bt_gatt_notify issues

[rtisys]

I am developing a simple ble peripheral device that will measure temperature and humidity.  For the most

part, it's working but I can't get the behavior I expect when enabling notifications.  I have searched various

web sites - including Zephyr - for info; but am coming up short.  Some help would be appreciated.

I am using latest nrf connect sdk to develop the app,  I have been having problems consistently using

the vscode front-end, so I am currently using the nrf connect cli for building the app and nrf downloading

tool to flash.  Will leave questions about vscode for another thread.  I can reliably build and flash apps 

using the above and am using the android nrf connect test app on smartphone for verifying correct

service behavior.  Service declaration is as follows:

BT_GATT_SERVICE_DEFINE(svc,

    BT_GATT_PRIMARY_SERVICE(BT_UUID_ESS),

    BT_GATT_CHARACTERISTIC(BT_UUID_TEMPERATURE,

        BT_GATT_CHRC_READ | BT_GATT_CHRC_NOTIFY,

        BT_GATT_PERM_READ,

        read_temperature, NULL, &temp_ble),

    BT_GATT_CCC(BLE_temperature_ccc_cfg_changed,

            BT_GATT_PERM_READ | BT_GATT_PERM_WRITE),

    BT_GATT_CHARACTERISTIC(BT_UUID_HUMIDITY,

        BT_GATT_CHRC_READ | BT_GATT_CHRC_NOTIFY,

        BT_GATT_PERM_READ,

        read_humidity, NULL, &hum_ble),

    BT_GATT_CCC(BLE_humidity_ccc_cfg_changed,

            BT_GATT_PERM_READ | BT_GATT_PERM_WRITE),

);

temp_ble and hum_ble are updated from a SHT4x sensor over i2c using

existing Zephyr driver.  Values are reasonable and that part appears to be

working reliably.  I interrogate the sensor every 2 sec, and set both characteristic

values.  After service discovery using android nrf connect tool, both can be read

out successfully.  I also issue a bt_gatt_notify() for each value.  It's my understanding

that, if a ble central node is connected AND notification has been enabled, then

these values will be reported to the client (i.e. ble central)  and displayed.  Here's

the main code:

........

int bt_svc_set_temperature(int temp_ble) {
int rc = bt_gatt_notify(NULL, &svc.attrs[1], &temp_ble, sizeof(temp_ble));
return rc == -ENOTCONN ? 0 : rc;
}

int bt_svc_set_humidity(int hum_ble) {
int rc = bt_gatt_notify(NULL, &svc.attrs[3], &hum_ble, sizeof(hum_ble));
return rc == -ENOTCONN ? 0 : rc;
}

int main(void) {
// start advertising
int error = bt_enable(NULL);
........
error = bt_le_adv_start(BT_LE_ADV_CONN, ad, ARRAY_SIZE(ad), NULL, 0);
.........
printk("Advertising successfully started\n");

while (1) {
k_sleep(K_SECONDS(2));
if (sensor_sample_fetch(sht)) {
printf("Failed to fetch sample from SHT4X device\n");
} else {
sensor_channel_get(sht, SENSOR_CHAN_AMBIENT_TEMP, &temp);
temp_ble = temp.val1; // Only integer part of temp for now
bt_svc_set_temperature(temp_ble);

sensor_channel_get(sht, SENSOR_CHAN_HUMIDITY, &hum);
hum_ble = hum.val1; // Only integer part of hum for now;
bt_svc_set_humidity(hum_ble);
}

As I mentioned, individual reads work fine.  But notify's yield weird

behavors.  In the above code, when temperature notifications are 

enabled, it works correctly.  When I subsequently enable humidity

notifications, the temperature value gets overwritten ( and contiinually

updated) with the humidity data.  Humidity values do not get updated

at all.  Various permutations of svc.attrs[x]

yield different - but also incorrect - behavior.

I clearly don't have the attrs[n] mapping to characteristics correct; but

can't find any discussion about it.  So:

-What's the correct usage of svc.attrs[x] for the above service descriptor

macro.

-Do I need to wait for a notification to complete before issuing another?

If so, how can I detect completion?

Or, do I have the whole model of notification behavior wrong?

Thanks

[Edvin]

Hello,

The svc.attrs[x] is a bit tricky in Zephyr (in my opinion). And I typically just try until I get it right, whenever I add a new characteristic. For more details, however, I can recommend the Bluetooth Low Energy Fundamentals course on Developer Academy:

https://academy.nordicsemi.com/courses/bluetooth-low-energy-fundamentals/lessons/lesson-4-bluetooth-le-data-exchange/topic/attribute-table/

Particularly, Lesson 4 will cover the attribute table, both on the theory part and in the exercise. Perhaps you can run through Lesson 4, and see if it is any clearer what number to use in the svc.attrs. As you can see, it depends on the characteristics before it, and how they are set up.

Best regards,

Edvin

[Robotics Gun]

For somebody who face the same issue, 
I put the lesson code solution's repo address here.

https://github.com/NordicDeveloperAcademy/bt-fund/blob/main/lesson4/blefund_less4_exer2_solution/src/my_lbs.c

selecting the right index is a bit tricky as Edvin mentioned.

[Robotics Gun]

What's the link "run3"?

Does this have something to do with the question?

[Edvin]

Spam bot. Deleting user.

[DiBosco]

A little trick to help with this ludicrous situation:

uint16_t i;

for ( i = 0; i < svc.attr_count; i++)
{
    printk("Attribute %d UUID: %s\n", i, bt_uuid_str(svc.attrs[i].uuid));

}

This will spit out what all the different attributes are assigned to as their array positions.