What variable and struct does the write characteristic store its data in after a write command from an application on nrf connect?

[RyanA]

Hello,

I have followed the BLE tutorial on characteristics to make my own 2 custom characteristics that successfully read and write uint8 data to my nRF52832, however I want to take that data that was written from each characteristic to my chip and store them in local variables in my main loop, how do I do this? 

[awneil]

When the Central (any Central - not specific to nRF Connect) writes a Characteristic, you will get a BLE Event - and the data is in that event.

[RyanA]

Ok, is there a specific function or struct that I can see that in?

[awneil]

Have you looked at the Service & Characteristic tutorials?

https://devzone.nordicsemi.com/nordic/short-range-guides/b/bluetooth-low-energy/posts/ble-services-a-beginners-tutorial

https://devzone.nordicsemi.com/nordic/short-range-guides/b/bluetooth-low-energy/posts/ble-characteristics-a-beginners-tutorial

they include the event handler ...

[RyanA]

I read the characteristics tutorial and I can see they have a handler for the BLE event in this function: 

In that handler we get this definition:

Where in here are we updating our actual write value data? I want to take that written value and print it in the debug window to verify my app write function is working. 

[awneil]

That's the one!

If you go to the definitions of those events, you will find that there are also other events - including one for 'Write'.

In the event structure, you get (a pointer to) the data that was written ...

Not as pictures!

[RyanA]

Ok, I am looking through all the events in the same location as BLE_GAP_EVT (ble_gap.h), and I see this:

/**@brief GAP Event IDs.
 * IDs that uniquely identify an event coming from the stack to the application.
 */
enum BLE_GAP_EVTS
{
  BLE_GAP_EVT_CONNECTED                   = BLE_GAP_EVT_BASE,       /**< Connected to peer.                              \n See @ref ble_gap_evt_connected_t             */
  BLE_GAP_EVT_DISCONNECTED                = BLE_GAP_EVT_BASE + 1,   /**< Disconnected from peer.                         \n See @ref ble_gap_evt_disconnected_t.         */
  BLE_GAP_EVT_CONN_PARAM_UPDATE           = BLE_GAP_EVT_BASE + 2,   /**< Connection Parameters updated.                  \n See @ref ble_gap_evt_conn_param_update_t.    */
  BLE_GAP_EVT_SEC_PARAMS_REQUEST          = BLE_GAP_EVT_BASE + 3,   /**< Request to provide security parameters.         \n Reply with @ref sd_ble_gap_sec_params_reply.  \n See @ref ble_gap_evt_sec_params_request_t. */
  BLE_GAP_EVT_SEC_INFO_REQUEST            = BLE_GAP_EVT_BASE + 4,   /**< Request to provide security information.        \n Reply with @ref sd_ble_gap_sec_info_reply.    \n See @ref ble_gap_evt_sec_info_request_t.   */
  BLE_GAP_EVT_PASSKEY_DISPLAY             = BLE_GAP_EVT_BASE + 5,   /**< Request to display a passkey to the user.       \n In LESC Numeric Comparison, reply with @ref sd_ble_gap_auth_key_reply. \n See @ref ble_gap_evt_passkey_display_t. */
  BLE_GAP_EVT_KEY_PRESSED                 = BLE_GAP_EVT_BASE + 6,   /**< Notification of a keypress on the remote device.\n See @ref ble_gap_evt_key_pressed_t           */
  BLE_GAP_EVT_AUTH_KEY_REQUEST            = BLE_GAP_EVT_BASE + 7,   /**< Request to provide an authentication key.       \n Reply with @ref sd_ble_gap_auth_key_reply.    \n See @ref ble_gap_evt_auth_key_request_t.   */
  BLE_GAP_EVT_LESC_DHKEY_REQUEST          = BLE_GAP_EVT_BASE + 8,   /**< Request to calculate an LE Secure Connections DHKey. \n Reply with @ref sd_ble_gap_lesc_dhkey_reply.  \n See @ref ble_gap_evt_lesc_dhkey_request_t */
  BLE_GAP_EVT_AUTH_STATUS                 = BLE_GAP_EVT_BASE + 9,   /**< Authentication procedure completed with status. \n See @ref ble_gap_evt_auth_status_t.          */
  BLE_GAP_EVT_CONN_SEC_UPDATE             = BLE_GAP_EVT_BASE + 10,  /**< Connection security updated.                    \n See @ref ble_gap_evt_conn_sec_update_t.      */
  BLE_GAP_EVT_TIMEOUT                     = BLE_GAP_EVT_BASE + 11,  /**< Timeout expired.                                \n See @ref ble_gap_evt_timeout_t.              */
  BLE_GAP_EVT_RSSI_CHANGED                = BLE_GAP_EVT_BASE + 12,  /**< RSSI report.                                    \n See @ref ble_gap_evt_rssi_changed_t.         */
  BLE_GAP_EVT_ADV_REPORT                  = BLE_GAP_EVT_BASE + 13,  /**< Advertising report.                             \n See @ref ble_gap_evt_adv_report_t.           */
  BLE_GAP_EVT_SEC_REQUEST                 = BLE_GAP_EVT_BASE + 14,  /**< Security Request.                               \n See @ref ble_gap_evt_sec_request_t.          */
  BLE_GAP_EVT_CONN_PARAM_UPDATE_REQUEST   = BLE_GAP_EVT_BASE + 15,  /**< Connection Parameter Update Request.            \n Reply with @ref sd_ble_gap_conn_param_update. \n See @ref ble_gap_evt_conn_param_update_request_t. */
  BLE_GAP_EVT_SCAN_REQ_REPORT             = BLE_GAP_EVT_BASE + 16,  /**< Scan request report.                            \n See @ref ble_gap_evt_scan_req_report_t. */
  BLE_GAP_EVT_PHY_UPDATE_REQUEST          = BLE_GAP_EVT_BASE + 17,  /**< PHY Update Request.                             \n Reply with @ref sd_ble_gap_phy_update. \n See @ref ble_gap_evt_phy_update_request_t. */
  BLE_GAP_EVT_PHY_UPDATE                  = BLE_GAP_EVT_BASE + 18,  /**< PHY Update Procedure is complete.               \n See @ref ble_gap_evt_phy_update_t.           */
  BLE_GAP_EVT_DATA_LENGTH_UPDATE_REQUEST = BLE_GAP_EVT_BASE + 19,   /**< Data Length Update Request.                     \n Reply with @ref sd_ble_gap_data_length_update.\n See @ref ble_gap_evt_data_length_update_request_t. */
  BLE_GAP_EVT_DATA_LENGTH_UPDATE         = BLE_GAP_EVT_BASE + 20,   /**< LL Data Channel PDU payload length updated.     \n See @ref ble_gap_evt_data_length_update_t. */
  BLE_GAP_EVT_QOS_CHANNEL_SURVEY_REPORT  = BLE_GAP_EVT_BASE + 21,   /**< Channel survey report.                          \n See @ref ble_gap_evt_qos_channel_survey_report_t. */
  BLE_GAP_EVT_ADV_SET_TERMINATED         = BLE_GAP_EVT_BASE + 22,   /**< Advertising set terminated.                     \n See @ref ble_gap_evt_adv_set_terminated_t. */
};

Looking through each of these I can't determine which one is applicable to reacting to "write" events.

[Vidar Berg]

Hi,

Write is a GATT event. I'd recommend you to take a look at existing services such as NUS and Blinky and see how these forward GATT "write" events to the application. 

Code snippet below is from the ble_nus.c service used by the ble_app_uart example

void ble_nus_on_ble_evt(ble_evt_t const * p_ble_evt, void * p_context)
{
    if ((p_context == NULL) || (p_ble_evt == NULL))
    {
        return;
    }

    ble_nus_t * p_nus = (ble_nus_t *)p_context;

    switch (p_ble_evt->header.evt_id)
    {
        case BLE_GAP_EVT_CONNECTED:
            on_connect(p_nus, p_ble_evt);
            break;

        case BLE_GATTS_EVT_WRITE:
            on_write(p_nus, p_ble_evt); //<-- Received write event. Run on_write to see if belongs to the NUS service
            break;

        case BLE_GATTS_EVT_HVN_TX_COMPLETE:
            on_hvx_tx_complete(p_nus, p_ble_evt);
            break;

        default:
            // No implementation needed.
            break;
    }
}

 

Here you can see how the BLE_GATTS_EVT_WRITE event is processed and how it's being forwarded to the event handler registered by the application during service init.