Development board nRF52832 with multilink + UART

[Yoann_F]

Hello everybody !

Here I am designing a system with 3 development boards nrf52832, where a card called "central" is connected to the computer by USB and this board will then communicate with the other 2 boards "device" with the code from this this tutorial: github.com/.../custom_ble_service_example.

So, my current problem is that I would like to have my computer communicate with the "central" development board. I then used the code example of the sdk "ble_app_multilink_central" and I added to it the parts of the code "ble_app_uart_c" to initialize the UART. During initialization, I get error 8 with APP_ERROR_CHECK (err_code). As I use the traces NFR_LOG I think that the problem thus comes from that. I would like to know if there is a way to debug (by seeing traces on the COM port). And for example what does it take to use printf? (I use Segger with the Nordic SDK 14.2).

Thank you in advance for your assistance !

[Yoann_F]

Here is part of my code:

#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include "nordic_common.h"
#include "nrf_sdh.h"
#include "nrf_sdh_ble.h"
#include "app_error.h"
#include "app_timer.h"
#include "app_uart.h"
#include "app_util_platform.h"
#include "bsp_btn_ble.h"
#include "ble.h"
#include "ble_hci.h"
#include "ble_advdata.h"
#include "ble_advertising.h"
#include "ble_conn_params.h"
#include "ble_db_discovery.h"
#include "ble_lbs_c.h"
#include "ble_conn_state.h"
#include "nrf_ble_gatt.h"

#if defined (UART_PRESENT)
#include "nrf_uart.h"
#endif
#if defined (UARTE_PRESENT)
#include "nrf_uarte.h"
#endif

#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 APP_BLE_CONN_CFG_TAG. */
#define APP_BLE_OBSERVER_PRIO     3                                     /**< Application's BLE observer priority. You shouldn't 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 the Button characteristic on the peer. */
#define LEDTEST                   BSP_BOARD_LED_3                       /**< LED to indicate a change of state of the the Button characteristic on the peer. */

#define LEDBUTTON_BUTTON          BSP_BUTTON_0                          /**< Button that will write 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). */

#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_TIMEOUT              0x0000                                /**< Timout when scanning. 0x0000 disables timeout. */

#define MIN_CONNECTION_INTERVAL   MSEC_TO_UNITS(7.5, UNIT_1_25_MS)      /**< Determines minimum connection interval in milliseconds. */
#define MAX_CONNECTION_INTERVAL   MSEC_TO_UNITS(30, UNIT_1_25_MS)       /**< Determines maximum connection interval in milliseconds. */
#define SLAVE_LATENCY             0                                     /**< Determines slave latency in terms of connection events. */
#define SUPERVISION_TIMEOUT       MSEC_TO_UNITS(4000, UNIT_10_MS)       /**< Determines supervision time-out in units of 10 milliseconds. */

#define UUID16_SIZE               2                                     /**< Size of a UUID, in bytes. */


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_DB_DISCOVERY_ARRAY_DEF(m_db_disc, NRF_SDH_BLE_CENTRAL_LINK_COUNT);  /**< Database discovery module instances. */

static char const m_target_periph_name[] = "L_PERIPH";             /**< Name of the device we try to connect to. This name is searched for in the scan report data*/
static char const m_target_periph_name2[] = "R_PERIPH";             /**< Name of the device we try to connect to. This name is searched for in the scan report data*/

#define UART_TX_BUF_SIZE        256                                     /**< UART TX buffer size. */
#define UART_RX_BUF_SIZE        256                                     /**< UART RX buffer size. */

static ble_gap_scan_params_t const m_scan_params =
{
    .active   = 0,
    .interval = SCAN_INTERVAL,
    .window   = SCAN_WINDOW,
    .timeout  = SCAN_TIMEOUT,
    #if (NRF_SD_BLE_API_VERSION <= 2)
        .selective   = 0,
        .p_whitelist = NULL,
    #endif
    #if (NRF_SD_BLE_API_VERSION >= 3)
        .use_whitelist  = 0,
        .adv_dir_report = 0,
    #endif
};

static ble_gap_conn_params_t const m_connection_param =
{
    (uint16_t)MIN_CONNECTION_INTERVAL,
    (uint16_t)MAX_CONNECTION_INTERVAL,
    (uint16_t)SLAVE_LATENCY,
    (uint16_t)SUPERVISION_TIMEOUT
};

void assert_nrf_callback(uint16_t line_num, const uint8_t * p_file_name)
{
    app_error_handler(0xDEADBEEF, line_num, p_file_name);
}

static void leds_init(void)
{
    bsp_board_leds_init();
}

static uint32_t adv_report_parse(uint8_t type, uint8_array_t * p_advdata, uint8_array_t * p_typedata)
{
    uint32_t  index = 0;
    uint8_t * p_data;

    p_data = p_advdata->p_data;

    while (index < p_advdata->size)
    {
        uint8_t field_length = p_data[index];
        uint8_t field_type   = p_data[index + 1];

        if (field_type == type)
        {
            p_typedata->p_data = &p_data[index + 2];
            p_typedata->size   = field_length - 1;
            return NRF_SUCCESS;
        }
        index += field_length + 1;
    }
    return NRF_ERROR_NOT_FOUND;
}

static void scan_start(void)
{
    ret_code_t ret;

    (void) sd_ble_gap_scan_stop();
    
    printf("Start scanning for device name %s.", (uint32_t)m_target_periph_name);
    ret = sd_ble_gap_scan_start(&m_scan_params);
    APP_ERROR_CHECK(ret);
    printf("\rscan error : %d", ret);    

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

static void lbs_c_evt_handler(ble_lbs_c_t * p_lbs_c, ble_lbs_c_evt_t * p_lbs_c_evt)
{
    switch (p_lbs_c_evt->evt_type)
    {
        case BLE_LBS_C_EVT_DISCOVERY_COMPLETE:
        {
            ret_code_t err_code;

            //NRF_LOG_INFO("LED Button service discovered on conn_handle 0x%x",
                 //        p_lbs_c_evt->conn_handle);

            err_code = app_button_enable();
            APP_ERROR_CHECK(err_code);

            // LED Button service discovered. Enable notification of Button.
            err_code = ble_lbs_c_button_notif_enable(p_lbs_c);
            APP_ERROR_CHECK(err_code);
        } break; // BLE_LBS_C_EVT_DISCOVERY_COMPLETE

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

            if (p_lbs_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;
    }
}

static void on_adv_report(ble_evt_t const * p_ble_evt)
{
    uint32_t      err_code;
    uint8_array_t adv_data;
    uint8_array_t dev_name;
    bool          do_connect = false;

    // For readibility.
    ble_gap_evt_t  const * p_gap_evt  = &p_ble_evt->evt.gap_evt;
    ble_gap_addr_t const * peer_addr  = &p_gap_evt->params.adv_report.peer_addr;

    // Prepare advertisement report for parsing.
    adv_data.p_data = (uint8_t *)p_gap_evt->params.adv_report.data;
    adv_data.size   = p_gap_evt->params.adv_report.dlen;

    // Search for advertising names.
    bool found_name = false;
    err_code = adv_report_parse(BLE_GAP_AD_TYPE_COMPLETE_LOCAL_NAME,
                                &adv_data,
                                &dev_name);
    if (err_code != NRF_SUCCESS)
    {
        // Look for the short local name if it was not found as complete.
        err_code = adv_report_parse(BLE_GAP_AD_TYPE_SHORT_LOCAL_NAME, &adv_data, &dev_name);
        if (err_code != NRF_SUCCESS)
        {
            // If we can't parse the data, then exit.
            return;
        }
        else
        {
            found_name = true;
        }
    }
    else
    {
        found_name = true;
    }

    if (found_name)
    {   
        printf("\rFound peripheral %s", (uint32_t)dev_name.p_data);
            if (
                (memcmp(m_target_periph_name, dev_name.p_data, dev_name.size) == 0)
                || (memcmp(m_target_periph_name2, dev_name.p_data, dev_name.size) == 0) )
            {
                printf("\rFaire la connexion...avec %s ou %s",m_target_periph_name,m_target_periph_name2);
                do_connect = true;
            }
    }

    if (do_connect)
    {
        // Initiate connection.
        err_code = sd_ble_gap_connect(peer_addr, &m_scan_params, &m_connection_param, APP_BLE_CONN_CFG_TAG);
        printf("\rerr_code : do_connect = %d ", err_code);
        if (err_code != NRF_SUCCESS)
        {
            //NRF_LOG_ERROR("Connection Request Failed, reason %d", 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)
{
    printf("\rEvenement sur le BLE");
    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 has connected, initiate DB
        // discovery, update LEDs status and resume scanning if necessary.
        case BLE_GAP_EVT_CONNECTED:
        {   
            printf("Connection 0x%x established, starting DB discovery.");
            //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_lbs_c_handles_assign(&m_lbs_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);
            }
            bsp_board_led_invert(LED_4);

            // Update LEDs status, and check if we should be looking for more
            // peripherals to connect to.
            bsp_board_led_on(CENTRAL_CONNECTED_LED);
            if (ble_conn_state_n_centrals() == NRF_SDH_BLE_CENTRAL_LINK_COUNT)
            {
                bsp_board_led_off(CENTRAL_SCANNING_LED);
                printf("left and right peripheral were found ! Stop Scan !");
            }
            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 which 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_n_centrals() == 0)
            {
                err_code = app_button_disable();
                APP_ERROR_CHECK(err_code);

                // Turn off connection indication LED
                bsp_board_led_off(CENTRAL_CONNECTED_LED);
            }

            // Start scanning
            scan_start();

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

        } break;

        case BLE_GAP_EVT_ADV_REPORT:
            on_adv_report(p_ble_evt);
            break;

        case BLE_GAP_EVT_TIMEOUT:
        {
            // We have not specified a timeout for scanning, so only connection attemps can timeout.
            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;

#ifndef S140
        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;
#endif

        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 lbs_c_init(void)
{
    ret_code_t       err_code;
    ble_lbs_c_init_t lbs_c_init_obj;

    lbs_c_init_obj.evt_handler = lbs_c_evt_handler;

    for (uint32_t i = 0; i < NRF_SDH_BLE_CENTRAL_LINK_COUNT; i++)
    {
        err_code = ble_lbs_c_init(&m_lbs_c[i], &lbs_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 if 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/release).
 *            Determines if the LEDs of the servers will be ON or OFF.
 *
 * @return If successful NRF_SUCCESS is returned. Otherwise, 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_lbs_led_status_send(&m_lbs_c[i], button_action);
        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/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 will be 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 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_lbs_on_db_disc_evt(&m_lbs_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 to sleep until a BLE event is received by the application.
 */
static void power_manage(void)
{
    ret_code_t err_code = sd_app_evt_wait();
    APP_ERROR_CHECK(err_code);
}


/** @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);
    printf("\rlog : %d",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;

    err_code = nrf_ble_gatt_att_mtu_periph_set(&m_gatt, 64);
    APP_ERROR_CHECK(err_code);
}


/**@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.
 */
/**@snippet [Handling the data received over UART] */
void uart_event_handle(app_uart_evt_t * p_event)
{   
//    NRF_LOG_INFO("UART EVENT");
    static uint8_t data_array[5];
    static uint8_t index = 0;
    uint32_t       err_code;

    switch (p_event->evt_type)
    {
        case APP_UART_DATA_READY:
            UNUSED_VARIABLE(app_uart_get(&data_array[index]));
            index++;

            if ((data_array[index - 1] == '\n') || (index >= ( 5 )))
            {
         //       NRF_LOG_DEBUG("Ready to send data over BLE NUS");
         //       NRF_LOG_HEXDUMP_DEBUG(data_array, index);

                do
                {
                    uint16_t length = (uint16_t)index;
                    //err_code = ble_nus_string_send(&m_nus, data_array, &length);
                    if ( (err_code != NRF_ERROR_INVALID_STATE) && (err_code != NRF_ERROR_BUSY) )
                    {
                        APP_ERROR_CHECK(err_code);
                    }
                } while (err_code == NRF_ERROR_BUSY);

                index = 0;
            }
            break;

        case APP_UART_COMMUNICATION_ERROR:
            APP_ERROR_HANDLER(p_event->data.error_communication);
            break;

        case APP_UART_FIFO_ERROR:
            APP_ERROR_HANDLER(p_event->data.error_code);
            break;

        default:
            break;
    }
}
/**@snippet [Handling the data received over UART] */


/**@brief  Function for initializing the UART module.
 */
/**@snippet [UART Initialization] */
static void uart_init(void)
{
    uint32_t                     err_code;
    const app_uart_comm_params_t comm_params =
    {
        RX_PIN_NUMBER,
        TX_PIN_NUMBER,
        RTS_PIN_NUMBER,
        CTS_PIN_NUMBER,
        APP_UART_FLOW_CONTROL_DISABLED,
        false,
        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);
    printf("error log : %d",err_code);
}
/**@snippet [UART Initialization] */

int main(void)
{
    uint32_t err_code;

    // Initialize.
    err_code = app_timer_init();
    APP_ERROR_CHECK(err_code);

    uart_init();
    log_init();
    
    timer_init();
    leds_init();
    buttons_init();
    ble_stack_init();
    gatt_init();
    db_discovery_init();
    lbs_c_init();
    ble_conn_state_init();
    
    printf("\rinit over\r");


    // Start scanning for peripherals and initiate connection to devices which  advertise.
    scan_start();

    // Turn on the LED to signal scanning.
    bsp_board_led_on(CENTRAL_SCANNING_LED);

    for (;;)
    {
        if (!NRF_LOG_PROCESS())
        {
            power_manage();
        }
    }
}

I noticed that when I comment gatt_init () in the main function, I can then display the printf ().
Problem: with the modifications made, the "central" no longer connects to the 2 devices and in addition I still can not communicate between PC and nRF52DK card...
Hoping, that a person can help me. Thank you !

[rraa]

Did you ever connect  central device 2 peripheral devices same time?

[rraa]

~

[Yoann_F]

Here are the different steps of design:
1 - Take the example ble_app_multilink_central for the central board: connection with the other 2 peripheral boards, its OK
2 - Add uart_init () and uart_event_handle (): problem error 8

[Kenneth]

Hi, do I understand it correctly that it's APP_ERROR_CHECK(err_code); in uart_init() that returns error code 8?

If yes, then it may be nrf_drv_uart_init() that return error code 8 (=NRF_ERROR_INVALID_STATE), this means that the UART driver is already initialized. You should be able to ignore this error code.