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SPI app: ERROR 17 [NRF_ERROR_BUSY] Only When Called by App Timer

SDK v15.2.0 using code from the peripheral spi example.  Linux Ubuntu 19.04, uploading to nRF52 DK.

When I call my get_bma253_accel_data in main, I get a continuous stream of data as expected

    // Enter main loop.
    for (;;)
    {
        UNUSED_PARAMETER(get_bma253_accel_data(&accel));     
        NRF_LOG_INFO("accel -- x: %d, y: %d, z: %d", accel.x, accel.y, accel.z);
        idle_state_handle();   
    }

When I use an app timer to call get_bma253_accel_data (Line 301)

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 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
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 * 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
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 *    such product, must reproduce the above copyright notice, this list of
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 *    contributors may be used to endorse or promote products derived from this
 *    software without specific prior written permission.
 *
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 *    Nordic Semiconductor ASA integrated circuit.
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 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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 */
/** @file
 *
 * @defgroup ble_sdk_app_hts_main main.c
 * @{
 * @ingroup ble_sdk_app_hts
 * @brief Health Thermometer Service Sample Application main file.
 *
 * This file contains the source code for a sample application using the Health Thermometer service
 * It also includes the sample code for Battery and Device Information services.
 * This application uses the @ref srvlib_conn_params module.
 */

#include "stdint.h"
#include "string.h"
#include "nordic_common.h"
#include "nrf.h"
#include "app_error.h"
#include "ble.h"
#include "ble_err.h"
#include "ble_hci.h"
#include "ble_srv_common.h"
#include "ble_advdata.h"
#include "ble_advertising.h"
#include "ble_bas.h"
#include "ble_hts_custom.h"
#include "ble_dis.h"
#include "ble_conn_params.h"
#include "sensorsim.h"
#include "nrf_sdh.h"
#include "nrf_sdh_soc.h"
#include "nrf_sdh_ble.h"
#include "app_timer.h"
#include "peer_manager.h"
#include "peer_manager_handler.h"
#include "bsp_btn_ble.h"
#include "fds.h"
#include "ble_conn_state.h"
#include "nrf_ble_gatt.h"
#include "nrf_ble_qwr.h"
#include "nrf_pwr_mgmt.h"
#include "config.h"
#include "battery.h"
#include "ble_nus.h"
#include "tension.h"
#include "temperature.h"
#include "nrf_drv_saadc.h"
#include "stdlib.h"
#include "accelerometer.h"

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

#define DEVICE_NAME "Nordic_HTS"                /**< Name of device. Will be included in the advertising data. */
#define MANUFACTURER_NAME "NordicSemiconductor" /**< Manufacturer. Will be passed to Device Information Service. */
#define MODEL_NUM "NS-HTS-EXAMPLE"              /**< Model number. Will be passed to Device Information Service. */
#define MANUFACTURER_ID 0x1122334455            /**< Manufacturer ID, part of System ID. Will be passed to Device Information Service. */
#define ORG_UNIQUE_ID 0x667788                  /**< Organizational Unique ID, part of System ID. Will be passed to Device Information Service. */

#define APP_BLE_OBSERVER_PRIO 3 /**< Application's BLE observer priority. You shouldn't need to modify this value. */
#define APP_BLE_CONN_CFG_TAG 1  /**< A tag identifying the SoftDevice BLE configuration. */

#define APP_ADV_INTERVAL 40 /**< The advertising interval (in units of 0.625 ms. This value corresponds to 25 ms). */

#define APP_ADV_DURATION 18000 /**< The advertising duration (180 seconds) in units of 10 milliseconds. */

#define BATTERY_LEVEL_MEAS_INTERVAL APP_TIMER_TICKS(2000) /**< Battery level measurement interval (ticks). */
#define MIN_BATTERY_LEVEL 81                              /**< Minimum battery level as returned by the simulated measurement function. */
#define MAX_BATTERY_LEVEL 100                             /**< Maximum battery level as returned by the simulated measurement function. */
#define BATTERY_LEVEL_INCREMENT 1                         /**< Value by which the battery level is incremented/decremented for each call to the simulated measurement function. */

#define TEMP_TYPE_AS_CHARACTERISTIC 0 /**< Determines if temperature type is given as characteristic (1) or as a field of measurement (0). */

#define MIN_CELCIUS_DEGREES 3688     /**< Minimum temperature in celcius for use in the simulated measurement function (multiplied by 100 to avoid floating point arithmetic). */
#define MAX_CELCIUS_DEGRESS 3972     /**< Maximum temperature in celcius for use in the simulated measurement function (multiplied by 100 to avoid floating point arithmetic). */
#define CELCIUS_DEGREES_INCREMENT 36 /**< Value by which temperature is incremented/decremented for each call to the simulated measurement function (multiplied by 100 to avoid floating point arithmetic). */

#define MIN_CONN_INTERVAL MSEC_TO_UNITS(50, UNIT_1_25_MS)  /**< Minimum acceptable connection interval (0.5 seconds) */
#define MAX_CONN_INTERVAL MSEC_TO_UNITS(100, UNIT_1_25_MS) /**< Maximum acceptable connection interval (1 second). */
#define SLAVE_LATENCY 0                                     /**< Slave latency. */
#define CONN_SUP_TIMEOUT MSEC_TO_UNITS(4000, UNIT_10_MS)    /**< Connection supervisory timeout (4 seconds). */

#define FIRST_CONN_PARAMS_UPDATE_DELAY APP_TIMER_TICKS(5000) /**< Time from initiating event (connect or start of indication) to first time sd_ble_gap_conn_param_update is called (5 seconds). */
#define NEXT_CONN_PARAMS_UPDATE_DELAY APP_TIMER_TICKS(30000) /**< Time between each call to sd_ble_gap_conn_param_update after the first call (30 seconds). */
#define MAX_CONN_PARAMS_UPDATE_COUNT 3                       /**< Number of attempts before giving up the connection parameter negotiation. */

#define SEC_PARAM_BOND 0                               /**< Perform bonding. */
#define SEC_PARAM_MITM 0                               /**< Man In The Middle protection not required. */
#define SEC_PARAM_LESC 0                               /**< LE Secure Connections not enabled. */
#define SEC_PARAM_KEYPRESS 0                           /**< Keypress notifications not enabled. */
#define SEC_PARAM_IO_CAPABILITIES BLE_GAP_IO_CAPS_NONE /**< No I/O capabilities. */
#define SEC_PARAM_OOB 0                                /**< Out Of Band data not available. */
#define SEC_PARAM_MIN_KEY_SIZE 7                       /**< Minimum encryption key size. */
#define SEC_PARAM_MAX_KEY_SIZE 16                      /**< Maximum encryption key size. */

#define DEAD_BEEF 0xDEADBEEF /**< Value used as error code on stack dump, can be used to identify stack location on stack unwind. */

#define ADC_REF_VOLTAGE_IN_MILLIVOLTS 600  //!< Reference voltage (in milli volts) used by ADC while doing conversion.
#define DIODE_FWD_VOLT_DROP_MILLIVOLTS 270 //!< Typical forward voltage drop of the diode (Part no: SD103ATW-7-F) that is connected in series with the voltage supply. This is the voltage drop when the forward current is 1mA. Source: Data sheet of 'SURFACE MOUNT SCHOTTKY BARRIER DIODE ARRAY' available at www.diodes.com.
#define ADC_RES_10BIT 1024                 //!< Maximum digital value for 10-bit ADC conversion.
#define ADC_PRE_SCALING_COMPENSATION 6     //!< The ADC is configured to use VDD with 1/3 prescaling as input. And hence the result of conversion is to be multiplied by 3 to get the actual value of the battery voltage.
#define ADC_RESULT_IN_MILLI_VOLTS(ADC_VALUE) \
    ((((ADC_VALUE)*ADC_REF_VOLTAGE_IN_MILLIVOLTS) / ADC_RES_10BIT) * ADC_PRE_SCALING_COMPENSATION)
#define MILLIVOLTS_TO_PERCENT(ADC_VALUE) (ADC_VALUE * 100 / PEAK_BATTERY_VOLTAGE_MV)

APP_TIMER_DEF(m_hts_timer_id);      // Health thermometer service timer
APP_TIMER_DEF(m_tension_timer_id);  // Tension timer
BLE_NUS_DEF(m_nus, 1);              // Nordic UART Service structure
APP_TIMER_DEF(m_battery_timer_id);  /**< Battery timer. */
BLE_BAS_DEF(m_bas);                 /**< Structure used to identify the battery service. */
BLE_HTS_DEF_CUSTOM(m_hts);                 /**< Structure used to identify the health thermometer service. */
NRF_BLE_GATT_DEF(m_gatt);           /**< GATT module instance. */
NRF_BLE_QWR_DEF(m_qwr);             /**< Context for the Queued Write module.*/
BLE_ADVERTISING_DEF(m_advertising); /**< Advertising module instance. */

static struct bma2x2_accel_data accel = {0, 0, 0};
static sensorsim_cfg_t m_tension_sim_cfg;                // Tension sensor simulator configuration
static sensorsim_state_t m_tension_sim_state;            // Tension sensor simulator state
static uint16_t m_conn_handle = BLE_CONN_HANDLE_INVALID; /**< Handle of the current connection. */
static uint16_t m_ble_nus_max_data_len = BLE_GATT_ATT_MTU_DEFAULT - 3;
static bool m_hts_meas_ind_conf_pending = false;   /**< Flag to keep track of when an indication confirmation is pending. */
static sensorsim_cfg_t m_battery_sim_cfg;          /**< Battery Level sensor simulator configuration. */
static sensorsim_state_t m_battery_sim_state;      /**< Battery Level sensor simulator state. */
static sensorsim_cfg_t m_temp_celcius_sim_cfg;     /**< Temperature simulator configuration. */
static sensorsim_state_t m_temp_celcius_sim_state; /**< Temperature simulator state. */

/* Not enough bytes left for us to include the NUS service in 
   the adv packet; once we connect the NUS is visible. */
static ble_uuid_t m_adv_uuids[] = /**< Universally unique service identifiers. */
    {
        {BLE_UUID_HEALTH_THERMOMETER_SERVICE, BLE_UUID_TYPE_BLE},
        {BLE_UUID_BATTERY_SERVICE, BLE_UUID_TYPE_BLE},
        {BLE_UUID_DEVICE_INFORMATION_SERVICE, BLE_UUID_TYPE_BLE}};

static void advertising_start(bool erase_bonds);
static void temperature_measurement_send(void);
static void hts_sim_measurement(ble_hts_meas_t *p_meas);

/**@brief Callback 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] 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(DEAD_BEEF, line_num, p_file_name);
}

/**@brief Function for handling Peer Manager events.
 *
 * @param[in] p_evt  Peer Manager event.
 */
static void pm_evt_handler(pm_evt_t const *p_evt)
{
    ret_code_t err_code;
    bool is_indication_enabled;

    pm_handler_on_pm_evt(p_evt);
    pm_handler_flash_clean(p_evt);

    switch (p_evt->evt_id)
    {
    case PM_EVT_CONN_SEC_SUCCEEDED:
        // Send a single temperature measurement if indication is enabled.
        // NOTE: For this to work, make sure ble_hts_on_ble_evt() is called before
        // pm_evt_handler() in ble_evt_dispatch().
        err_code = ble_hts_is_indication_enabled(&m_hts, &is_indication_enabled);
        APP_ERROR_CHECK(err_code);
        if (is_indication_enabled)
        {
            temperature_measurement_send();
        }
        break;

    case PM_EVT_PEERS_DELETE_SUCCEEDED:
        advertising_start(false);
        break;

    default:
        break;
    }
}

static void TensionLevelUpdate(void)
{
    ret_code_t err_code;
    uint32_t tension_level;
    // largest value to be sent is 2^23 = 8388608, which is 7 bytes. Add one for EOL character
    static uint32_t max_n_digits = 8;
    uint8_t tension[max_n_digits];
    uint16_t len;

    if (simEnabled)
    {
        tension_level = sensorsim_measure(&m_tension_sim_state, &m_tension_sim_cfg);
        len = uint32_encode(tension_level, tension);
        NRF_LOG_INFO("Sending tension measurement: %d", *tension);

        err_code = ble_nus_data_send(&m_nus, tension, &len, m_conn_handle);
        if (err_code != NRF_ERROR_INVALID_STATE) // TODO: remove this quick fix (used in other send functions too)
        {
            APP_ERROR_CHECK(err_code);
        }
    }
    else
    {
        if (Hx711SampleConvert(&tension_level) == NRFX_SUCCESS)
        {
            len = (uint16_t) max_n_digits;
            itoa(tension_level, tension, 10);
            NRF_LOG_INFO("Sending tension measurement: %d of length: %d", tension_level, len);

            err_code = ble_nus_data_send(&m_nus, tension, &len, m_conn_handle);
            if (err_code != NRF_ERROR_INVALID_STATE) // TODO: remove this quick fix (used in other send functions too)
            {
                APP_ERROR_CHECK(err_code);
            }
        }
    }
}

/**@brief Function for performing a battery measurement, and update the Battery Level characteristic in the Battery Service.
 */
static void battery_level_update(void)
{
    ret_code_t err_code;
    nrf_saadc_value_t battery_level;

    if (simEnabled)
    {
        battery_level = (nrf_saadc_value_t)sensorsim_measure(&m_battery_sim_state, &m_battery_sim_cfg);
    }
    else
    {
        /* nrfx_saadc_sample_convert will put a value between 0-1023 in battery_level */
        err_code = nrfx_saadc_sample_convert(NRF_SAADC_INPUT_VDD, &battery_level);
        APP_ERROR_CHECK(err_code);

        /* ADC_RESULTS_IN_MILLI_VOLTS will return a value between 0-3600. Evidently, the value in
           battery_level should not be larger than the actual possible voltage of your battery */
        battery_level = ADC_RESULT_IN_MILLI_VOLTS(battery_level) + DIODE_FWD_VOLT_DROP_MILLIVOLTS;
        battery_level = MILLIVOLTS_TO_PERCENT(battery_level);
    }

    NRF_LOG_INFO("Sending battery measurement: %d", battery_level);

    err_code = ble_bas_battery_level_update(&m_bas, (uint8_t)battery_level, m_conn_handle);
    if ((err_code != NRF_SUCCESS) &&
        (err_code != NRF_ERROR_INVALID_STATE) &&
        (err_code != NRF_ERROR_RESOURCES) &&
        (err_code != NRF_ERROR_BUSY) &&
        (err_code != BLE_ERROR_GATTS_SYS_ATTR_MISSING))
    {
        APP_ERROR_HANDLER(err_code);
    }
    UNUSED_PARAMETER(get_bma253_accel_data(&accel));     
}

/**@brief Function for handling the Battery measurement timer timeout.
 *
 * @details This function will be called each time the battery level measurement timer expires.
 *
 * @param[in] p_context   Pointer used for passing some arbitrary information (context) from the
 *                        app_start_timer() call to the timeout handler.
 */
static void battery_level_meas_timeout_handler(void *p_context)
{
    UNUSED_PARAMETER(p_context);
    battery_level_update();
}

static void tension_timer_timeout_handler(void *p_context)
{
    UNUSED_PARAMETER(p_context);
    TensionLevelUpdate();
}

static void hts_timer_timeout_handler(void *p_context)
{
    UNUSED_PARAMETER(p_context);
    temperature_measurement_send();
}

/**@brief Function for populating simulated health thermometer measurement.
 */
static void hts_sim_measurement(ble_hts_meas_t *p_meas)
{
    static ble_date_time_t time_stamp = {2012, 12, 5, 11, 50, 0};

    uint32_t celciusX100;

    p_meas->temp_in_fahr_units = false;
    p_meas->time_stamp_present = true;
    p_meas->temp_type_present = (TEMP_TYPE_AS_CHARACTERISTIC ? false : true);

    celciusX100 = sensorsim_measure(&m_temp_celcius_sim_state, &m_temp_celcius_sim_cfg);

    p_meas->temp_in_celcius.exponent = -2;
    p_meas->temp_in_celcius.mantissa = celciusX100;
    p_meas->temp_in_fahr.exponent = -2;
    p_meas->temp_in_fahr.mantissa = (32 * 100) + ((celciusX100 * 9) / 5);
    p_meas->time_stamp = time_stamp;
    p_meas->temp_type = BLE_HTS_TEMP_TYPE_FINGER;

    // update simulated time stamp
    time_stamp.seconds += 27;
    if (time_stamp.seconds > 59)
    {
        time_stamp.seconds -= 60;
        time_stamp.minutes++;
        if (time_stamp.minutes > 59)
        {
            time_stamp.minutes = 0;
        }
    }
}

/**@brief Function for the Timer initialization.
 *
 * @details Initializes the timer module. This creates and starts application timers.
 */
static void timers_init(void)
{
    ret_code_t err_code;

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

    // Create timers.
    err_code = app_timer_create(&m_battery_timer_id,
                                APP_TIMER_MODE_REPEATED,
                                battery_level_meas_timeout_handler);
    APP_ERROR_CHECK(err_code);

    err_code = app_timer_create(&m_tension_timer_id,
                                APP_TIMER_MODE_REPEATED,
                                tension_timer_timeout_handler);
    APP_ERROR_CHECK(err_code);

    err_code = app_timer_create(&m_hts_timer_id,
                                APP_TIMER_MODE_REPEATED,
                                hts_timer_timeout_handler);
    APP_ERROR_CHECK(err_code);
}

/**@brief Function for the GAP initialization.
 *
 * @details This function sets up all the necessary GAP (Generic Access Profile) parameters of the
 *          device including the device name, appearance, and the preferred connection parameters.
 */
static void gap_params_init(void)
{
    ret_code_t err_code;
    ble_gap_conn_params_t gap_conn_params;
    ble_gap_conn_sec_mode_t sec_mode;

    BLE_GAP_CONN_SEC_MODE_SET_OPEN(&sec_mode);

    err_code = sd_ble_gap_device_name_set(&sec_mode,
                                          (const uint8_t *)DEVICE_NAME,
                                          strlen(DEVICE_NAME));
    APP_ERROR_CHECK(err_code);

    err_code = sd_ble_gap_appearance_set(BLE_APPEARANCE_GENERIC_THERMOMETER);
    APP_ERROR_CHECK(err_code);

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

    gap_conn_params.min_conn_interval = MIN_CONN_INTERVAL;
    gap_conn_params.max_conn_interval = MAX_CONN_INTERVAL;
    gap_conn_params.slave_latency = SLAVE_LATENCY;
    gap_conn_params.conn_sup_timeout = CONN_SUP_TIMEOUT;

    err_code = sd_ble_gap_ppcp_set(&gap_conn_params);
    APP_ERROR_CHECK(err_code);
}

/**@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 ((m_conn_handle == p_evt->conn_handle) && (p_evt->evt_id == NRF_BLE_GATT_EVT_ATT_MTU_UPDATED))
    {
        m_ble_nus_max_data_len = p_evt->params.att_mtu_effective - OPCODE_LENGTH - HANDLE_LENGTH;
        NRF_LOG_INFO("Data len is set to 0x%X(%d)", m_ble_nus_max_data_len, m_ble_nus_max_data_len);
    }
    NRF_LOG_DEBUG("ATT MTU exchange completed. central 0x%x peripheral 0x%x",
                  p_gatt->att_mtu_desired_central,
                  p_gatt->att_mtu_desired_periph);
}

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

/**@brief Function for simulating and sending one Temperature Measurement.
 */
static void temperature_measurement_send(void)
{
    ble_hts_meas_t temperature_meas;
    ret_code_t err_code;

        if (simEnabled)
        {
            hts_sim_measurement(&temperature_meas);
        }
        else
        {
            ReadTemperature(&temperature_meas);
        }

        /* For logging instead of including the entire math library I just assume .exponent = -2 (ie mantissa x 0.01)*/
        NRF_LOG_INFO("Sending temperature measurement: %d", temperature_meas.temp_in_celcius.mantissa * 0.01);

    err_code = ble_hts_measurement_send_custom(&m_hts, &temperature_meas);

        switch (err_code)
        {
        case NRF_SUCCESS:
            // Measurement was successfully sent, wait for confirmation.
            m_hts_meas_ind_conf_pending = true;
            break;

        case NRF_ERROR_INVALID_STATE:
            // Ignore error.
            break;

        default:
            APP_ERROR_HANDLER(err_code);
            break;
        }
    }

/**@brief Function for handling the Health Thermometer Service events.
 *
 * @details This function will be called for all Health Thermometer Service events which are passed
 *          to the application.
 *
 * @param[in] p_hts  Health Thermometer Service structure.
 * @param[in] p_evt  Event received from the Health Thermometer Service.
 */
static void on_hts_evt(ble_hts_t *p_hts, ble_hts_evt_t *p_evt)
{
    switch (p_evt->evt_type)
    {
    case BLE_HTS_EVT_INDICATION_ENABLED:
        // Indication has been enabled, send a single temperature measurement
        temperature_measurement_send();
        break;

    case BLE_HTS_EVT_INDICATION_CONFIRMED:
        m_hts_meas_ind_conf_pending = false;
        break;

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

/**@brief Function for handling Queued Write Module errors.
 *
 * @details A pointer to this function will be passed to each service which may need to inform the
 *          application about an error.
 *
 * @param[in]   nrf_error   Error code containing information about what went wrong.
 */
static void nrf_qwr_error_handler(uint32_t nrf_error)
{
    APP_ERROR_HANDLER(nrf_error);
}

/**@brief Function for handling the data from the Nordic UART Service.
 *
 * @details This function will process the data received from the Nordic UART BLE Service and send
 *          it to the UART module.
 *
 * @param[in] p_evt       Nordic UART Service event.
 */
/**@snippet [Handling the data received over BLE] */
static void nus_data_handler(ble_nus_evt_t *p_evt)
{

    if (p_evt->type == BLE_NUS_EVT_RX_DATA)
    {
    }
}
/**@snippet [Handling the data received over BLE] */

/**@brief Function for initializing services that will be used by the application.
 *
 * @details Initialize the Health Thermometer, Battery and Device Information services.
 */
static void services_init(void)
{
    ret_code_t err_code;
    ble_hts_init_t hts_init;
    ble_bas_init_t bas_init;
    ble_dis_init_t dis_init;
    nrf_ble_qwr_init_t qwr_init = {0};
    ble_dis_sys_id_t sys_id;
    ble_nus_init_t nus_init;

    // Initialize Queued Write Module.
    qwr_init.error_handler = nrf_qwr_error_handler;

    err_code = nrf_ble_qwr_init(&m_qwr, &qwr_init);
    APP_ERROR_CHECK(err_code);

    // Initialize Health Thermometer Service
    memset(&hts_init, 0, sizeof(hts_init));

    hts_init.evt_handler = on_hts_evt;
    hts_init.temp_type_as_characteristic = TEMP_TYPE_AS_CHARACTERISTIC;
    hts_init.temp_type = BLE_HTS_TEMP_TYPE_BODY;

    // Here the sec level for the Health Thermometer Service can be changed/increased.
    hts_init.ht_meas_cccd_wr_sec = SEC_JUST_WORKS;
    hts_init.ht_type_rd_sec = SEC_OPEN;

    err_code = ble_hts_init(&m_hts, &hts_init);
    APP_ERROR_CHECK(err_code);

    // Initialize Battery Service.
    memset(&bas_init, 0, sizeof(bas_init));

    // Here the sec level for the Battery Service can be changed/increased.
    bas_init.bl_rd_sec = SEC_OPEN;
    bas_init.bl_cccd_wr_sec = SEC_OPEN;
    bas_init.bl_report_rd_sec = SEC_OPEN;

    bas_init.evt_handler = NULL;
    bas_init.support_notification = true;
    bas_init.p_report_ref = NULL;
    bas_init.initial_batt_level = 100;

    err_code = ble_bas_init(&m_bas, &bas_init);
    APP_ERROR_CHECK(err_code);

    // Initialize Device Information Service.
    memset(&dis_init, 0, sizeof(dis_init));

    ble_srv_ascii_to_utf8(&dis_init.manufact_name_str, MANUFACTURER_NAME);
    ble_srv_ascii_to_utf8(&dis_init.model_num_str, MODEL_NUM);

    sys_id.manufacturer_id = MANUFACTURER_ID;
    sys_id.organizationally_unique_id = ORG_UNIQUE_ID;
    dis_init.p_sys_id = &sys_id;

    dis_init.dis_char_rd_sec = SEC_OPEN;

    err_code = ble_dis_init(&dis_init);
    APP_ERROR_CHECK(err_code);

    // Initialise Nordic UART Service.
    memset(&nus_init, 0, sizeof(nus_init));

    /* For received data, which we don't care about right now */
    nus_init.data_handler = nus_data_handler;

    err_code = ble_nus_init(&m_nus, &nus_init);
    APP_ERROR_CHECK(err_code);
}

/**@brief Function for initializing the sensor simulators.
 */
static void sensor_simulator_init(void)
{
    m_tension_sim_cfg.min = 5000;
    m_tension_sim_cfg.max = 30000;
    m_tension_sim_cfg.incr = 1000;
    m_tension_sim_cfg.start_at_max = true;

    sensorsim_init(&m_tension_sim_state, &m_tension_sim_cfg);

    m_battery_sim_cfg.min = MIN_BATTERY_LEVEL;
    m_battery_sim_cfg.max = MAX_BATTERY_LEVEL;
    m_battery_sim_cfg.incr = BATTERY_LEVEL_INCREMENT;
    m_battery_sim_cfg.start_at_max = true;

    sensorsim_init(&m_battery_sim_state, &m_battery_sim_cfg);

    // Temperature is in celcius (it is multiplied by 100 to avoid floating point arithmetic).
    m_temp_celcius_sim_cfg.min = MIN_CELCIUS_DEGREES;
    m_temp_celcius_sim_cfg.max = MAX_CELCIUS_DEGRESS;
    m_temp_celcius_sim_cfg.incr = CELCIUS_DEGREES_INCREMENT;
    m_temp_celcius_sim_cfg.start_at_max = false;

    sensorsim_init(&m_temp_celcius_sim_state, &m_temp_celcius_sim_cfg);
}

/**@brief Function for starting application timers.
 */
static void application_timers_start(void)
{
    ret_code_t err_code;

    // Start application timers.
    err_code = app_timer_start(m_battery_timer_id, BATTERY_LEVEL_MEAS_INTERVAL, NULL);
    APP_ERROR_CHECK(err_code);

    err_code = app_timer_start(m_tension_timer_id, APP_TIMER_TICKS(2000), NULL);
    APP_ERROR_CHECK(err_code);

    err_code = app_timer_start(m_hts_timer_id, APP_TIMER_TICKS(2000), NULL);
    APP_ERROR_CHECK(err_code);
}

/**@brief Function for stopping application timers.
 */
static void application_timers_stop(void)
{
    ret_code_t err_code;

    // Start application timers.
    err_code = app_timer_stop(m_battery_timer_id);
    APP_ERROR_CHECK(err_code);

    err_code = app_timer_stop(m_tension_timer_id);
    APP_ERROR_CHECK(err_code);

    err_code = app_timer_stop(m_hts_timer_id);
    APP_ERROR_CHECK(err_code);
}

/**@brief Function for handling the Connection Parameters Module.
 *
 * @details This function will be called for all events in the Connection Parameters Module which
 *          are passed to the application.
 *          @note All this function does is to disconnect. This could have been done by simply
 *                setting the disconnect_on_fail config parameter, but instead we use the event
 *                handler mechanism to demonstrate its use.
 *
 * @param[in] p_evt  Event received from the Connection Parameters Module.
 */
static void on_conn_params_evt(ble_conn_params_evt_t *p_evt)
{
    ret_code_t err_code;

    if (p_evt->evt_type == BLE_CONN_PARAMS_EVT_FAILED)
    {
        err_code = sd_ble_gap_disconnect(m_conn_handle, BLE_HCI_CONN_INTERVAL_UNACCEPTABLE);
        APP_ERROR_CHECK(err_code);
    }
}

/**@brief Function for handling a Connection Parameters error.
 *
 * @param[in] nrf_error  Error code containing information about what went wrong.
 */
static void conn_params_error_handler(uint32_t nrf_error)
{
    APP_ERROR_HANDLER(nrf_error);
}

/**@brief Function for initializing the Connection Parameters module.
 */
static void conn_params_init(void)
{
    ret_code_t err_code;
    ble_conn_params_init_t cp_init;

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

    cp_init.p_conn_params = NULL;
    cp_init.first_conn_params_update_delay = FIRST_CONN_PARAMS_UPDATE_DELAY;
    cp_init.next_conn_params_update_delay = NEXT_CONN_PARAMS_UPDATE_DELAY;
    cp_init.max_conn_params_update_count = MAX_CONN_PARAMS_UPDATE_COUNT;
    cp_init.start_on_notify_cccd_handle = BLE_GATT_HANDLE_INVALID;
    cp_init.disconnect_on_fail = false;
    cp_init.evt_handler = on_conn_params_evt;
    cp_init.error_handler = conn_params_error_handler;

    err_code = ble_conn_params_init(&cp_init);
    APP_ERROR_CHECK(err_code);
}

/**@brief Function for putting the chip into sleep mode.
 *
 * @note This function will not return.
 */
static void sleep_mode_enter(void)
{
    ret_code_t err_code;

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

    // Prepare wakeup buttons.
    err_code = bsp_btn_ble_sleep_mode_prepare();
    APP_ERROR_CHECK(err_code);

    // Go to system-off mode (this function will not return; wakeup will cause a reset).
    err_code = sd_power_system_off();
    APP_ERROR_CHECK(err_code);
}

/**@brief Function for handling advertising events.
 *
 * @details This function will be called for advertising events which are passed to the application.
 *
 * @param[in] ble_adv_evt  Advertising event.
 */
static void on_adv_evt(ble_adv_evt_t ble_adv_evt)
{
    ret_code_t err_code;

    switch (ble_adv_evt)
    {
    case BLE_ADV_EVT_FAST:
        NRF_LOG_INFO("Fast advertising.");
        err_code = bsp_indication_set(BSP_INDICATE_ADVERTISING);
        APP_ERROR_CHECK(err_code);
        break;

    case BLE_ADV_EVT_IDLE:
        sleep_mode_enter();
        break;

    default:
        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)
{
    uint32_t err_code = NRF_SUCCESS;

    switch (p_ble_evt->header.evt_id)
    {
    case BLE_GAP_EVT_CONNECTED:
        NRF_LOG_INFO("Connected.");
        err_code = bsp_indication_set(BSP_INDICATE_CONNECTED);
        APP_ERROR_CHECK(err_code);
        m_conn_handle = p_ble_evt->evt.gap_evt.conn_handle;
        err_code = nrf_ble_qwr_conn_handle_assign(&m_qwr, m_conn_handle);
        APP_ERROR_CHECK(err_code);
        err_code = sd_ble_gatts_sys_attr_set(m_conn_handle, NULL, 0, 0);
        APP_ERROR_CHECK(err_code);
        application_timers_start();
        break;

    case BLE_GAP_EVT_DISCONNECTED:
        NRF_LOG_INFO("Disconnected.");
        m_conn_handle = BLE_CONN_HANDLE_INVALID;
        m_hts_meas_ind_conf_pending = false;
        application_timers_stop();
        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;

    case BLE_GATTS_EVT_SYS_ATTR_MISSING:
        // No system attributes have been stored.
        err_code = sd_ble_gatts_sys_attr_set(m_conn_handle, NULL, 0, 0);
        APP_ERROR_CHECK(err_code);
        break;

    default:
        // No implementation needed.
        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 BSP module.
 *
 * @param[in]   event   Event generated by button press.
 */
static void bsp_event_handler(bsp_event_t event)
{
    ret_code_t err_code;

    switch (event)
    {
    case BSP_EVENT_SLEEP:
        sleep_mode_enter();
        break;

    case BSP_EVENT_DISCONNECT:
        err_code = sd_ble_gap_disconnect(m_conn_handle,
                                         BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);
        if (err_code != NRF_ERROR_INVALID_STATE)
        {
            APP_ERROR_CHECK(err_code);
        }
        break;

    case BSP_EVENT_WHITELIST_OFF:
        if (m_conn_handle == BLE_CONN_HANDLE_INVALID)
        {
            err_code = ble_advertising_restart_without_whitelist(&m_advertising);
            if (err_code != NRF_ERROR_INVALID_STATE)
            {
                APP_ERROR_CHECK(err_code);
            }
        }
        break;

    case BSP_EVENT_KEY_0:
        if (m_conn_handle != BLE_CONN_HANDLE_INVALID)
        {
            temperature_measurement_send();
        }
        break;

    default:
        break;
    }
}

/**@brief Function for the Peer Manager initialization.
 */
static void peer_manager_init(void)
{
    ble_gap_sec_params_t sec_param;
    ret_code_t err_code;

    err_code = pm_init();
    APP_ERROR_CHECK(err_code);

    memset(&sec_param, 0, sizeof(ble_gap_sec_params_t));

    // Security parameters to be used for all security procedures.
    sec_param.bond = SEC_PARAM_BOND;
    sec_param.mitm = SEC_PARAM_MITM;
    sec_param.lesc = SEC_PARAM_LESC;
    sec_param.keypress = SEC_PARAM_KEYPRESS;
    sec_param.io_caps = SEC_PARAM_IO_CAPABILITIES;
    sec_param.oob = SEC_PARAM_OOB;
    sec_param.min_key_size = SEC_PARAM_MIN_KEY_SIZE;
    sec_param.max_key_size = SEC_PARAM_MAX_KEY_SIZE;
    sec_param.kdist_own.enc = 0;
    sec_param.kdist_own.id = 0;
    sec_param.kdist_peer.enc = 0;
    sec_param.kdist_peer.id = 0;

    err_code = pm_sec_params_set(&sec_param);
    APP_ERROR_CHECK(err_code);

    err_code = pm_register(pm_evt_handler);
    APP_ERROR_CHECK(err_code);
}

/**@brief Clear bond information from persistent storage.
 */
static void delete_bonds(void)
{
    ret_code_t err_code;

    NRF_LOG_INFO("Erase bonds!");

    err_code = pm_peers_delete();
    APP_ERROR_CHECK(err_code);
}

/**@brief Function for initializing the Advertising functionality.
 *
 * @details Encodes the required advertising data and passes it to the stack.
 *          Also builds a structure to be passed to the stack when starting advertising.
 */
static void advertising_init(void)
{
    ret_code_t err_code;
    ble_advertising_init_t init;

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

    init.advdata.name_type = BLE_ADVDATA_FULL_NAME;
    init.advdata.include_appearance = true;
    init.advdata.flags = BLE_GAP_ADV_FLAGS_LE_ONLY_GENERAL_DISC_MODE;
    init.advdata.uuids_complete.uuid_cnt = sizeof(m_adv_uuids) / sizeof(m_adv_uuids[0]);
    init.advdata.uuids_complete.p_uuids = m_adv_uuids;

    init.config.ble_adv_fast_enabled = true;
    init.config.ble_adv_fast_interval = APP_ADV_INTERVAL;
    init.config.ble_adv_fast_timeout = APP_ADV_DURATION;

    init.evt_handler = on_adv_evt;

    err_code = ble_advertising_init(&m_advertising, &init);
    APP_ERROR_CHECK(err_code);

    ble_advertising_conn_cfg_tag_set(&m_advertising, APP_BLE_CONN_CFG_TAG);
}

/**@brief Function for initializing buttons and leds.
 *
 * @param[out] p_erase_bonds  Will be true if the clear bonding button was pressed to wake the application up.
 */
static void buttons_leds_init(bool *p_erase_bonds)
{
    ret_code_t err_code;
    bsp_event_t startup_event;

    err_code = bsp_init(BSP_INIT_LEDS | 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);

    *p_erase_bonds = (startup_event == BSP_EVENT_CLEAR_BONDING_DATA);
}

/**@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 handling the idle state (main loop).
 *
 * @details If there is no pending log operation, then sleep until next the next event occurs.
 */
static void idle_state_handle(void)
{
    if (NRF_LOG_PROCESS() == false)
    {
        nrf_pwr_mgmt_run();
    }
}

/**@brief Function for starting advertising.
 */
static void advertising_start(bool erase_bonds)
{
    if (erase_bonds == true)
    {
        delete_bonds();
        // Advertising is started by PM_EVT_PEERS_DELETE_SUCCEEDED event.
    }
    else
    {
        uint32_t err_code = ble_advertising_start(&m_advertising, BLE_ADV_MODE_FAST);
        APP_ERROR_CHECK(err_code);
    }
}

/**@brief Function for application main entry.
 */
int main(void)
{
    bool erase_bonds;

    // Initialize.
    log_init();
    timers_init();
    buttons_leds_init(&erase_bonds);
    power_management_init();
    ble_stack_init();
    gap_params_init();
    gatt_init();
    services_init();
    advertising_init();
    sensor_simulator_init();
    conn_params_init();
    peer_manager_init();
    BatteryADCInit();
    Hx711Init(INPUT_CH_A_128);
    bma_spi_init();


    if (debugEnabled)
    {
        NRF_LOG_INFO("This is a debug build!!");
    }
    if (simEnabled)
    {
        NRF_LOG_INFO("This is a simulated build!!");
    }

    // Start execution.
    NRF_LOG_INFO("Health Thermometer example started.");
    advertising_start(erase_bonds);


    // Enter main loop.
    for (;;)
    {
        NRF_LOG_INFO("accel -- x: %d, y: %d, z: %d", accel.x, accel.y, accel.z);
        idle_state_handle();   
    }
}

/**
 * @}
 */

I get the following error (upon connection via nRFConnect):

<error> app: ERROR 17 [NRF_ERROR_BUSY] at ../../../Src/accelerometer.c:92
PC at: 0x00027717

Here's my accelerometer .c file

#include "accelerometer.h"
#include "bma2x2.h" 
#include "nrf_drv_spi.h"
#include "nrf_delay.h"

#define SPI_INSTANCE 0

static const nrf_drv_spi_t spi = NRF_DRV_SPI_INSTANCE(SPI_INSTANCE);
struct bma2x2_t bma2x2;
static volatile bool spi_xfer_done = false;


#define BMA2x2_SPI_BUS_WRITE_CONTROL_BYTE	0x7F
#define BMA2x2_SPI_BUS_READ_CONTROL_BYTE	0x80

#ifdef DEVKIT
    #define BMA_SPI_SS_PIN 19
    #define BMA_SPI_MISO_PIN 23
    #define BMA_SPI_MOSI_PIN 22
    #define BMA_SPI_SCK_PIN 20
#else
    #define BMA_SPI_SS_PIN 17
    #define BMA_SPI_MISO_PIN 8
    #define BMA_SPI_MOSI_PIN 6
    #define BMA_SPI_SCK_PIN 15
#endif

void spi_event_handler(nrf_drv_spi_evt_t const * p_event,
                       void *                    p_context)
{
    spi_xfer_done = true;
}

void bma_spi_init()
{
    nrf_drv_spi_config_t spi_config = NRF_DRV_SPI_DEFAULT_CONFIG;
    spi_config.ss_pin   = BMA_SPI_SS_PIN;
    spi_config.miso_pin = BMA_SPI_MISO_PIN;
    spi_config.mosi_pin = BMA_SPI_MOSI_PIN;
    spi_config.sck_pin  = BMA_SPI_SCK_PIN;
    spi_config.mode = NRF_DRV_SPI_MODE_3;
    spi_config.bit_order = NRF_SPI_BIT_ORDER_MSB_FIRST;
    APP_ERROR_CHECK(nrf_drv_spi_init(&spi, &spi_config, spi_event_handler, NULL));

    bma2x2.chip_id = BMA2x2_INIT_VALUE;
    bma2x2.bus_write = &bma_spi_write;
    bma2x2.bus_read = &bma_spi_read;
    bma2x2.delay_msec = &spi_delay;
    BMA_ERROR_CHECK(bma2x2_init(&bma2x2));

    if (bma2x2.chip_id == 0xFA)
    {
        NRF_LOG_INFO("BMA initialised.");
    }
    else
    {
        NRF_LOG_INFO("BMA initialisation failed. Received chip id: %x", bma2x2.chip_id);
    }
}

s8 bma_spi_write(u8 dev_addr, u8 reg_addr, u8 *reg_data, u8 cnt)
{
    s8 error = NO_ERROR;
    uint8_t tx_buf[cnt+1];

    // place write bit + address in first byte
    tx_buf[0] = (reg_addr++) & BMA2x2_SPI_BUS_WRITE_CONTROL_BYTE;

    for (uint8_t string_pos = 1; string_pos < cnt+1; string_pos++)
    {   
        // place data in second byte        
        tx_buf[string_pos] = *(reg_data + string_pos - 1);
    }

    spi_xfer_done = false;
    error = (s8) nrf_drv_spi_transfer(&spi, tx_buf, cnt+1, NULL, 0);
    BMA_ERROR_CHECK(error);

    nrf_delay_us(100);

    return error;
}

s8 bma_spi_read(u8 dev_addr, u8 reg_addr, u8 *reg_data, u8 cnt)
{
    s8 error = NO_ERROR;
    uint8_t tx_buf[1] = {reg_addr|BMA2x2_SPI_BUS_READ_CONTROL_BYTE};
    u8 array[cnt+1];

    spi_xfer_done = false;
    error = (s8) nrf_drv_spi_transfer(&spi, tx_buf, 1, array, cnt+1);
    BMA_ERROR_CHECK(error);

    nrf_delay_us(100);

    for (uint32_t i=0; i < cnt; i++)
    {
        *(reg_data + i) = array[i+1];
    }

    return error;
}

void spi_delay(u32 millis_time)
{
    nrf_delay_ms(millis_time);
}

s32 bma2x2_data_readout()
{
	/*Local variables for reading accel x, y and z data*/
	s16	accel_x_s16, accel_y_s16, accel_z_s16 = BMA2x2_INIT_VALUE;
	/* Local variable used to assign the bandwidth value*/
	u8 bw_value_u8 = BMA2x2_INIT_VALUE;
	/* Local variable used to set the bandwidth value*/
	u8 banwid = BMA2x2_INIT_VALUE;
	/* status of communication*/
	s32 com_rslt = NO_ERROR;

    com_rslt += bma2x2_set_power_mode(BMA2x2_MODE_NORMAL);


    bw_value_u8 = 0x08;/* set bandwidth of 7.81Hz*/
	com_rslt += bma2x2_set_bw(bw_value_u8);

	/* This API used to read back the written value of bandwidth*/
	com_rslt += bma2x2_get_bw(&banwid);

   	/* Read the accel X data*/
	com_rslt += bma2x2_read_accel_x(&accel_x_s16);
	/* Read the accel Y data*/
	com_rslt += bma2x2_read_accel_y(&accel_y_s16);
	/* Read the accel Z data*/
	com_rslt += bma2x2_read_accel_z(&accel_z_s16);
    NRF_LOG_INFO("accel -- x: %d, y: %d, z: %d", accel_x_s16, accel_y_s16, accel_y_s16);

    com_rslt += bma2x2_set_power_mode(BMA2x2_MODE_DEEP_SUSPEND);

    BMA_ERROR_CHECK(com_rslt);
        
    return com_rslt;
}

s32 get_bma253_accel_data(struct bma2x2_accel_data *accel)
{
    s32 com_rslt = NO_ERROR;

    com_rslt = bma2x2_read_accel_x(&(accel->x));

    com_rslt = bma2x2_read_accel_y(&(accel->y));

    com_rslt = bma2x2_read_accel_z(&(accel->z));
    
    BMA_ERROR_CHECK(com_rslt);

    return com_rslt;
}

I thought it might have something to do with the spi_xfer_done flag, but I'm having a hard time debugging. Can someone point me in the right direction? Here's one similar question

Here's what the output looks like

<info> app: accel -- x: 0, y: 0, z: 0
<info> app: accel -- x: 0, y: 0, z: 0
<info> app: accel -- x: 0, y: 0, z: 0
<info> app: accel -- x: 0, y: 0, <info> app: accel -- x: 0, y: 0, z: 0
<info> app: accel -- x: 0, y: 0, z: 0
<info> app: accel -- x: 0, y: 0, z: 0
<info> app: accel -- x: 0, y: 0, z: 0
<info> app: accel -- x: 0, y: 0, z: 0
<info> app: accel -- x: 0, y: 0, z: 0
<info> app: Sending temperature measurement: 25
<info> app: Sending battery measurement: 105
<error> app: ERROR 17 [NRF_ERROR_BUSY] at ../../../Src/accelerometer.c:92
PC at: 0x00027717
<error> app: End of error report

Lastly, I'm using the BMA2x2 driver. The .h/.c files can be found here

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  • Without looking at the code in detail it is clear that processes which generate interrupts are being instigated from within other interrupts, which raises scary priority issues, race hazards and the like. I would suggest starting by removing all the functions invoked by the different timer interrupts into the main loop and set volatile bool flags to initiate the functions from outside the interrupts, which in turn clear the flags when invoked.

    A nice touch is to use counters instead of bool flags, then with logs or breakpoints it is possible to see what would have been multiple invocation attempts when the functions were invoked from within the interrupts, but which are now safely invoked in the main loop without being invoked again whilst already running from the previous invocation. Multiple simultaneous invocations will cause stuff such as NRF_ERROR_BUSY.

    As an aside, using incrementing counters instead of bool flags shows up a double-interrupt in the SPI handler, which I never tracked down. Maybe you will see that as well, although I use both a counter and a bool event so I don't reset the counter.

    volatile uint32_t mSpiInterruptCounter = 0UL;
    
    void SPIM0_SPIS0_TWIM0_TWIS0_SPI0_TWI0_IRQHandler(void)
    {
      mSpiInterruptCounter++;
      NRF_SPIM0->EVENTS_END = 0;
    }

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