How to correctly setup a BLE Notify characteristic using NCS?

[MehdiZ]

Hello, I followed this tutorial nRF Connect SDK Bluetooth Low Energy tutorial part 1: Custom Service in Peripheral role  in order to setup custom BLE service. The tutorial as is works correctly and defines one service with an RX write characteristic and a TX notify characteristic.

BT_GATT_SERVICE_DEFINE(my_service,
BT_GATT_PRIMARY_SERVICE(BT_UUID_MY_SERVICE),
BT_GATT_CHARACTERISTIC(BT_UUID_MY_SERVICE_RX,
BT_GATT_CHRC_WRITE | BT_GATT_CHRC_WRITE_WITHOUT_RESP,
BT_GATT_PERM_READ | BT_GATT_PERM_WRITE, 
NULL, on_receive, NULL),
BT_GATT_CHARACTERISTIC(BT_UUID_MY_SERVICE_TX,
BT_GATT_CHRC_NOTIFY,
BT_GATT_PERM_READ,
NULL, NULL, NULL),
BT_GATT_CCC(on_cccd_changed,
BT_GATT_PERM_READ | BT_GATT_PERM_WRITE),
);

What I tried to do is delete the RX characteristic and keep the TX one in both read and notify

BT_GATT_SERVICE_DEFINE(my_service,
BT_GATT_PRIMARY_SERVICE(BT_UUID_MY_SERVICE),
BT_GATT_CHARACTERISTIC(BT_UUID_MY_SERVICE_TX,
			       BT_GATT_CHRC_NOTIFY | BT_GATT_CHRC_READ,
			       BT_GATT_PERM_READ | BT_GATT_PERM_WRITE,
                   on_read, NULL, &number),
BT_GATT_CCC(on_cccd_changed,
        BT_GATT_PERM_READ | BT_GATT_PERM_WRITE),
);

The read aspect works correctly and I'm able to get the data on the client interface (PC side, LightBlue)

But when I try to activate the notification, I have the following error on the Serial Monitor (nRF53 side):

[00:59:48.195,129] <wrn> bt_l2cap: Ignoring data for unknown channel ID 0x003a
Error, unable to send notification err= -2

which corresponds to ENOENT 2 /**< No such file or directory */

I do not understand what is the problem.

The idea I have is that a write characteristic is needed in order for the client to enable the notification. Something like the following:

Client -> enable notification through write/RX

Peripheral -> send data through TX

is my understanding correct?

[Simon]

Try the following sample:ble_peripheral_cus_service.zip

Did you remember to change attrs[3] to attrs[2]

const struct bt_gatt_attr *attr = &my_service.attrs[2]; 

Best regards,

Simon

[MehdiZ]

Nope... I totally forgot about that array.

Thank you very much.

If I understand correctly, once the service is create by the macro, the characteristics are stored in the attribute array and can be accessed afterwards from there?

[MehdiZ]

Would you kindly point me in the right direction to setup multiple notifications?

I found the configuration to activate it  CONFIG_BT_GATT_NOTIFY_MULTIPLE but I don't see further documentation on it

Thank you for your help.

[Simon]

Take a look at this case. Test that out for example with this sample: https://github.com/simon-iversen/ncs_samples/tree/master/ble_peripheral_cus_service and try to add notifications to two characteristics.

Best regards,

Simon

[MehdiZ]

Thank you for you help.
I got it working by adding a CCC for each characteristic

BT_GATT_SERVICE_DEFINE(my_service,
BT_GATT_PRIMARY_SERVICE(BT_UUID_MY_SERVICE),
BT_GATT_CHARACTERISTIC(BT_UUID_MY_SERVICE_TX,
                   BT_GATT_CHRC_NOTIFY,
                   BT_GATT_PERM_READ,
                   on_read, NULL, &number),
BT_GATT_CCC(on_cccd_changed,
        BT_GATT_PERM_READ | BT_GATT_PERM_WRITE),
BT_GATT_CHARACTERISTIC(BT_UUID_MY_SERVICE_TX2,
                   BT_GATT_CHRC_NOTIFY,
                   BT_GATT_PERM_READ,
                   on_read, NULL, &number),
BT_GATT_CCC(on_cccd_changed,
        BT_GATT_PERM_READ | BT_GATT_PERM_WRITE),
);