BLE_NUS_C_EVT_NUS_TX_EVT has handle of 0xffff

[Andreas48752]

Hello,

I have following problem:

I am using a multilink NUS Central. With one peripheral connection everything works fine. But when make a second connection I sometimes get an error when I try to send data back in received event.

In ble_nus_c_evt_handler event with event type BLE_NUS_C_EVT_NUS_TX_EVT the p_ble_nus_c sometimes has an invalid connection handle (0xFFFF).

If I try to send data back I get following warning: <warning> ble_nus_c: Connection handle invalid.

How is this possible? I am assigning the handles with  ble_nus_c_handles_assign on discovery complete.

Does anyone know about this issue?

Thanks (again)...

[Edvin]

Hello,

What SDK version do you use?

Try to add this line in on_hvx() in ble_nus_c.c:

ble_nus_c_evt.conn_handle = p_ble_evt->evt.gap_evt.conn_handle;

But only if it is in fact a BLE_NUS_C_EVT_NUS_TX_EVT.

So something like this:

static void on_hvx(ble_nus_c_t * p_ble_nus_c, ble_evt_t const * p_ble_evt)
{
    // HVX can only occur from client sending.
    if (   (p_ble_nus_c->handles.nus_tx_handle != BLE_GATT_HANDLE_INVALID)
        && (p_ble_evt->evt.gattc_evt.params.hvx.handle == p_ble_nus_c->handles.nus_tx_handle)
        && (p_ble_nus_c->evt_handler != NULL))
    {
        ble_nus_c_evt_t ble_nus_c_evt;

        ble_nus_c_evt.evt_type = BLE_NUS_C_EVT_NUS_TX_EVT;
        ble_nus_c_evt.p_data   = (uint8_t *)p_ble_evt->evt.gattc_evt.params.hvx.data;
        ble_nus_c_evt.data_len = p_ble_evt->evt.gattc_evt.params.hvx.len;
        ble_nus_c_evt.conn_handle = p_ble_evt->evt.gap_evt.conn_handle; // <-- ADD THIS LINE

        p_ble_nus_c->evt_handler(p_ble_nus_c, &ble_nus_c_evt);
        NRF_LOG_DEBUG("Client sending data.");
    }
}

Does that change the behavior?

Best regards,

Edvin

[Andreas48752]

Thanks for the reply. I added the line but it did not change. I am using SDK 15.2

following a small snippet of my code:

static void ble_nus_c_evt_handler(ble_nus_c_t * p_ble_nus_c, ble_nus_c_evt_t const * p_ble_nus_evt)
{
   static uint8_t data_array[sendData_length];
   uint32_t ret_val;

    ret_code_t err_code;
    switch (p_ble_nus_evt->evt_type)
    {       
        case BLE_NUS_C_EVT_DISCOVERY_COMPLETE:
            err_code = ble_nus_c_handles_assign(p_ble_nus_c, p_ble_nus_evt->conn_handle, &p_ble_nus_evt->handles);
            APP_ERROR_CHECK(err_code);
            err_code = ble_nus_c_tx_notif_enable(p_ble_nus_c);
            APP_ERROR_CHECK(err_code);
            break;
        case BLE_NUS_C_EVT_NUS_TX_EVT:                 
            NRF_LOG_INFO("received data from peripheral %i %i",  p_ble_nus_evt->conn_handle,p_ble_nus_evt->data_len);                       
            // send some data back to client          
            NRF_LOG_INFO("send back data %i, %i", p_ble_nus_evt->conn_handle, p_ble_nus_c->conn_handle);
            ret_val = ble_nus_c_string_send(p_ble_nus_c/*&p_ble_nus_c[ p_ble_nus_evt->conn_handle]*/, data_array, sendData_length);
            if (ret_val == NRF_ERROR_INVALID_STATE)
            {
                NRF_LOG_INFO("send back break point");
            }
            ...

It seems that p_ble_nus_evt->conn_handle could be good (0 or 1 if two peripherals are connected, but p_ble_nus_c->conn_handle is sometimes 0xffff)

[Edvin]

I remember that I did this a while back. You have to be careful when using the handles, so that everything is exactly like it was in the ble_lbs_c.c/h files, and with the conn_handles. 

Without knowing exactly what the issue is, can you try the attached main.c file, together with the attached ble_nus_c_cust.h and ble_nuc_c_cust.c files, instead of the ble_nuc_c.h/c. 

I remember that there was some issues regarding the handles. You need to assign handles both in the BLE_NUS_C_DISCOVERY_COMPLETE event, and in the BLE_GAP_EVT_CONNECTED event.

Try the attached files (they were modified from the default main and ble_nus_c files in SDK15.2.0) 

Fullscreen ble_nus_c_cust.c Download

/**
 * Copyright (c) 2012 - 2018, Nordic Semiconductor ASA
 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice, this
 *    list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form, except as embedded into a Nordic
 *    Semiconductor ASA integrated circuit in a product or a software update for
 *    such product, must reproduce the above copyright notice, this list of
 *    conditions and the following disclaimer in the documentation and/or other
 *    materials provided with the distribution.
 *
 * 3. Neither the name of Nordic Semiconductor ASA nor the names of its
 *    contributors may be used to endorse or promote products derived from this
 *    software without specific prior written permission.
 *
 * 4. This software, with or without modification, must only be used with a
 *    Nordic Semiconductor ASA integrated circuit.
 *
 * 5. Any software provided in binary form under this license must not be reverse
 *    engineered, decompiled, modified and/or disassembled.
 *
 * THIS SOFTWARE IS PROVIDED BY NORDIC SEMICONDUCTOR ASA "AS IS" AND ANY EXPRESS
 * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY, NONINFRINGEMENT, AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL NORDIC SEMICONDUCTOR ASA OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */

#include "sdk_common.h"
#if NRF_MODULE_ENABLED(BLE_NUS_C)
#include <stdlib.h>

#include "ble.h"
#include "ble_nus_c_cust.h"
#include "ble_gattc.h"
#include "ble_srv_common.h"
#include "app_error.h"

#define NRF_LOG_MODULE_NAME ble_nus_c
#include "nrf_log.h"
NRF_LOG_MODULE_REGISTER();

void ble_nus_c_on_db_disc_evt(ble_nus_c_t * p_ble_nus_c, ble_db_discovery_evt_t * p_evt)
{
    ble_nus_c_evt_t nus_c_evt;
    memset(&nus_c_evt,0,sizeof(ble_nus_c_evt_t));

    ble_gatt_db_char_t * p_chars = p_evt->params.discovered_db.charateristics;

    // Check if the NUS was discovered.
    if (    (p_evt->evt_type == BLE_DB_DISCOVERY_COMPLETE)
        &&  (p_evt->params.discovered_db.srv_uuid.uuid == BLE_UUID_NUS_SERVICE)
        &&  (p_evt->params.discovered_db.srv_uuid.type == p_ble_nus_c->uuid_type))
    {
        for (uint32_t i = 0; i < p_evt->params.discovered_db.char_count; i++)
        {
            switch (p_chars[i].characteristic.uuid.uuid)
            {
                case BLE_UUID_NUS_RX_CHARACTERISTIC:
                    nus_c_evt.handles.nus_rx_handle = p_chars[i].characteristic.handle_value;
                    break;

                case BLE_UUID_NUS_TX_CHARACTERISTIC:
                    nus_c_evt.handles.nus_tx_handle = p_chars[i].characteristic.handle_value;
                    nus_c_evt.handles.nus_tx_cccd_handle = p_chars[i].cccd_handle;
                    break;

                default:
                    break;
            }
        }
        if (p_ble_nus_c->evt_handler != NULL)
        {
            nus_c_evt.conn_handle = p_evt->conn_handle;
            nus_c_evt.evt_type    = BLE_NUS_C_EVT_DISCOVERY_COMPLETE;
            p_ble_nus_c->evt_handler(p_ble_nus_c, &nus_c_evt);
        }
    }
}

/**@brief     Function for handling Handle Value Notification received from the SoftDevice.
 *
 * @details   This function will uses the Handle Value Notification received from the SoftDevice
 *            and checks if it is a notification of the NUS TX characteristic from the peer. If
 *            it is, this function will decode the data and send it to the
 *            application.
 *
 * @param[in] p_ble_nus_c Pointer to the NUS Client structure.
 * @param[in] p_ble_evt   Pointer to the BLE event received.
 */
static void on_hvx(ble_nus_c_t * p_ble_nus_c, ble_evt_t const * p_ble_evt)
{
    if (p_ble_nus_c->conn_handle != p_ble_evt->evt.gattc_evt.conn_handle)
    {
        return;
    }
    // HVX can only occur from client sending.
    if (   (p_ble_nus_c->handles.nus_tx_handle != BLE_GATT_HANDLE_INVALID)
        && (p_ble_evt->evt.gattc_evt.params.hvx.handle == p_ble_nus_c->handles.nus_tx_handle)
        && (p_ble_nus_c->evt_handler != NULL))
    {
        ble_nus_c_evt_t ble_nus_c_evt;
        ble_nus_c_evt.conn_handle = p_ble_nus_c->conn_handle;

        ble_nus_c_evt.evt_type = BLE_NUS_C_EVT_NUS_TX_EVT;
        ble_nus_c_evt.p_data   = (uint8_t *)p_ble_evt->evt.gattc_evt.params.hvx.data;
        ble_nus_c_evt.data_len = p_ble_evt->evt.gattc_evt.params.hvx.len;

        p_ble_nus_c->evt_handler(p_ble_nus_c, &ble_nus_c_evt);
        NRF_LOG_DEBUG("Client sending data.");
    }
}

uint32_t ble_nus_c_init(ble_nus_c_t * p_ble_nus_c, ble_nus_c_init_t * p_ble_nus_c_init)
{
    uint32_t      err_code;
    ble_uuid_t    uart_uuid;
    ble_uuid128_t nus_base_uuid = NUS_BASE_UUID;

    VERIFY_PARAM_NOT_NULL(p_ble_nus_c);
    VERIFY_PARAM_NOT_NULL(p_ble_nus_c_init);

    err_code = sd_ble_uuid_vs_add(&nus_base_uuid, &p_ble_nus_c->uuid_type);
    VERIFY_SUCCESS(err_code);

    uart_uuid.type = p_ble_nus_c->uuid_type;
    uart_uuid.uuid = BLE_UUID_NUS_SERVICE;

    p_ble_nus_c->conn_handle           = BLE_CONN_HANDLE_INVALID;
    p_ble_nus_c->evt_handler           = p_ble_nus_c_init->evt_handler;
    p_ble_nus_c->handles.nus_tx_handle = BLE_GATT_HANDLE_INVALID;
    p_ble_nus_c->handles.nus_rx_handle = BLE_GATT_HANDLE_INVALID;

    return ble_db_discovery_evt_register(&uart_uuid);
}

void ble_nus_c_on_ble_evt(ble_evt_t const * p_ble_evt, void * p_context)
{
    ble_nus_c_t * p_ble_nus_c = (ble_nus_c_t *)p_context;

    if ((p_ble_nus_c == NULL) || (p_ble_evt == NULL))
    {
        return;
    }

    if ( (p_ble_nus_c->conn_handle != BLE_CONN_HANDLE_INVALID)
       &&(p_ble_nus_c->conn_handle != p_ble_evt->evt.gap_evt.conn_handle)
       )
    {
        return;
    }

    switch (p_ble_evt->header.evt_id)
    {
        case BLE_GATTC_EVT_HVX:
            on_hvx(p_ble_nus_c, p_ble_evt);
            break;

        case BLE_GAP_EVT_DISCONNECTED:
            if (p_ble_evt->evt.gap_evt.conn_handle == p_ble_nus_c->conn_handle
                    && p_ble_nus_c->evt_handler != NULL)
            {
                ble_nus_c_evt_t nus_c_evt;

                nus_c_evt.evt_type = BLE_NUS_C_EVT_DISCONNECTED;

                p_ble_nus_c->conn_handle = BLE_CONN_HANDLE_INVALID;
                p_ble_nus_c->evt_handler(p_ble_nus_c, &nus_c_evt);
            }
            break;

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

/**@brief Function for creating a message for writing to the CCCD. */
static uint32_t cccd_configure(uint16_t conn_handle, uint16_t cccd_handle, bool enable)
{
    uint8_t buf[BLE_CCCD_VALUE_LEN];

    buf[0] = enable ? BLE_GATT_HVX_NOTIFICATION : 0;
    buf[1] = 0;

    ble_gattc_write_params_t const write_params =
    {
        .write_op = BLE_GATT_OP_WRITE_REQ,
        .flags    = BLE_GATT_EXEC_WRITE_FLAG_PREPARED_WRITE,
        .handle   = cccd_handle,
        .offset   = 0,
        .len      = sizeof(buf),
        .p_value  = buf
    };

    return sd_ble_gattc_write(conn_handle, &write_params);
}


uint32_t ble_nus_c_tx_notif_enable(ble_nus_c_t * p_ble_nus_c)
{
    VERIFY_PARAM_NOT_NULL(p_ble_nus_c);

    if ( (p_ble_nus_c->conn_handle == BLE_CONN_HANDLE_INVALID)
       ||(p_ble_nus_c->handles.nus_tx_cccd_handle == BLE_GATT_HANDLE_INVALID)
       )
    {
        return NRF_ERROR_INVALID_STATE;
    }
    return cccd_configure(p_ble_nus_c->conn_handle,p_ble_nus_c->handles.nus_tx_cccd_handle, true);
}


uint32_t ble_nus_c_string_send(ble_nus_c_t * p_ble_nus_c, uint8_t * p_string, uint16_t length)
{
    VERIFY_PARAM_NOT_NULL(p_ble_nus_c);

    if (length > BLE_NUS_MAX_DATA_LEN)
    {
        NRF_LOG_WARNING("Content too long.");
        return NRF_ERROR_INVALID_PARAM;
    }
    if (p_ble_nus_c->conn_handle == BLE_CONN_HANDLE_INVALID)
    {
        NRF_LOG_WARNING("Connection handle invalid.");
        return NRF_ERROR_INVALID_STATE;
    }

    ble_gattc_write_params_t const write_params =
    {
        .write_op = BLE_GATT_OP_WRITE_CMD,
        .flags    = BLE_GATT_EXEC_WRITE_FLAG_PREPARED_WRITE,
        .handle   = p_ble_nus_c->handles.nus_rx_handle,
        .offset   = 0,
        .len      = length,
        .p_value  = p_string
    };

    return sd_ble_gattc_write(p_ble_nus_c->conn_handle, &write_params);
}


uint32_t ble_nus_c_handles_assign(ble_nus_c_t               * p_ble_nus,
                                  uint16_t                    conn_handle,
                                  ble_nus_c_handles_t const * p_peer_handles)
{
    VERIFY_PARAM_NOT_NULL(p_ble_nus);

    p_ble_nus->conn_handle = conn_handle;
    if (p_peer_handles != NULL)
    {
        p_ble_nus->handles.nus_tx_cccd_handle = p_peer_handles->nus_tx_cccd_handle;
        p_ble_nus->handles.nus_tx_handle      = p_peer_handles->nus_tx_handle;
        p_ble_nus->handles.nus_rx_handle      = p_peer_handles->nus_rx_handle;
    }
    return NRF_SUCCESS;
}
#endif // NRF_MODULE_ENABLED(BLE_NUS_C)

ble_nus_c_cust.h

Fullscreen main.c Download

/**
 * Copyright (c) 2014 - 2018, Nordic Semiconductor ASA
 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice, this
 *    list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form, except as embedded into a Nordic
 *    Semiconductor ASA integrated circuit in a product or a software update for
 *    such product, must reproduce the above copyright notice, this list of
 *    conditions and the following disclaimer in the documentation and/or other
 *    materials provided with the distribution.
 *
 * 3. Neither the name of Nordic Semiconductor ASA nor the names of its
 *    contributors may be used to endorse or promote products derived from this
 *    software without specific prior written permission.
 *
 * 4. This software, with or without modification, must only be used with a
 *    Nordic Semiconductor ASA integrated circuit.
 *
 * 5. Any software provided in binary form under this license must not be reverse
 *    engineered, decompiled, modified and/or disassembled.
 *
 * THIS SOFTWARE IS PROVIDED BY NORDIC SEMICONDUCTOR ASA "AS IS" AND ANY EXPRESS
 * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY, NONINFRINGEMENT, AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL NORDIC SEMICONDUCTOR ASA OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */
/**
 * @brief BLE LED Button Service central and client application main file.
 *
 * This example can be a central for up to 8 peripherals.
 * The peripheral is called ble_app_blinky and can be found in the ble_peripheral
 * folder.
 */

#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include "nordic_common.h"
#include "nrf_sdh.h"
#include "nrf_sdh_ble.h"
#include "app_timer.h"
#include "bsp_btn_ble.h"
#include "ble.h"
#include "ble_hci.h"
#include "ble_advertising.h"
#include "ble_conn_params.h"
#include "ble_db_discovery.h"
//#include "ble_lbs_c.h"
#include "ble_nus_c_cust.h"
#include "ble_conn_state.h"
#include "nrf_ble_gatt.h"
#include "nrf_pwr_mgmt.h"
#include "nrf_ble_scan.h"

#include "nrf_log.h"
#include "nrf_log_ctrl.h"
#include "nrf_log_default_backends.h"


#define APP_BLE_CONN_CFG_TAG      1                                     /**< Tag that refers to the BLE stack configuration that is set with @ref sd_ble_cfg_set. The default tag is @ref APP_BLE_CONN_CFG_TAG. */
#define APP_BLE_OBSERVER_PRIO     3                                     /**< BLE observer priority of the application. There is no need to modify this value. */

#define CENTRAL_SCANNING_LED      BSP_BOARD_LED_0
#define CENTRAL_CONNECTED_LED     BSP_BOARD_LED_1
#define LEDBUTTON_LED             BSP_BOARD_LED_2                       /**< LED to indicate a change of state of the Button characteristic on the peer. */

#define LEDBUTTON_BUTTON          BSP_BUTTON_0                          /**< Button that writes to the LED characteristic of the peer. */
#define BUTTON_DETECTION_DELAY    APP_TIMER_TICKS(50)                   /**< Delay from a GPIOTE event until a button is reported as pushed (in number of timer ticks). */

NRF_BLE_GATT_DEF(m_gatt);                                               /**< GATT module instance. */
//BLE_LBS_C_ARRAY_DEF(m_lbs_c, NRF_SDH_BLE_CENTRAL_LINK_COUNT);           /**< LED button client instances. */
BLE_NUS_C_ARRAY_DEF(m_nus_c, NRF_SDH_BLE_CENTRAL_LINK_COUNT);           /**< LED button client instances. */
BLE_DB_DISCOVERY_ARRAY_DEF(m_db_disc, NRF_SDH_BLE_CENTRAL_LINK_COUNT);  /**< Database discovery module instances. */
NRF_BLE_SCAN_DEF(m_scan);                                               /**< Scanning Module instance. */

//static char const m_target_periph_name[] = "Nordic_Blinky";             /**< Name of the device to try to connect to. This name is searched for in the scanning report data. */
static char const m_target_periph_name[] = "Nordic_UART";             /**< Name of the device to try to connect to. This name is searched for in the scanning report data. */


/**@brief Function for handling asserts in the SoftDevice.
 *
 * @details This function is called in case of an assert in the SoftDevice.
 *
 * @warning This handler is only an example and is not meant for the final product. You need to analyze
 *          how your product is supposed to react in case of an assert.
 * @warning On assert from the SoftDevice, the system can only recover on reset.
 *
 * @param[in] line_num     Line number of the failing assert call.
 * @param[in] p_file_name  File name of the failing assert call.
 */
void assert_nrf_callback(uint16_t line_num, const uint8_t * p_file_name)
{
    app_error_handler(0xDEADBEEF, line_num, p_file_name);
}


/**@brief Function for initializing the LEDs.
 *
 * @details Initializes all LEDs used by the application.
 */
static void leds_init(void)
{
    bsp_board_init(BSP_INIT_LEDS);
}


static void scan_evt_handler(scan_evt_t const * p_scan_evt)
{
    ret_code_t err_code;

    switch(p_scan_evt->scan_evt_id)
    {
        case NRF_BLE_SCAN_EVT_CONNECTING_ERROR:
        {
            err_code = p_scan_evt->params.connecting_err.err_code;
            APP_ERROR_CHECK(err_code);
        } break;

        default:
            break;
    }
}


/**@brief Function for initializing the scanning and setting the filters.
 */
static void scan_init(void)
{
    ret_code_t          err_code;
    nrf_ble_scan_init_t init_scan;

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

    init_scan.connect_if_match = true;
    init_scan.conn_cfg_tag     = APP_BLE_CONN_CFG_TAG;

    err_code = nrf_ble_scan_init(&m_scan, &init_scan, scan_evt_handler);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_ble_scan_filter_set(&m_scan, SCAN_NAME_FILTER, m_target_periph_name);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_ble_scan_filters_enable(&m_scan, NRF_BLE_SCAN_NAME_FILTER, false);
    APP_ERROR_CHECK(err_code);
}


/**@brief Function for starting scanning. */
static void scan_start(void)
{
    ret_code_t ret;

    NRF_LOG_INFO("Start scanning for device name %s.", (uint32_t)m_target_periph_name);
    ret = nrf_ble_scan_start(&m_scan);
    APP_ERROR_CHECK(ret);
    // Turn on the LED to signal scanning.
    bsp_board_led_on(CENTRAL_SCANNING_LED);
}


/**@brief Handles events coming from the LED Button central module.
 *
 * @param[in] p_lbs_c     The instance of LBS_C that triggered the event.
 * @param[in] p_lbs_c_evt The LBS_C event.
 */
//static void nus_c_evt_handler(ble_nus_c_t * p_nus_c, ble_nus_c_evt_t * p_nus_c_evt)
static void ble_nus_c_evt_handler(ble_nus_c_t * p_ble_nus_c, ble_nus_c_evt_t const * p_ble_nus_evt)
{
    switch (p_ble_nus_evt->evt_type)
    {
        case BLE_NUS_C_EVT_DISCOVERY_COMPLETE:
        {
            ret_code_t err_code;

            NRF_LOG_INFO("Nordic UART Service discovered on conn_handle 0x%x",
                         p_ble_nus_evt->conn_handle);

            err_code = app_button_enable();
            APP_ERROR_CHECK(err_code);
            
            err_code = ble_nus_c_handles_assign(p_ble_nus_c, p_ble_nus_evt->conn_handle, &p_ble_nus_evt->handles);
            APP_ERROR_CHECK(err_code);

            // LED Button Service discovered. Enable notification of Button.
            err_code = ble_nus_c_tx_notif_enable(p_ble_nus_c);
            APP_ERROR_CHECK(err_code);
        } break; // BLE_LBS_C_EVT_DISCOVERY_COMPLETE

        case BLE_NUS_C_EVT_NUS_TX_EVT:
        {
//            NRF_LOG_INFO("Link 0x%x, Button state changed on peer to 0x%x",
//                         p_nus_c_evt->conn_handle,
//                         p_nus_c_evt->params.button.button_state);

            NRF_LOG_INFO("notification from conn handle 0x%04x", p_ble_nus_evt->conn_handle);
            NRF_LOG_RAW_INFO("data: ");
            for (int i=0; i<p_ble_nus_evt->data_len; i++)
            {
                NRF_LOG_RAW_INFO("%c", p_ble_nus_evt->p_data[i]);
            }
            NRF_LOG_RAW_INFO("\r\n");
//            if (p_nus_c_evt->params.button.button_state)
//            {
//                bsp_board_led_on(LEDBUTTON_LED);
//            }
//            else
//            {
//                bsp_board_led_off(LEDBUTTON_LED);
//            }
        } break; // BLE_LBS_C_EVT_BUTTON_NOTIFICATION

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


/**@brief Function for handling BLE events.
 *
 * @param[in]   p_ble_evt   Bluetooth stack event.
 * @param[in]   p_context   Unused.
 */
static void ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context)
{
    ret_code_t err_code;

    // For readability.
    ble_gap_evt_t const * p_gap_evt = &p_ble_evt->evt.gap_evt;

    switch (p_ble_evt->header.evt_id)
    {
        // Upon connection, check which peripheral is connected, initiate DB
        // discovery, update LEDs status, and resume scanning, if necessary.
        case BLE_GAP_EVT_CONNECTED:
        {
            NRF_LOG_INFO("Connection 0x%x established, starting DB discovery.",
                         p_gap_evt->conn_handle);

            APP_ERROR_CHECK_BOOL(p_gap_evt->conn_handle < NRF_SDH_BLE_CENTRAL_LINK_COUNT);

            err_code = ble_nus_c_handles_assign(&m_nus_c[p_gap_evt->conn_handle],
                                                p_gap_evt->conn_handle,
                                                NULL);
            APP_ERROR_CHECK(err_code);

            err_code = ble_db_discovery_start(&m_db_disc[p_gap_evt->conn_handle],
                                              p_gap_evt->conn_handle);
            if (err_code != NRF_ERROR_BUSY)
            {
                APP_ERROR_CHECK(err_code);
            }

            // Update LEDs status and check whether it is needed to look for more
            // peripherals to connect to.
            bsp_board_led_on(CENTRAL_CONNECTED_LED);
            if (ble_conn_state_central_conn_count() == NRF_SDH_BLE_CENTRAL_LINK_COUNT)
            {
                bsp_board_led_off(CENTRAL_SCANNING_LED);
            }
            else
            {
                // Resume scanning.
                bsp_board_led_on(CENTRAL_SCANNING_LED);
                scan_start();
            }
        } break; // BLE_GAP_EVT_CONNECTED

        // Upon disconnection, reset the connection handle of the peer that disconnected, update
        // the LEDs status and start scanning again.
        case BLE_GAP_EVT_DISCONNECTED:
        {
            NRF_LOG_INFO("LBS central link 0x%x disconnected (reason: 0x%x)",
                         p_gap_evt->conn_handle,
                         p_gap_evt->params.disconnected.reason);

            if (ble_conn_state_central_conn_count() == 0)
            {
                err_code = app_button_disable();
                APP_ERROR_CHECK(err_code);

                // Turn off the LED that indicates the connection.
                bsp_board_led_off(CENTRAL_CONNECTED_LED);
            }

            // Start scanning.
            scan_start();

            // Turn on the LED for indicating scanning.
            bsp_board_led_on(CENTRAL_SCANNING_LED);

        } break;

        case BLE_GAP_EVT_TIMEOUT:
        {
            // Timeout for scanning is not specified, so only the connection requests can time out.
            if (p_gap_evt->params.timeout.src == BLE_GAP_TIMEOUT_SRC_CONN)
            {
                NRF_LOG_DEBUG("Connection request timed out.");
            }
        } break;

        case BLE_GAP_EVT_CONN_PARAM_UPDATE_REQUEST:
        {
            NRF_LOG_DEBUG("BLE_GAP_EVT_CONN_PARAM_UPDATE_REQUEST.");
            // Accept parameters requested by peer.
            err_code = sd_ble_gap_conn_param_update(p_gap_evt->conn_handle,
                                        &p_gap_evt->params.conn_param_update_request.conn_params);
            APP_ERROR_CHECK(err_code);
        } break;

        case BLE_GAP_EVT_PHY_UPDATE_REQUEST:
        {
            NRF_LOG_DEBUG("PHY update request.");
            ble_gap_phys_t const phys =
            {
                .rx_phys = BLE_GAP_PHY_AUTO,
                .tx_phys = BLE_GAP_PHY_AUTO,
            };
            err_code = sd_ble_gap_phy_update(p_ble_evt->evt.gap_evt.conn_handle, &phys);
            APP_ERROR_CHECK(err_code);
        } break;

        case BLE_GATTC_EVT_TIMEOUT:
        {
            // Disconnect on GATT client timeout event.
            NRF_LOG_DEBUG("GATT client timeout.");
            err_code = sd_ble_gap_disconnect(p_ble_evt->evt.gattc_evt.conn_handle,
                                             BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);
            APP_ERROR_CHECK(err_code);
        } break;

        case BLE_GATTS_EVT_TIMEOUT:
        {
            // Disconnect on GATT server timeout event.
            NRF_LOG_DEBUG("GATT server timeout.");
            err_code = sd_ble_gap_disconnect(p_ble_evt->evt.gatts_evt.conn_handle,
                                             BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);
            APP_ERROR_CHECK(err_code);
        } break;

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


/**@brief LED Button collector initialization. */
static void nus_c_init(void)
{
    ret_code_t       err_code;
    ble_nus_c_init_t nus_c_init_obj;

    nus_c_init_obj.evt_handler = ble_nus_c_evt_handler;

    for (uint32_t i = 0; i < NRF_SDH_BLE_CENTRAL_LINK_COUNT; i++)
    {
        err_code = ble_nus_c_init(&m_nus_c[i], &nus_c_init_obj);
        APP_ERROR_CHECK(err_code);
    }
}


/**@brief Function for initializing the BLE stack.
 *
 * @details Initializes the SoftDevice and the BLE event interrupts.
 */
static void ble_stack_init(void)
{
    ret_code_t err_code;

    err_code = nrf_sdh_enable_request();
    APP_ERROR_CHECK(err_code);

    // Configure the BLE stack using the default settings.
    // Fetch the start address of the application RAM.
    uint32_t ram_start = 0;
    err_code = nrf_sdh_ble_default_cfg_set(APP_BLE_CONN_CFG_TAG, &ram_start);
    APP_ERROR_CHECK(err_code);

    // Enable BLE stack.
    err_code = nrf_sdh_ble_enable(&ram_start);
    APP_ERROR_CHECK(err_code);

    // Register a handler for BLE events.
    NRF_SDH_BLE_OBSERVER(m_ble_observer, APP_BLE_OBSERVER_PRIO, ble_evt_handler, NULL);
}


/**@brief Function for writing to the LED characteristic of all connected clients.
 *
 * @details Based on whether the button is pressed or released, this function writes a high or low
 *          LED status to the server.
 *
 * @param[in] button_action The button action (press or release).
 *            Determines whether the LEDs of the servers are ON or OFF.
 *
 * @return If successful, NRF_SUCCESS is returned. Otherwise, returns the error code from @ref ble_lbs_led_status_send.
 */
static ret_code_t led_status_send_to_all(uint8_t button_action)
{
    ret_code_t err_code;

    for (uint32_t i = 0; i< NRF_SDH_BLE_CENTRAL_LINK_COUNT; i++)
    {
        //err_code = ble_nus_c_string_send(&m_nus_c[i], button_action);
        err_code = NRF_SUCCESS;
        NRF_LOG_INFO("not actually sending anything");
        if (err_code != NRF_SUCCESS &&
            err_code != BLE_ERROR_INVALID_CONN_HANDLE &&
            err_code != NRF_ERROR_INVALID_STATE)
        {
            return err_code;
        }
    }
        return NRF_SUCCESS;
}


/**@brief Function for handling events from the button handler module.
 *
 * @param[in] pin_no        The pin that the event applies to.
 * @param[in] button_action The button action (press or release).
 */
static void button_event_handler(uint8_t pin_no, uint8_t button_action)
{
    ret_code_t err_code;

    switch (pin_no)
    {
        case LEDBUTTON_BUTTON:
            err_code = led_status_send_to_all(button_action);
            if (err_code == NRF_SUCCESS)
            {
                NRF_LOG_INFO("LBS write LED state %d", button_action);
            }
            break;

        default:
            APP_ERROR_HANDLER(pin_no);
            break;
    }
}


/**@brief Function for initializing the button handler module.
 */
static void buttons_init(void)
{
    ret_code_t err_code;

   // The array must be static because a pointer to it is saved in the button handler module.
    static app_button_cfg_t buttons[] =
    {
        {LEDBUTTON_BUTTON, false, BUTTON_PULL, button_event_handler}
    };

    err_code = app_button_init(buttons, ARRAY_SIZE(buttons), BUTTON_DETECTION_DELAY);
    APP_ERROR_CHECK(err_code);
}


/**@brief Function for handling database discovery events.
 *
 * @details This function is a callback function to handle events from the database discovery module.
 *          Depending on the UUIDs that are discovered, this function forwards the events
 *          to their respective services.
 *
 * @param[in] p_event  Pointer to the database discovery event.
 */
static void db_disc_handler(ble_db_discovery_evt_t * p_evt)
{
    NRF_LOG_DEBUG("call to ble_nus_on_db_disc_evt for instance %d and link 0x%x!",
                  p_evt->conn_handle,
                  p_evt->conn_handle);

    ble_nus_c_on_db_disc_evt(&m_nus_c[p_evt->conn_handle], p_evt);
}


/** @brief Database discovery initialization.
 */
static void db_discovery_init(void)
{
    ret_code_t err_code = ble_db_discovery_init(db_disc_handler);
    APP_ERROR_CHECK(err_code);
}


/**@brief Function for initializing power management.
 */
static void power_management_init(void)
{
    ret_code_t err_code;
    err_code = nrf_pwr_mgmt_init();
    APP_ERROR_CHECK(err_code);
}


/**@brief Function for handling the idle state (main loop).
 *
 * @details This function handles any pending log operations, then sleeps until the next event occurs.
 */
static void idle_state_handle(void)
{
    if (NRF_LOG_PROCESS() == false)
    {
        nrf_pwr_mgmt_run();
    }
}


/** @brief Function for initializing the log module.
 */
static void log_init(void)
{
    ret_code_t err_code = NRF_LOG_INIT(NULL);
    APP_ERROR_CHECK(err_code);

    NRF_LOG_DEFAULT_BACKENDS_INIT();
}


/** @brief Function for initializing the timer.
 */
static void timer_init(void)
{
    ret_code_t err_code = app_timer_init();
    APP_ERROR_CHECK(err_code);
}


/**@brief Function for initializing the GATT module.
 */
static void gatt_init(void)
{
    ret_code_t err_code = nrf_ble_gatt_init(&m_gatt, NULL);
    APP_ERROR_CHECK(err_code);
}


int main(void)
{
    // Initialize.
    log_init();
    timer_init();
    leds_init();
    buttons_init();
    power_management_init();
    ble_stack_init();
    gatt_init();
    db_discovery_init();
    nus_c_init();
    ble_conn_state_init();
    scan_init();

    // Start execution.
    NRF_LOG_INFO("Multilink example started.");
    scan_start();

    for (;;)
    {
        idle_state_handle();
    }
}

Best regards,

Edvin

[droberson]

For anyone else who comes across this, Edvin's on_hvx() patch resolved the issue for me in SDK 15.2.

[MassimoF]

Assigning the conn_handle to the nus_handle also in the BLE_GAP_EVT_CONNECTED event resolved my issues.