Device name too short?

[maxv]

When trying out the ble_app_hrs_freertos example in nRF5 SDK 11.0.0-2.alpha_bc3f6a0, I seem to only get 14-character long device name announcements.

This happens when using components/softdevice/s132/hex/s132_nrf52_2.0.0-7.alpha_softdevice.hex.

I haven't had a chance yet to try it with s132_nrf52_1.0.0-3.alpha_softdevice.hex, which I had experience using from the nRF52 SDK 0.9.2.

[Anders Strand]

I reproduced the behavior, and did some investigation.

The SDK module ble_advertising.c shortens the name if there is not room for the full name in the advertisement packet. (Added in SDK 11). The total amount of bytes in the advertisement cannot exceed 31 bytes.

From ble_advdata.c:

else
    {
        // Else short name needs to be used. Or application has requested use of short name.
        adv_data_format = BLE_GAP_AD_TYPE_SHORT_LOCAL_NAME;

        // If application has set a preference on the short name size, it needs to be considered,
        // else fit what can be fit.
        if ((BLE_ADVDATA_SHORT_NAME == p_advdata->name_type) &&
                (p_advdata->short_name_len <= rem_adv_data_len))
        {
            // Short name fits available size.
            actual_length = p_advdata->short_name_len;
        }
        // Else whatever can fit the data buffer will be packed.
        else
        {
            actual_length = rem_adv_data_len;
        }
    }

[maxv]

What accounts for the other 17 bytes, if the device name has to be shortened to 14 characters?

[maxv]

Does it have to do with the number of services being announced? Each standard service has a 2-byte service UUID? If I use a custom service UUID, that's 16-bytes. So there's 14 + 16, plus 1 byte for something else...

I noticed that if I do this:

#if 0
    {BLE_UUID_HEART_RATE_SERVICE,         BLE_UUID_TYPE_BLE},
    {BLE_UUID_BATTERY_SERVICE,            BLE_UUID_TYPE_BLE},
#endif

in the heart rate sample, the full Device Name is now visible again.

[Anders Strand]

All fields get 2 bytes header (length+type). We have 4 fields. (name, uuid's, flags, appearance.) This gives 8 bytes in header. Then we have 3 uuids of 2 bytes = 6 bytes. Then the flags are 1 byte, and the appearance is 2 bytes. This totals on 17 bytes, which leaves 14 bytes for the name (31 in total).