Session Run Time BLE CGMS characteristic

[pedrovfr_LNNano]

Hello all,

I'm developing a BLE CGMS application based on the experimental example on the SDK folder.

Now I working in the timing characteristics and I could see that the session run time (SRT) is not implemented on the example. When I try to use the function nrf_ble_cgms_srt_set() the input value parameter is in hours units, but my application runs in the seconds time base and I can't see where can I customize this characteristic.

Is this established inside the Softdevices? Can I change this for minutes for example? Are there other examples that implements similar characteristics with the Softdevices?

Following the functions that access this characteristic:

/**@brief Function for adding a characteristic for the Session Run Time.
 *
 * @param[in]   p_cgms   Service instance.
 *
 * @return      NRF_SUCCESS if characteristic was successfully added, otherwise an error code.
 */
static uint32_t srt_char_add(nrf_ble_cgms_t * p_cgms)
{
    uint8_t               len = 0;
    uint8_t               encoded_initial_srt[NRF_BLE_CGMS_SRT_LEN];
    ble_add_char_params_t add_char_params;

    memset(&add_char_params, 0, sizeof(add_char_params));


    len += uint16_encode(p_cgms->session_run_time, &(encoded_initial_srt[len]));

    add_char_params.uuid            = BLE_UUID_CGM_SESSION_RUN_TIME;
    add_char_params.max_len         = NRF_BLE_CGMS_SRT_LEN;
    add_char_params.init_len        = len;
    add_char_params.p_init_value    = encoded_initial_srt;
    add_char_params.read_access     = SEC_JUST_WORKS;
    add_char_params.write_access    = SEC_NO_ACCESS;
    add_char_params.char_props.read = true;

    return characteristic_add(p_cgms->service_handle,
                              &add_char_params,
                              &p_cgms->char_handles.srt);
}

ret_code_t nrf_ble_cgms_srt_set(nrf_ble_cgms_t * p_cgms, uint16_t run_time)
{
    ble_gatts_value_t srt_val;
    uint8_t           encoded_session_run_time[NRF_BLE_CGMS_SRT_LEN];
    uint8_t           gatts_value_set_len = 0;

    gatts_value_set_len = uint16_encode(run_time, encoded_session_run_time); // (p_sst, encoded_start_session_time);

    memset(&srt_val, 0, sizeof(ble_gatts_value_t));
    srt_val.len     = gatts_value_set_len;
    srt_val.p_value = encoded_session_run_time;
    srt_val.offset  = 0;

    return (sd_ble_gatts_value_set(p_cgms->conn_handle, p_cgms->char_handles.srt.value_handle,
                                   &srt_val));
}

[Kenneth]

The softdevice doesn't really care what data that is passed here, and the application will try to follow the BLE specification on the format of the data that should be passed. From Figure 1 in the BLE service specifications it seems this should be in hours:minutes:
https://www.bluetooth.com/specifications/specs/continuous-glucose-monitoring-service-1-0-1/

Kenneth

[pedrovfr_LNNano]

Ok, but how can I change this or even update the minutes on this characteristic?

Also the Figure 1 on the document refers to the Session Start Time, my issue is more related to the Session Run Time.

[Kenneth]

In GSS (current version is v5, available here https://www.bluetooth.com/specifications/specs/gatt-specification-supplement-5/ ) you can find the description. In short, it has 1 16bit mandatory field for the session run time  and one optional field for the CRC.

unit: org.bluetooth.unit.time.hour

[pedrovfr_LNNano]

Ok, what I am understanding is that the structure of this characteristic is implemented by the Softdevices (such as the "hours" text displayed in the peer) following the SIG's specification, and I can't actually change any of that and just pass the mandatory value through the 16bit field.

Is that right?

To present the value in minutes should I implement a custom service or can I modify the present CGM SRT service somehow?

[Kenneth]

pedrovfr_LNNano said:

Ok, what I am understanding is that the structure of this characteristic is implemented by the Softdevices (such as the "hours" text displayed in the peer) following the SIG's specification, and I can't actually change any of that and just pass the mandatory value through the 16bit field.

That is not technically correct, the softdfevice doesn't care about the data written, the data is just passed to the peer if read. However the whole point with having a Bluetooth profile is interoperability, if you change the data then you are no longer complying to the Bluetooth specifications. So if you need to have data that does not comply to a specific profile, then you will need to add this as a custom profile.

Kenneth