Read/write chracteristics by handle value without service discovery.

[Shishir]

ble_app_wbh.zipble_app_wbh_c.zip

Hi Nordic,

I am trying an application where both central and peripheral devices are based on nRF52.

I have to create custom services and characteristics for the same.

These are going to be battery operated and power sensitive devices.

Hence as part of power saving we want to avoid service discovery process at central side.

Our handle values are more or less going to locked once development is over hence we do not see ant reason to have service discovery either.

I have attached sample app of my central and peripheral device here both written for nRF52840 with SDK v15.0.0.

While I am able to connect my central device with peripheral device but I am getting error 17 when I try to read by handle directly.

The final intent is to be able to read/write chars by handle value.

Can you pin point the mistakes I am making or perhaps share a similar working example which can help with my purpose.

Cheers,

Shishir

[Shishir]

Hi Hung,

Sorry for the delayed response but I tried putting 1 - 5 sec delay before and after read request for handle number but still the same error. From the logs it looks like MTU size change is being made after my handle read request so tries delay after read handle request also. still I am getting the same error.

Can you point me any working sample application where we do a read by handle or write by handle without service discovery.

Cheers,

Shishir

[Hung Bui]

Hi Shishir, 

Attached is the main.c file of the ble_app_uart_c project I modified. 

It will execute a sd_ble_gattc_read() to read the ble_app_uart device name (handle 03) when BLE_GAP_EVT_DATA_LENGTH_UPDATE event occurs. 

You just need to replace main.c in ble_app_uart_c and test it with a ble_app_uart peripheral. You can see the log in RTT. (SDK 15.0)

Fullscreen main.c Download

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 *    Nordic Semiconductor ASA integrated circuit.
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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_log.h"
#include "nrf_log_ctrl.h"
#include "nrf_log_default_backends.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        256                                     /**< UART TX buffer size. */
#define UART_RX_BUF_SIZE        256                                     /**< 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_DEF(m_ble_nus_c);                                             /**< BLE NUS service client instance. */
NRF_BLE_GATT_DEF(m_gatt);                                               /**< GATT module instance. */
BLE_DB_DISCOVERY_DEF(m_db_disc);                                        /**< DB discovery module instance. */

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. */

/**@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
};


/**@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;

    ret = sd_ble_gap_scan_start(&m_scan_params, &m_scan_buffer);
    APP_ERROR_CHECK(ret);

    ret = bsp_indication_set(BSP_INDICATE_SCANNING);
    APP_ERROR_CHECK(ret);
}


/**@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)
{
    ble_nus_c_on_db_disc_evt(&m_ble_nus_c, 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 void ble_nus_chars_received_uart_print(uint8_t * p_data, uint16_t data_len)
{
    ret_code_t ret_val;

    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);
    }
}


/**@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 uint8_t data_array[BLE_NUS_MAX_DATA_LEN];
    static uint16_t index = 0;
    uint32_t ret_val;

    switch (p_event->evt_type)
    {
        /**@snippet [Handling data from UART] */
        case APP_UART_DATA_READY:
            UNUSED_VARIABLE(app_uart_get(&data_array[index]));
            index++;

            if ((data_array[index - 1] == '\n') || (index >= (m_ble_nus_max_data_len)))
            {
                NRF_LOG_DEBUG("Ready to send data over BLE NUS");
                NRF_LOG_HEXDUMP_DEBUG(data_array, index);

                do
                {
                    ret_val = ble_nus_c_string_send(&m_ble_nus_c, data_array, index);
                    if ( (ret_val != NRF_ERROR_INVALID_STATE) && (ret_val != NRF_ERROR_BUSY) )
                    {
                        APP_ERROR_CHECK(ret_val);
                    }
                } while (ret_val == NRF_ERROR_BUSY);

                index = 0;
            }
            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("Connected to target");
            err_code = ble_nus_c_handles_assign(&m_ble_nus_c, 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);
        
            // 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);
            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_INFO("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_INFO("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;
        case BLE_GATTC_EVT_READ_RSP:
            NRF_LOG_INFO("GATT Read RSP");
            NRF_LOG_INFO("Device Name:%s",p_ble_evt->evt.gattc_evt.params.read_rsp.data) ;
            
            break;
        case BLE_GAP_EVT_DATA_LENGTH_UPDATE:
            NRF_LOG_INFO("DLE Updated"); 
             err_code = sd_ble_gattc_read( p_ble_evt->evt.gap_evt.conn_handle,3,0);
             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);
    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();
    }
}

I guess the trick is to wait until MTU exchange and DLE Update is finished. 

[Shishir]

checking for BLE_GAP_EVT_DATA_LENGTH_UPDATE and than attempting to access the parameter did the trick and now I am able to run in my attached sample app as wel.

Thanks a lot Hung Bui !!!