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

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

Vidar Berg — Mon, 14 Oct 2019 12:43:44 GMT

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.

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

RyanA — Sat, 12 Oct 2019 18:59:13 GMT

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

data-mode="c_cpp">/**@brief GAP Event IDs. identify an event coming from the stack to the application. = BLE_GAP_EVT_BASE, /**< Connected to peer. \n See @ref ble_gap_evt_connected_t */ = BLE_GAP_EVT_BASE + 1, /**< Disconnected from peer. \n See @ref ble_gap_evt_disconnected_t. */ = BLE_GAP_EVT_BASE + 2, /**< Connection Parameters updated. \n See @ref ble_gap_evt_conn_param_update_t. */ = 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_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_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_BASE + 6, /**< Notification of a keypress on the remote device.\n See @ref ble_gap_evt_key_pressed_t */ = 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_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_BASE + 9, /**< Authentication procedure completed with status. \n See @ref ble_gap_evt_auth_status_t. */ = BLE_GAP_EVT_BASE + 10, /**< Connection security updated. \n See @ref ble_gap_evt_conn_sec_update_t. */ = BLE_GAP_EVT_BASE + 11, /**< Timeout expired. \n See @ref ble_gap_evt_timeout_t. */ = BLE_GAP_EVT_BASE + 12, /**< RSSI report. \n See @ref ble_gap_evt_rssi_changed_t. */ = BLE_GAP_EVT_BASE + 13, /**< Advertising report. \n See @ref ble_gap_evt_adv_report_t. */ = BLE_GAP_EVT_BASE + 14, /**< Security Request. \n See @ref ble_gap_evt_sec_request_t. */ = 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_BASE + 16, /**< Scan request report. \n See @ref ble_gap_evt_scan_req_report_t. */ = 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_BASE + 18, /**< PHY Update Procedure is complete. \n See @ref ble_gap_evt_phy_update_t. */ = 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_BASE + 20, /**< LL Data Channel PDU payload length updated. \n See @ref ble_gap_evt_data_length_update_t. */ = BLE_GAP_EVT_BASE + 21, /**< Channel survey report. \n See @ref ble_gap_evt_qos_channel_survey_report_t. */ = 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.

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

awneil — Wed, 09 Oct 2019 17:46:34 GMT

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!

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

RyanA — Wed, 09 Oct 2019 17:35:18 GMT

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.

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

awneil — Mon, 07 Oct 2019 10:55:33 GMT

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 ...

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

RyanA — Mon, 07 Oct 2019 03:29:11 GMT

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

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

awneil — Thu, 03 Oct 2019 08:56:49 GMT

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.