Custom Notification Characteristic Not Updating

RE: Custom Notification Characteristic Not Updating

paulcardno — Mon, 21 Nov 2022 19:16:17 GMT

thanks Hung
Much appreciate the feedback and you comment about the characteristic declare and value did highlight some learning that I needed to do, so was well worth hearing. And the other comments and links where very help.

RE: Custom Notification Characteristic Not Updating

Hung Bui — Mon, 21 Nov 2022 08:38:37 GMT

Hi Paul,

I think it's my mistake. Either handle, characteristic declare or characteristic value would work. So in your case adc_svc.attrs[1] and adc_svc.attrs[2] would work.
I was having the impression that adc_svc.attrs[1] doesn't work but it was because the length was 21 at that point. It was not because of adc_svc.attrs[1].

RE: Custom Notification Characteristic Not Updating

paulcardno — Sat, 19 Nov 2022 03:04:49 GMT

Hi Hung

Thanks you very much for this information. I was missing a couple of knowledge points and I have studied these, so thanks for this. Much appreciated.

with the characteristic declaration versus characteristic value

I tried

rc = bt_gatt_notify(NULL, &adc_svc.attrs[2], &level, sizeof(level));

which worked but I also tried

rc = bt_gatt_notify(NULL, &adc_svc.attrs[1], &level, sizeof(level));

and this worked also (checked that the firmware was updated by changing the service UUID).

I assume this is what you are referring to. Or am a I missing something here.

I also read your blog (thanks for the link), you say "For example in the following service, the characteristic value of BT_UUID_MY_SERVICE_TX and BT_UUID_MY_SERVICE_TEMPERATURE are attrs[3] and attrs[6] respectively."

and then you say "...in the above example we have the ATT table for the service as follows:"

with "

4	TX Char value

So in my mind it looks like the first sentence should be attrs[4] for service_TX, am I missing something here also?

RE: Custom Notification Characteristic Not Updating

Hung Bui — Fri, 18 Nov 2022 12:26:17 GMT

Hi Paul,
From what I can see in the code is that you have a characteristic with notification permission. But you haven't defined a CCCD characteristic so that the client can enable notification for the characteristic above it.

If you have a look at the NUS service (\nrf\subsys\bluetooth\services\nus.c) in peripheral_uart sample you can find this:

Also there were two more issue with notify_adc(), first is that you are sending to attribute handle &adc_svc.attrs[1] which is the declaration of the characteristic, not the value. And secondly, you are trying to send 21 bytes when the maximum is 20 bytes (if you don't use data length extension, max data length is 23 bytes and with 3 bytes overhead you have 20 bytes left )
This would work:

static void notify_adc(void){
	printk("notify adc2 ");
	static uint8_t level[20] = { 'a', 'b', 'a', 'a', '8', 'a', 'a', 'a'};

	int rc;
	rc = bt_gatt_notify(NULL, &adc_svc.attrs[2], &level, sizeof(level));

	return rc == -ENOTCONN ? 0 : rc;
}

With this in the declaration:

BT_GATT_SERVICE_DEFINE(adc_svc,
	BT_GATT_PRIMARY_SERVICE(&notification_uuid),
	BT_GATT_CHARACTERISTIC(&adc_notify_uuid.uuid, BT_GATT_CHRC_NOTIFY,
				BT_GATT_PERM_NONE, NULL, NULL, NULL),
	BT_GATT_CCC(NULL,     BT_GATT_PERM_READ | BT_GATT_PERM_WRITE),

);

I would suggest to have a look at my blog here. In the blog I explained why you should use correct attribute handle.