Service Changed Characteristic not enabled on iOS

[AntonHellbe]

Hi,

My current project make use of pairing/bonding and at certain times I change the attribute table. To notify the central device about this change I have added the Service Changed Characteristic. It works good on Android after enabling notifications from the characteristic. However on iOS I cannot see the Service Changed Characteristic (iOS filter these out according to their guidelines), and I always receive error: NRF_ERROR_INVALID_STATE when I try to send I notification to an iOS device. I  seems like iOS is not enabling notifications on the Service Changed characteristic.

Is there something more to be done in order to make iOS enable notification for the Service Changed Characteristic?

I send the notification from the BLE_GAP_EVT_CONN_SEC_UPDATE event:

        err_code = sd_ble_gatts_service_changed(
            p_ble_evt->evt.gap_evt.conn_handle,
            system_service_get_service_handle(),
            0xFFFF
        );

        if (err_code != NRF_SUCCESS)
        {
            NRF_LOG_INFO("Failed to send service changed indication!, err_code: %d", err_code);
        }

Edit:

After pairing I can send a Service Changed Notification. When the device eventually get disconnected after pairing/bonding I reconnect and after that the nRF52832 can no longer send Service Changed Notifications (NRF_ERROR_INVALID_STATE). Do I need to restore the CCCDs or what is happening here?

[AntonHellbe]

Hi,

I think I have found the issue. After pairing/bonding another CPU resets the nRF52832, but before that happens I can see that the application can send one notification. When the nRF52832 boots up again and I reconnect I receive error NRF_ERROR_INVALID_STATE. It seems like the CCCDs are not saved on Central side it seems like.

[AntonHellbe]

Further investigation shows that it happens regardless of reset or not.

I pair / bond using my iPhone 6s running iOS 12.4, after pairing/bonding is complete my application running on the nRF52832 sends one notification successfully. Then the iPhone gets disconnected regardless, when I reconnect I receive NRF_ERROR_INVALID_STATE when trying to send Service Changed notifications

[AntonHellbe]

Hi Vidar,

I checked the source files for SDK v15.0 (gatt_cache_manager.c) and I cannot find the function service_changed_cccd function that you mention in the other thread.

[Vidar Berg]

That is correct, the function was added in SDK 15.3.0 and is used to check whether the SC indication is enabled or if it even exists. You need to add this function in the NRF_ERROR_INVALID_STATE case to determine if is "invalid state" because of an ongoing MTU exchange or simply because the indication is not enabled.

[AntonHellbe]

I see, I was a bit unsure about that part. Thanks for clarifying!

[Vidar Berg]

No problem. Just let me know if it's not working. 

[AntonHellbe]

Hi Vidar,

adding the service_changed_cccd and modifying the send_in_evt function solved it. Thanks.

/ Anton