Bluetooth: Characteristic Value Showing up as N/A on nRF Connect App

[SarikaK]

I'm unable to load values into characteristics—they show up as N/A on the nRF Connect app. I tried to enable notifications using the Client Characteristic Configuration, but when I click on the second arrow button (which I think is for enabling notifications, but I could be wrong), I am getting an error that says "Reading is not permitted". I thought I had set the necessary read permissions, but is there something I need to fix here?

This is the function which is passed the value that I want to load into the characteristic:

What I see in the app:

[Susheel Nuguru]

What is AFE_GATT_PERM_DEFAULT? are you sure if there are read permissions there?

[SarikaK]

My understanding is that AFE_GATT_PERM_DEFAULT should set the default GATT permissions for both read and write:

[SarikaK]

peripheral_nfc_pairing.zip

Thank you!

[SarikaK]

Just wanted to follow up on this!

[Susheel Nuguru]

I am sorry Sarika, Did not reproduce the issue yet, 

I will spend time tomorrow to reproduce this and if I manage to reproduce it, it will be easy to give a workaround/fix.

[Susheel Nuguru]

sorry for testing this a bit late Sarika, 

I see no issues with the read permissions testing this on iPhone14 with nRF connect for mobile app.

scratch that ..I am able to reproduce that it is not able to read the characteristic when pressing the down arrow. I will look into it and see if it is missing any permissions.

[Susheel Nuguru]

Just realized that you have not implemented the gatt read function for reading the attribute. Please fix your service definition as below 

static ssize_t read_nfc_feature(struct bt_conn *conn,
				const struct bt_gatt_attr *attr, void *buf,
				uint16_t len, uint16_t offset)
{
	return bt_gatt_attr_read(conn, attr, buf, len, offset, NULL, 0);
}

/* Sine Wave Service Declaration */
BT_GATT_SERVICE_DEFINE(afe_svc,
	BT_GATT_PRIMARY_SERVICE(BT_UUID_AFE_DATA),
	BT_GATT_CHARACTERISTIC(BT_UUID_SINE_WAVE,
				BT_GATT_CHRC_READ | BT_GATT_CHRC_NOTIFY,
				BT_GATT_PERM_READ, read_nfc_feature, NULL,
				NULL),
	BT_GATT_CCC(afe_ccc_cfg_changed,
			 AFE_GATT_PERM_DEFAULT),
);

This should allow your app to read the attribute from nrf connect app. 

[SarikaK]

Thanks, Susheel! I'll try this out and get back to you.

[SarikaK]

So I was able to see that notifications were enabled, but I could only read the value 00 for the attribute in the nRF Connect app. I am passing this function varying sine_value values from an array, so I was expecting to see the attribute value change accordingly in the app.

 

Not sure what else needs to be modified here—does attr_count need to be assigned a value?

[Susheel Nuguru]

When I tested this a month ago, the attribute value was changing for the one for which you were sending notification. After implementing the read function, the read in the app was also working fine.

Please note that reading in app and getting notified are two different things. If the read is not working correct, then I guess something with read function is not correct.