Uart BLE multilink

Question

I have integrated the code of the multilink into the uart central example. The project can be built without error. however, it is not possible to establish a connection to the peripheral devices on which no code change has been made. In the SDK-config, I set NRF_SDH_BLE_CENTRAL_LINK_COUNT and NRF_SDH_BLE_TOTAL_LINK_COUNT to 8, respectively. I have adjusted the main following. I use the 6.1.0 stack.

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*/
#include <stdio.h>
#include <stdint.h>
#include <stdbool.h>
#include "nordic_common.h"
#include "app_error.h"
#include "app_uart.h"
#include "ble_db_discovery.h"
#include "app_timer.h"
#include "app_util.h"
#include "bsp_btn_ble.h"
#include "ble.h"
#include "ble_gap.h"
#include "ble_hci.h"
#include "nrf_sdh.h"
#include "nrf_sdh_ble.h"
#include "nrf_sdh_soc.h"
#include "ble_advdata.h"
#include "ble_nus_c.h"
#include "nrf_ble_gatt.h"
#include "nrf_pwr_mgmt.h"
#include "nrf_drv_uart.h"
#include "nrf_log.h"
#include "nrf_log_ctrl.h"
#include "nrf_log_default_backends.h"
#include "nrf_ble_scan.h"


#define APP_BLE_CONN_CFG_TAG 1 /**< A tag that refers to the BLE stack configuration we set with @ref sd_ble_cfg_set. Default tag is @ref BLE_CONN_CFG_TAG_DEFAULT. */
#define APP_BLE_OBSERVER_PRIO 3 /**< Application's BLE observer priority. You shoulnd't need to modify this value. */

#define UART_TX_BUF_SIZE 1024 /**< UART TX buffer size. */
#define UART_RX_BUF_SIZE 1024 /**< UART RX buffer size. */

#define NUS_SERVICE_UUID_TYPE BLE_UUID_TYPE_VENDOR_BEGIN /**< UUID type for the Nordic UART Service (vendor specific). */

#define SCAN_INTERVAL 0x00A0 /**< Determines scan interval in units of 0.625 millisecond. */
#define SCAN_WINDOW 0x0050 /**< Determines scan window in units of 0.625 millisecond. */
#define SCAN_DURATION 0x0000 /**< Timout when scanning. 0x0000 disables timeout. */

#define MIN_CONNECTION_INTERVAL MSEC_TO_UNITS(20, UNIT_1_25_MS) /**< Determines minimum connection interval in millisecond. */
#define MAX_CONNECTION_INTERVAL MSEC_TO_UNITS(75, UNIT_1_25_MS) /**< Determines maximum connection interval in millisecond. */
#define SLAVE_LATENCY 0 /**< Determines slave latency in counts of connection events. */
#define SUPERVISION_TIMEOUT MSEC_TO_UNITS(4000, UNIT_10_MS) /**< Determines supervision time-out in units of 10 millisecond. */

#define ECHOBACK_BLE_UART_DATA 1 /**< Echo the UART data that is received over the Nordic UART Service back to the sender. */


BLE_NUS_C_ARRAY_DEF(m_ble_nus_c, NRF_SDH_BLE_CENTRAL_LINK_COUNT); /**< BLE NUS service client instance. */
NRF_BLE_GATT_DEF(m_gatt); /**< GATT module instance. */
BLE_DB_DISCOVERY_ARRAY_DEF(m_db_disc, NRF_SDH_BLE_CENTRAL_LINK_COUNT); /**< Database discovery module instances. */
NRF_BLE_SCAN_DEF(m_scan); 

static uint16_t m_ble_nus_max_data_len = BLE_GATT_ATT_MTU_DEFAULT - OPCODE_LENGTH - HANDLE_LENGTH; /**< Maximum length of data (in bytes) that can be transmitted to the peer by the Nordic UART service module. */
static uint8_t data_array[BLE_NUS_MAX_DATA_LEN];
static uint8_t data_crc[BLE_NUS_MAX_DATA_LEN];
void uart_event_handle(app_uart_evt_t * p_event);
static char const m_target_periph_name[] = "Nordic_UART"; 

/**@brief Connection parameters requested for connection. */
static ble_gap_conn_params_t const m_connection_param =
{
    (uint16_t)MIN_CONNECTION_INTERVAL, // Minimum connection
    (uint16_t)MAX_CONNECTION_INTERVAL, // Maximum connection
    (uint16_t)SLAVE_LATENCY, // Slave latency
    (uint16_t)SUPERVISION_TIMEOUT // Supervision time-out
};

static uint8_t m_scan_buffer_data[BLE_GAP_SCAN_BUFFER_MIN]; /**< buffer where advertising reports will be stored by the SoftDevice. */

/**@brief Pointer to the buffer where advertising reports will be stored by the SoftDevice. */
static ble_data_t m_scan_buffer =
{
    m_scan_buffer_data,
    BLE_GAP_SCAN_BUFFER_MIN
};

/** @brief Parameters used when scanning. */
static ble_gap_scan_params_t const m_scan_params =
{
    .active = 1,
    .interval = SCAN_INTERVAL,
    .window = SCAN_WINDOW,
    .timeout = SCAN_DURATION,
    .scan_phys = BLE_GAP_PHY_1MBPS,
    .filter_policy = BLE_GAP_SCAN_FP_ACCEPT_ALL,
};

/**@brief NUS uuid. */
static ble_uuid_t const m_nus_uuid =
{
    .uuid = BLE_UUID_NUS_SERVICE,
    .type = NUS_SERVICE_UUID_TYPE
};

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;
    }
}

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 asserts in the SoftDevice.
*
* @details This function will be called in case of an assert in the SoftDevice.
*
* @warning This handler is an example only and does not fit a final product. You need to analyze
* how your product is supposed to react in case of 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 to start 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.
}


/**@brief Function for handling database discovery events.
*
* @details This function is callback function to handle events from the database discovery module.
* Depending on the UUIDs that are discovered, this function should forward 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_lbs_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_ble_nus_c[p_evt->conn_handle], p_evt);
}


/**@brief Function for handling characters received by the Nordic UART Service.
*
* @details This function takes a list of characters of length data_len and prints the characters out on UART.
* If @ref ECHOBACK_BLE_UART_DATA is set, the data is sent back to sender.
*/
static uint32_t ble_nus_chars_received_uart_print(uint8_t * p_data, uint16_t data_len)
{
    ret_code_t ret_val;
    uint8_t byte[1024] = {0}; 
    uint32_t check = 0;
    
    NRF_LOG_DEBUG("Receiving data.");
    NRF_LOG_HEXDUMP_DEBUG(p_data, data_len);


    for (uint32_t i = 0; i < data_len; i++)
    {
        do
        {
            ret_val = app_uart_put(p_data[i]);
            if ((ret_val != NRF_SUCCESS) && (ret_val != NRF_ERROR_BUSY))
            {
                NRF_LOG_ERROR("app_uart_put failed for index 0x%04x.", i);
                APP_ERROR_CHECK(ret_val);
            }
        } while (ret_val == NRF_ERROR_BUSY);
    }
    if (p_data[data_len-1] == '\r')
    {
        while (app_uart_put('\n') == NRF_ERROR_BUSY);
    }
    if (ECHOBACK_BLE_UART_DATA)
    {
    // Send data back to peripheral.
    do
    {
        ret_val = ble_nus_c_string_send(&m_ble_nus_c, p_data, data_len);
        if ((ret_val != NRF_SUCCESS) && (ret_val != NRF_ERROR_BUSY))
        {
            NRF_LOG_ERROR("Failed sending NUS message. Error 0x%x. ", ret_val);
            APP_ERROR_CHECK(ret_val);
        }
    } while (ret_val == NRF_ERROR_BUSY);
    }
    for (uint32_t i = 0; i < data_len; i++)
    {
        byte[i] = p_data[i];
    }
    bsp_board_led_invert(BSP_BOARD_LED_0);
    return nrf_drv_uart_tx(&app_uart_inst,byte,data_len); 

}


/**@brief Function for handling app_uart events.
*
* @details This function will receive a single character from the app_uart module and append it to
* a string. The string will be be sent over BLE when the last character received was a
* 'new line' '\n' (hex 0x0A) or if the string has reached the maximum data length.
*/
void uart_event_handle(app_uart_evt_t * p_event)
{

    static uint16_t index = 0;
    uint8_t rx_byte;
    uint32_t ret_val;
    uint32_t get_state;
    
    switch (p_event->evt_type)
    {
    /**@snippet [Handling data from UART] */
    case APP_UART_DATA_READY:

    do
    {
        get_state = app_uart_get(&rx_byte);
        if(get_state == NRF_SUCCESS)
        {
            data_array[index] = rx_byte;
            if( data_array[index] == 0x03 || index + 1 >= m_ble_nus_max_data_len )
        {
    
        bsp_board_led_invert(BSP_BOARD_LED_0);
        NRF_LOG_DEBUG("Ready to send data over BLE NUS");
        NRF_LOG_HEXDUMP_DEBUG(data_array, index);
    
        if(m_ble_nus_c->conn_handle != BLE_CONN_HANDLE_INVALID)
        {
        
        do
    {
        ret_val = ble_nus_c_string_send(&m_ble_nus_c, data_array, index);
    if(0)
    {
    APP_ERROR_CHECK(ret_val);

}
} while (ret_val == NRF_ERROR_BUSY); 
}
index = 0; 
}
else
{ 
index++;
if(index > sizeof(data_array))
{
index = 0;
} 
}
}
}
while(get_state == NRF_SUCCESS);
break;

/**@snippet [Handling data from UART] */
case APP_UART_COMMUNICATION_ERROR:
NRF_LOG_ERROR("Communication error occurred while handling UART.");
APP_ERROR_HANDLER(p_event->data.error_communication);
break;

case APP_UART_FIFO_ERROR:
NRF_LOG_ERROR("Error occurred in FIFO module used by UART.");
APP_ERROR_HANDLER(p_event->data.error_code);
break;

default:
break;
}
}


/**@brief Callback handling NUS Client events.
*
* @details This function is called to notify the application of NUS client events.
*
* @param[in] p_ble_nus_c NUS Client Handle. This identifies the NUS client
* @param[in] p_ble_nus_evt Pointer to the NUS Client event.
*/

/**@snippet [Handling events from the ble_nus_c module] */
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)
{
ret_code_t err_code;

switch (p_ble_nus_evt->evt_type)
{
case BLE_NUS_C_EVT_DISCOVERY_COMPLETE:
NRF_LOG_INFO("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);
NRF_LOG_INFO("Connected to device with Nordic UART Service.");
break;

case BLE_NUS_C_EVT_NUS_TX_EVT:
ble_nus_chars_received_uart_print(p_ble_nus_evt->p_data, p_ble_nus_evt->data_len);
break;

case BLE_NUS_C_EVT_DISCONNECTED:
NRF_LOG_INFO("Disconnected.");
scan_start();
break;
}
}
/**@snippet [Handling events from the ble_nus_c module] */


/**
* @brief Function for shutdown events.
*
* @param[in] event Shutdown type.
*/
static bool shutdown_handler(nrf_pwr_mgmt_evt_t event)
{
ret_code_t err_code;

err_code = bsp_indication_set(BSP_INDICATE_IDLE);
APP_ERROR_CHECK(err_code);

switch (event)
{
case NRF_PWR_MGMT_EVT_PREPARE_WAKEUP:
// Prepare wakeup buttons.
err_code = bsp_btn_ble_sleep_mode_prepare();
APP_ERROR_CHECK(err_code);
break;

default:
break;
}

return true;
}

NRF_PWR_MGMT_HANDLER_REGISTER(shutdown_handler, APP_SHUTDOWN_HANDLER_PRIORITY);


/**@brief Function for handling the advertising report BLE event.
*
* @param[in] p_adv_report Advertising report from the SoftDevice.
*/
static void on_adv_report(ble_gap_evt_adv_report_t const * p_adv_report)
{
ret_code_t err_code;

if (ble_advdata_uuid_find(p_adv_report->data.p_data, p_adv_report->data.len, &m_nus_uuid))
{
err_code = sd_ble_gap_connect(&p_adv_report->peer_addr,
&m_scan_params,
&m_connection_param,
APP_BLE_CONN_CFG_TAG);

if (err_code == NRF_SUCCESS)
{
// scan is automatically stopped by the connect
err_code = bsp_indication_set(BSP_INDICATE_IDLE);
APP_ERROR_CHECK(err_code);
NRF_LOG_INFO("Connecting to target %02x%02x%02x%02x%02x%02x",
p_adv_report->peer_addr.addr[0],
p_adv_report->peer_addr.addr[1],
p_adv_report->peer_addr.addr[2],
p_adv_report->peer_addr.addr[3],
p_adv_report->peer_addr.addr[4],
p_adv_report->peer_addr.addr[5]
);
}
}
else
{
err_code = sd_ble_gap_scan_start(NULL, &m_scan_buffer);
APP_ERROR_CHECK(err_code);
}
}


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

switch (p_ble_evt->header.evt_id)
{
case BLE_GAP_EVT_ADV_REPORT:
on_adv_report(&p_gap_evt->params.adv_report);
break; // BLE_GAP_EVT_ADV_REPORT

case BLE_GAP_EVT_CONNECTED:
NRF_LOG_INFO("Connection 0x%x established, starting DB discovery.",
p_gap_evt->conn_handle);
err_code = ble_nus_c_handles_assign(&m_ble_nus_c[p_gap_evt->conn_handle], p_ble_evt->evt.gap_evt.conn_handle, NULL);
APP_ERROR_CHECK(err_code);

err_code = bsp_indication_set(BSP_INDICATE_CONNECTED);
APP_ERROR_CHECK(err_code);
err_code = ble_db_discovery_start(&m_db_disc[p_gap_evt->conn_handle],
p_gap_evt->conn_handle);
// start discovery of services. The NUS Client waits for a discovery result
err_code = ble_db_discovery_start(&m_db_disc, p_ble_evt->evt.gap_evt.conn_handle);
if (err_code != NRF_ERROR_BUSY)
{
APP_ERROR_CHECK(err_code);
}
break;

case BLE_GAP_EVT_TIMEOUT:
if (p_gap_evt->params.timeout.src == BLE_GAP_TIMEOUT_SRC_SCAN)
{
NRF_LOG_INFO("Scan timed out.");
scan_start();
}
else if (p_gap_evt->params.timeout.src == BLE_GAP_TIMEOUT_SRC_CONN)
{
NRF_LOG_INFO("Connection Request timed out.");
}
break;

case BLE_GAP_EVT_SEC_PARAMS_REQUEST:
// Pairing not supported
err_code = sd_ble_gap_sec_params_reply(p_ble_evt->evt.gap_evt.conn_handle, BLE_GAP_SEC_STATUS_PAIRING_NOT_SUPP, NULL, NULL);
APP_ERROR_CHECK(err_code);
break;

case BLE_GAP_EVT_CONN_PARAM_UPDATE_REQUEST:
// Accepting 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:
break;
}
}


/**@brief Function for initializing the BLE stack.
*
* @details Initializes the SoftDevice and the BLE event interrupt.
*/
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 handling events from the GATT library. */
void gatt_evt_handler(nrf_ble_gatt_t * p_gatt, nrf_ble_gatt_evt_t const * p_evt)
{
if (p_evt->evt_id == NRF_BLE_GATT_EVT_ATT_MTU_UPDATED)
{
NRF_LOG_INFO("ATT MTU exchange completed.");

m_ble_nus_max_data_len = p_evt->params.att_mtu_effective - OPCODE_LENGTH - HANDLE_LENGTH;
NRF_LOG_INFO("Ble NUS max data length set to 0x%X(%d)", m_ble_nus_max_data_len, m_ble_nus_max_data_len);
}
}


/**@brief Function for initializing the GATT library. */
void gatt_init(void)
{
ret_code_t err_code;

err_code = nrf_ble_gatt_init(&m_gatt, gatt_evt_handler);
APP_ERROR_CHECK(err_code);

err_code = nrf_ble_gatt_att_mtu_central_set(&m_gatt, NRF_SDH_BLE_GATT_MAX_MTU_SIZE);
APP_ERROR_CHECK(err_code);
}


/**@brief Function for handling events from the BSP module.
*
* @param[in] event Event generated by button press.
*/
void bsp_event_handler(bsp_event_t event)
{
ret_code_t err_code;

switch (event)
{
case BSP_EVENT_SLEEP:
nrf_pwr_mgmt_shutdown(NRF_PWR_MGMT_SHUTDOWN_GOTO_SYSOFF);
break;

case BSP_EVENT_DISCONNECT:
err_code = sd_ble_gap_disconnect(m_ble_nus_c->conn_handle,
BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);
if (err_code != NRF_ERROR_INVALID_STATE)
{
APP_ERROR_CHECK(err_code);
}
break;

default:
break;
}
}

/**@brief Function for initializing the UART. */
static void uart_init(void)
{
ret_code_t err_code;

app_uart_comm_params_t const comm_params =
{
.rx_pin_no = RX_PIN_NUMBER,
.tx_pin_no = TX_PIN_NUMBER,
.rts_pin_no = RTS_PIN_NUMBER,
.cts_pin_no = CTS_PIN_NUMBER,
.flow_control = APP_UART_FLOW_CONTROL_DISABLED,
.use_parity = false,
.baud_rate = UART_BAUDRATE_BAUDRATE_Baud115200
};

APP_UART_FIFO_INIT(&comm_params,
UART_RX_BUF_SIZE,
UART_TX_BUF_SIZE,
uart_event_handle,
APP_IRQ_PRIORITY_LOWEST,
err_code);

APP_ERROR_CHECK(err_code);
}

/**@brief Function for initializing the NUS Client. */
static void nus_c_init(void)
{
ret_code_t err_code;
ble_nus_c_init_t init;

init.evt_handler = ble_nus_c_evt_handler;

err_code = ble_nus_c_init(&m_ble_nus_c, &init);
APP_ERROR_CHECK(err_code);
}


/**@brief Function for initializing buttons and leds. */
static void buttons_leds_init(void)
{
ret_code_t err_code;
bsp_event_t startup_event;

err_code = bsp_init(BSP_INIT_LEDS, bsp_event_handler);
err_code = bsp_init(BSP_INIT_BUTTONS, bsp_event_handler);
APP_ERROR_CHECK(err_code);

err_code = bsp_btn_ble_init(NULL, &startup_event);
APP_ERROR_CHECK(err_code);
}


/**@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 nrf 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 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 initializing the Database Discovery Module. */
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 handling the idle state (main loop).
*
* @details Handle any pending log operation(s), then sleep until the next event occurs.
*/
static void idle_state_handle(void)
{
if (NRF_LOG_PROCESS() == false)
{
nrf_pwr_mgmt_run();
}
}


int main(void)
{
// Initialize.
log_init();
timer_init();
uart_init();
buttons_leds_init();
db_discovery_init();
power_management_init();
ble_stack_init();
gatt_init();
nus_c_init();

// Start execution.
printf("BLE UART central example started.\r\n");
NRF_LOG_INFO("BLE UART central example started.");
scan_start();

// Enter main loop.
for (;;)
{
idle_state_handle();
}
}

For whatever reason, you can not establish a connection.
the debug terminal shows the following:

<info> app: BLE UART central example started.
<info> app: Start scanning for device name Nordic_UART.
<error> ble_scan: sd_ble_gap_scan_start returned 0x10
<error> app: Fatal error