_db_disc_evt() within db_disc_handler()[/quote]

Hypothetically, if I develop a custom service for the central, should I put something equivalent to custom_service_on_db_disc_evt() into the db_disc_handler() as well, just like how it is for CTS?

[quote userid="7377" url="~/f/nordic-q-a/65209/how-to-configure-gatt-server-in-nrf-app-to-test-cts-example/266715"]you will see the ble_cts_c_on_ble_evt() handler which handled events, and it calls current_time_read() do process this before it calls the applications event handler in the end. That is where you see this in your application. In the example, the event handler is on_cts_c_evt() in the main., and that is where you read it.[/quote]

ble_cts_c_on_ble_evt() seems like a callback function too, but it doesn't seem to be registered anywhere. What passes it the events?

To clarify, the applications event handler is on_cts_c_evt(), and ble_cts_c_on_ble_evt() is the SoftDevice event handler?

Thank you again for the helpful and detailed answers!

RE: How to configure GATT server in NRF app to test CTS example

Einar Thorsrud — Thu, 27 Aug 2020 11:37:07 GMT

Hi,

[quote user="nordev"]Do you mean that these have to be added on the phone side in the NRF app, or on the firmware side? If the former, how do you add a service and characteristic in the app -- I did not see an option to do so...[/quote]

You are right, I was writing too fast. That is only available on the Android version. So you have to use nRF Connect for desktop or nRF Connect for Android to do this test.

[quote user="nordev"]Do you mean that the CTS instance and anything else registered as an observer each gets ALL the BLE events that occur and has to filter internally which events are relevant / can be handled by itself?[/quote]

Yes, that is how it is done.

[quote user="nordev"]Does it handle all BLE events, or only events associated with services?[/quote]

The service implementation only handles what it cares about and ignore other events.

[quote user="nordev"]If my previous statement is true, does that mean ble_cts_c_current_time_read() passes an event to the db discovery module?[/quote]

No. The discovery module is used for service discovery, and no longer used when that is done. ble_cts_c_current_time_read() is implemented in the CTS client implementation (components\ble\ble_services\ble_cts_c\ble_cts_c.c), and that just makes a GATTC read operation. That is queued and handled by the SoftDevice, which will generate an event when the read is completed. If you look more in ble_cts_c.c, you will see the ble_cts_c_on_ble_evt() handler which handled events, and it calls current_time_read() do process this before it calls the applications event handler in the end. That is where you see this in your application. In the example, the event handler is on_cts_c_evt() in the main., and that is where you read it.

RE: How to configure GATT server in NRF app to test CTS example

nordev — Thu, 27 Aug 2020 10:08:48 GMT

Thank you so much for this thorough reply!

[quote userid="7377" url="~/f/nordic-q-a/65209/how-to-configure-gatt-server-in-nrf-app-to-test-cts-example/266411"]You can set this up for iOS also, but then you need to configure it manually by adding the CTS service (UUID 0x1805) and the current time characteristic (UUID 0x2A2B)[/quote]

Do you mean that these have to be added on the phone side in the NRF app, or on the firmware side? If the former, how do you add a service and characteristic in the app -- I did not see an option to do so...

[quote userid="7377" url="~/f/nordic-q-a/65209/how-to-configure-gatt-server-in-nrf-app-to-test-cts-example/266411"]The CTS implementation gets all BLE events as it is registered as an observer by the NRF_SDH_BLE_OBSERVERS macro which is part of the BLE_CTS_C_DEF macro.[/quote]

Do you mean that the CTS instance and anything else registered as an observer each gets ALL the BLE events that occur and has to filter internally which events are relevant / can be handled by itself? Or if not, do you mean that the db_discovery handles the filtering of events to the right modules? Does it handle all BLE events, or only events associated with services?

[quote userid="7377" url="~/f/nordic-q-a/65209/how-to-configure-gatt-server-in-nrf-app-to-test-cts-example/266411"]Once the service is discovered, you can get the current time by calling ble_cts_c_current_time_read().[/quote]

If my previous statement is true, does that mean ble_cts_c_current_time_read() passes an event to the db discovery module?

RE: How to configure GATT server in NRF app to test CTS example

Einar Thorsrud — Wed, 26 Aug 2020 09:15:02 GMT

Hi,

[quote user=""]Question 1: How do I configure the GATT server on the NRF app to make my phone a Current Time server?[/quote]

The instructions in the example documentation are for nRF Connect for Desktop. Using that you simply import the cts_central.ncs by selecting Server Setup and loading the file:

You can set this up for iOS also, but then you need to configure it manually by adding the CTS service (UUID 0x1805) and the current time characteristic (UUID 0x2A2B).

[quote user=""]Question 2: How does the current time service get called?[/quote]

You can see this from the example in the SDK (examples\ble_peripheral\ble_app_cts_c\main.c), and this follows the same approach as all other services in the SDK.

[quote user=""]In the second screenshot (see references in left sidebar), it is evident that the event handler is never explicitly called. So how is an event generated?[/quote]

The on_cts_c_evt() function is a callback function / event handler that is registered when you initialize the CTS service (see the services_init() function in the example). The CTS implementation gets all BLE events as it is registered as an observer by the NRF_SDH_BLE_OBSERVERS macro which is part of the BLE_CTS_C_DEF macro. So when there is a BLE event that is handled by the service implementation (components\ble\ble_services\ble_cts_c\ble_cts_c.c), which calls the registered event handler when appropriate.

[quote user=""]What does the client (NRF device) have to do to discover the Current Time Server[/quote]

That is part of the service discovery procedure which is handled by the DB discovery module. Looking at main.c again, you can see in pm_evt_handler() that service discovery is started once the link is encrypted, by the call to ble_db_discovery_start(). Events from the discovery module is passed to the CTS implementation by the call to ble_cts_c_on_db_disc_evt() within db_disc_handler(). And here the CTS module does as described before, handles the event, and if relevant, passes an event back to the registered event handler (BLE_CTS_C_EVT_DISCOVERY_COMPLETE or BLE_CTS_C_EVT_DISCOVERY_FAILED in this case).

[quote user=""]or to request a Current Time reading from the server?[/quote]

Once the service is discovered, you can get the current time by calling ble_cts_c_current_time_read(). Then wait for the BLE_CTS_C_EVT_CURRENT_TIME event and do something with it (the example prints it using current_time_print()).