Service and Characteristic discovery of GATT server on central

[demJanus]

I have implemented a Peripheral on an PCA10040 based on one of the SDK (v15) examples. When using nrf Connect with a nRF51 dongle I can connect to the peripheral and interact with my services on the peripherals GATT server.

I now want to do the same thing vice versa, meaning I want to query services on a GATT server on the central from the Peripheral.

So I created a DIS (0x180A) with a manufacturer name sting (0x2A29) and a custom service (0000AABBCCDD11223344556677889900) with one characteristic (0000AABCCCDD11223344556677889900) on the nrf connect application.

During initialization on the NRF52-DK I do:

static void discovery_init(void)
{
    ble_db_discovery_init_t db_init;
    ret_code_t err_code;
    ble_uuid_t uuid;

    memset(&db_init, 0, sizeof(db_init));
    db_init.evt_handler  = discovery_handler;
    db_init.p_gatt_queue = &m_ble_gatt_queue;

    err_code = ble_db_discovery_init(&db_init);
    APP_ERROR_CHECK(err_code);

    uuid.uuid = 0x2A29;  // Manufacturer String
    uuid.type = BLE_UUID_TYPE_BLE;
    err_code = ble_db_discovery_evt_register(&uuid);
    APP_ERROR_CHECK(err_code);

    uuid.uuid = 0xAABC;
    uuid.type = BLE_UUID_TYPE_VENDOR_BEGIN;
    err_code = ble_db_discovery_evt_register(&uuid);
    APP_ERROR_CHECK(err_code);
}

after the connect event BLE_GAP_EVT_CONNECTED I start the discovery:

err_code = ble_db_discovery_start(&m_db_disc, p_ble_evt->evt.gap_evt.conn_handle);
APP_ERROR_CHECK(err_code);

Unfortunately none of the registered services are discovered.

My discovery handler is called three times with the events:

BLE_DB_DISCOVERY_SRV_NOT_FOUND
BLE_DB_DISCOVERY_SRV_NOT_FOUND
BLE_DB_DISCOVERY_AVAILABLE

If I use the 0x180A for the DIS service instead of the characteristic 0x2A29 the handler is called only twice

BLE_DB_DISCOVERY_SRV_NOT_FOUND
BLE_DB_DISCOVERY_AVAILABLE

but I never get a

BLE_DB_DISCOVERY_COMPLETE

What am I missing?

How can I discover custom services?

And can't I just discover all services available on the central and have a look on them somehow?

Thanks for the support.

[Vidar Berg]

Hello,

If I use the 0x180A for the DIS service instead of the characteristic 0x2A29 the handler is called only twice

Yes, it's the service UUID you need to register with the DB discovery module. The characteristics are then found by you in the discovery_handler() after the service has been discovered. I posted an example of this here: https://devzone.nordicsemi.com/f/nordic-q-a/49837/reading-the-central-device-name-smart-phone-in-nrf52dk-peripheral-and-printing-on-the-terminal/199126#199126

custom service (0000AABBCCDD11223344556677889900) with one characteristic (0000AABCCCDD11223344556677889900) on the nrf connect application.

Did you register this UUID with sd_ble_uuid_vs_add() on the peripheral?

Thanks,

Vidar

[demJanus]

Yes I did register it. Menwhile I found out that I need to use the handle that gets provided when registering the service and not BLE_UUID_TYPE_VENDOR_BEGIN.

This could be pointed out a bit more clearly. Not clear to me from the tutorials.

The following threads gave an indication to the reason for me:

https://devzone.nordicsemi.com/f/nordic-q-a/206/128-bit-uuid-s

https://devzone.nordicsemi.com/f/nordic-q-a/52390/understanding-advertisement-of-multiple-128-bit-custom-uuid/212429

https://devzone.nordicsemi.com/f/nordic-q-a/21152/db-discovery-failing-with-ble_db_discovery_srv_not_found

Thanks for your support anyway.

[Vidar Berg]

Thanks for the update and feedback. Yes, I agree that it may not be immediately obvious that you have to reference the UUID using the tag returned by sd_ble_uuid_vs_add().