How to know when a GATT client characteristic is updated

RE: How to know when a GATT client characteristic is updated

julienh — Tue, 13 Sep 2016 15:45:50 GMT

Thank you Petter. I had created the same server setup yesterday. Take a little bit of time for me to understand notification on client side but now, everything is clear for me.

RE: How to know when a GATT client characteristic is updated

Petter Myhre — Tue, 13 Sep 2016 11:44:35 GMT

This server setup should send notifications when updated.

RE: How to know when a GATT client characteristic is updated

RK — Sun, 11 Sep 2016 11:05:18 GMT

I actually don't know the answer to that- having never used the MCP. Technically any CTS peripheral should have the time characteristic as notify, it's mandatory, however if the MCP is just for testing purposes with the examples in the SDK, perhaps it isn't.

Can you use one of the little btle things runs on a phone? I know LIghtblue can pretend to be various sorts of peripheral, CTS is probably one of them and I'd assume they wrote it to be notifyable.

RE: How to know when a GATT client characteristic is updated

julienh — Sun, 11 Sep 2016 09:22:29 GMT

I use the server setup from cts example for MCP to test my App. It seems that the characteristic is not configured as Notify. I think that I have to create my own server setup. Is that right?

RE: How to know when a GATT client characteristic is updated

RK — Sat, 10 Sep 2016 01:58:15 GMT

well do you know that your server properly supports notifications? Is it sending them out? Have you written the CCCD with the notification/indication value?

RE: How to know when a GATT client characteristic is updated

julienh — Fri, 09 Sep 2016 23:21:03 GMT

Following advises, I have tried to use the alert notification example without success. I added :

CCCD configuration to enable notification for the characteristic.
BLE_GATTC_EVT_HVX event to the on_ble_event

but nothing happen in the client side when I update the value in the server.

Help please

Julien

RE: How to know when a GATT client characteristic is updated

julienh — Thu, 08 Sep 2016 07:10:13 GMT

Hi RK, thanks for your answer.

I don't understand very well the service initialization on the Current Time Service client code. It seems that everything is managed in the db_discover_evt_handler(ble_db_discovery_evt_t * p_evt) and that I can get back cccd handle here :

if (p_evt->params.discovered_db.charateristics[i].characteristic.uuid.uuid ==
            BLE_UUID_CURRENT_TIME_CHAR)
        {
            // Found Current Time characteristic. Store CCCD and value handle and break.
            mp_ble_cts->cts_cccd_handle =
                p_evt->params.discovered_db.charateristics[i].cccd_handle;
            mp_ble_cts->current_time_handle =
                p_evt->params.discovered_db.charateristics[i].characteristic.handle_value;
            break;

Do I have to add a BLE CTS event type when I get back notification information from the DB?

RE: How to know when a GATT client characteristic is updated

RK — Wed, 07 Sep 2016 23:46:57 GMT

If you want to notify a peer of changes to the value of a characteristic then you make the characteristic notifyable or indicatable. The peer then subscribes to the characteristic and you send out notifications or indications each time you make a change.

If you look at the description of the current time service you'll see the Current Time characteristic is notifyable (and it's mandatory to be).

As far as I can see, the Current Time Service client code (ble_cts_c) doesn't have any code to support subscribing to the characteristic, so you'll have to write your own/add your own. If you look at the code for the ble_hrs_c service however, that does show how to subscribe to notifications, you have to write to the 'cccd' characteristic to enable and disable them, then handle the resultant messages.

So you need to integrate that subscription and handling code into the Current Time Service code.