RTC in repeat mode with no CPU involvement ( SDK v13 )

[bryanhsieh]

Hi Everyone,

The custom device I am working on is a BLE peripheral device and uses softdevice S132,

therefore to get accurate triggering of ADC sampling and to avoid periodic CPU blocking by the radio, 

I am trying to get RTC to trigger the ADC without any CPU involvement. Also currently, the custom function

I use for ADC sampling is a blocking function ( increased power consumption from CPU idling ) therefore I hope by using PPI and RTC, I can also reduce the

overall power consumption.

Looking at the RTC example provided in SDK v13, I can see that that rtc in repeat mode is done by calling the rtc_cc_set function

in the rtc interrupt on compare event. 

My question is this, will I be able to run the RTC in repeat mode without the use of the interrupt function as it can be blocked by the radio interrupt?

I can disable the rtc interrupt ( SDK v13 requires an interrupt handler to be passed ) and use rtc clear task with PPI to reset the rtc counter , but I can see that the cc value is set in the rtc_cc_set function in the interrupt.

Is this required ? if I set the cc value once, I presume it will be retained even after calling the rtc clear task?

In the IRQ handler for rtc, the compare event register is cleared, if I were to disable the interrupt, then there would be no way of clearing this register using PPI.

However, is it necessary to clear the compare event register manually, would it go low if were to clear the counter and then high again?

In summary what I would likely to do is to set the cc value once, clear the counter using ppi on compare event ( forked to trigger adc sampling ) and wait for the next compare event.

Alternatively, I can leave the interrupt as it is with the rtc_cc_set function but then do the clearing with PPI, as long as the counter is cleared straightaway and the compare event register

and cc value is cleared/set a few cycles before the counter reaches the cc value ( ie cc time interval <  1/sampling_freq ) then this would be fine?

Thanks.

[Jørgen Holmefjord]

Hi,

It is not required to call nrf_drv_rtc_cc_set() in the event handler to set the CC value, the reason that this function is called in the handler is to re-enable the event and interrupts. In the IRQ Handler in the RTC driver, the events and interrupts are disabled when the COMPARE event is detected:

nrf_rtc_event_disable(p_reg,int_mask);
nrf_rtc_int_disable(p_reg,int_mask);

If you disable the interrupt and clear the RTC through PPI (fork), the code in the interrupt handler will not be run, and the CC register will not be changed.

Note that you will likely not be able to achieve low power SAADC sampling and sampling with precise timing at the same time. The SAADC will have high current consumption when it is STARTED, which is required in order to sample through PPI. We have some low power examples, but this will stop the SAADC between each sample, and it must be restarted by the software.

You may be able to achieve something semi-precise and low power by triggering the START task through PPI and connect the SAADC STARTED event to the SAMPLE task, but I have not tested this. This approach is also not supported by the SAADC driver, so you would need to write your own custom implementation. In this solution, you need to make sure that the CPU provides a new buffer for the SAADC to sample to between the STARTED event and the next triggering of START task over PPI, to prevent the previous buffer from being overwritten.

Best regards,
Jørgen

[bryanhsieh]

Fullscreen 6661.main.c Download

/* Copyright (c) 2014 Nordic Semiconductor. All Rights Reserved.
 *
 * The information contained herein is property of Nordic Semiconductor ASA.
 * Terms and conditions of usage are described in detail in NORDIC
 * SEMICONDUCTOR STANDARD SOFTWARE LICENSE AGREEMENT.
 *
 * Licensees are granted free, non-transferable use of the information. NO
 * WARRANTY of ANY KIND is provided. This heading must NOT be removed from
 * the file.
 *
 */

/** @file
 *
 * @defgroup ble_sdk_app_template_main main.c
 * @{
 * @ingroup ble_sdk_app_template
 * @brief Template project main file.
 *
 * This file contains a template for creating a new application. It has the code necessary to wakeup
 * from button, advertise, get a connection restart advertising on disconnect and if no new
 * connection created go back to system-off mode.
 * It can easily be used as a starting point for creating a new application, the comments identified
 * with 'YOUR_JOB' indicates where and how you can customize.
 */

#include <stdbool.h>
#include <stdint.h>
#include <string.h>

#include "nordic_common.h"
#include "nrf.h"
#include "app_error.h"
#include "ble.h"
#include "ble_hci.h"
#include "ble_srv_common.h"
#include "ble_advdata.h"
#include "ble_advertising.h"
#include "ble_conn_params.h"
#include "boards.h"
#include "softdevice_handler.h"
#include "app_timer.h"
#include "fstorage.h"
#include "fds.h"
#include "peer_manager.h"

#include "bsp.h"
#include "sensorsim.h"
#include "nrf_gpio.h"
#include "ble_hci.h"
#include "ble_advdata.h"
#include "ble_advertising.h"
#include "ble_conn_state.h"
#include "nrf_drv_gpiote.h"
#include "nrf_drv_rtc.h"
#include "nrf_drv_saadc.h"
#include "nrf_saadc.h"
#include "nrf_drv_ppi.h"
#include "nrf_drv_clock.h"
#include "sdk_config.h"

#define NRF_LOG_MODULE_NAME "APP"
#include "nrf_log.h"
#include "nrf_log_ctrl.h"

#define IS_SRVC_CHANGED_CHARACT_PRESENT 1                                           /**< Include or not the service_changed characteristic. if not enabled, the server's database cannot be changed for the lifetime of the device*/

#if (NRF_SD_BLE_API_VERSION <= 3)
    #define NRF_BLE_MAX_MTU_SIZE        GATT_MTU_SIZE_DEFAULT                   /**< MTU size used in the softdevice enabling and to reply to a BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST event. */
#else
    #define NRF_BLE_MAX_MTU_SIZE        BLE_GATT_MTU_SIZE_DEFAULT               /**< MTU size used in the softdevice enabling and to reply to a BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST event. */
#endif

#define APP_FEATURE_NOT_SUPPORTED       BLE_GATT_STATUS_ATTERR_APP_BEGIN + 2        /**< Reply when unsupported features are requested. */

#define CENTRAL_LINK_COUNT              0                                           /**< Number of central links used by the application. When changing this number remember to adjust the RAM settings*/
#define PERIPHERAL_LINK_COUNT           1                                           /**< Number of peripheral links used by the application. When changing this number remember to adjust the RAM settings*/

#define DEVICE_NAME                     "Bryan"                           /**< Name of device. Will be included in the advertising data. */
#define MANUFACTURER_NAME               "NordicSemiconductor"                       /**< Manufacturer. Will be passed to Device Information Service. */
#define APP_ADV_INTERVAL                1000                                         /**< The advertising interval (in units of 0.625 ms. This value corresponds to 187.5 ms). */
#define APP_ADV_TIMEOUT_IN_SECONDS      0                                         /**< The advertising timeout in units of seconds. */

#define APP_TIMER_PRESCALER             0                                           /**< Value of the RTC1 PRESCALER register. */
#define APP_TIMER_OP_QUEUE_SIZE         4                                           /**< Size of timer operation queues. */

#define MIN_CONN_INTERVAL               MSEC_TO_UNITS(7.5, UNIT_1_25_MS)            /**< Minimum acceptable connection interval (0.1 seconds). */
#define MAX_CONN_INTERVAL               MSEC_TO_UNITS(20, UNIT_1_25_MS)            /**< Maximum acceptable connection interval (0.2 second). */
#define SLAVE_LATENCY                   0                                           /**< Slave latency. */
#define CONN_SUP_TIMEOUT                MSEC_TO_UNITS(500, UNIT_10_MS)             /**< Connection supervisory timeout (4 seconds). */

#define FIRST_CONN_PARAMS_UPDATE_DELAY  APP_TIMER_TICKS(500, APP_TIMER_PRESCALER)  /**< Time from initiating event (connect or start of notification) to first time sd_ble_gap_conn_param_update is called (5 seconds). */
#define NEXT_CONN_PARAMS_UPDATE_DELAY   APP_TIMER_TICKS(10000, APP_TIMER_PRESCALER) /**< 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                  1                                           /**< 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          15                                         /**< 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. */
#include "nrf_delay.h"

#define ANALOG_VREF_PIN NRF_SAADC_INPUT_AIN7
#define ANALOG_VREF_PIN2 NRF_SAADC_INPUT_AIN5
#define ANALOG_TEMPERATURE_SENSOR_PIN NRF_SAADC_INPUT_AIN1
#define ANALOG_CH0_PIN NRF_SAADC_INPUT_AIN6
#define ANALOG_CH1_PIN NRF_SAADC_INPUT_AIN4


#define RTC_CC_VALUE 128
#define RTC_PRESCALER 0  

static uint16_t m_conn_handle = BLE_CONN_HANDLE_INVALID;                            /**< Handle of the current connection. */
const  nrf_drv_rtc_t           rtc_adc = NRF_DRV_RTC_INSTANCE(2);
static  nrf_ppi_channel_t rtc_ppi_channel, saadc_ppi_channel1, saadc_ppi_channel2, saadc_ppi_channel3;

#define NUM_OF_CHANNELS 4

static bool m_saadc_initialized = false;

static int16_t result[3]={0};

/* YOUR_JOB: Declare all services structure your application is using
   static ble_xx_service_t                     m_xxs;
   static ble_yy_service_t                     m_yys;
 */

// YOUR_JOB: Use UUIDs for service(s) used in your application.
static ble_uuid_t m_adv_uuids[] = {{BLE_UUID_DEVICE_INFORMATION_SERVICE, BLE_UUID_TYPE_BLE}}; /**< Universally unique service identifiers. */

static void advertising_start(void);

/**@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;

    switch (p_evt->evt_id)
    {
        case PM_EVT_BONDED_PEER_CONNECTED:
        {
            NRF_LOG_INFO("Connected to a previously bonded device.\r\n");
        } break;

        case PM_EVT_CONN_SEC_SUCCEEDED:
        {
            NRF_LOG_INFO("Connection secured. Role: %d. conn_handle: %d, Procedure: %d\r\n",
                         ble_conn_state_role(p_evt->conn_handle),
                         p_evt->conn_handle,
                         p_evt->params.conn_sec_succeeded.procedure);
        } break;

        case PM_EVT_CONN_SEC_FAILED:
        {
            /* Often, when securing fails, it shouldn't be restarted, for security reasons.
             * Other times, it can be restarted directly.
             * Sometimes it can be restarted, but only after changing some Security Parameters.
             * Sometimes, it cannot be restarted until the link is disconnected and reconnected.
             * Sometimes it is impossible, to secure the link, or the peer device does not support it.
             * How to handle this error is highly application dependent. */
        } break;

        case PM_EVT_CONN_SEC_CONFIG_REQ:
        {
            // Reject pairing request from an already bonded peer.
            pm_conn_sec_config_t conn_sec_config = {.allow_repairing = false};
            pm_conn_sec_config_reply(p_evt->conn_handle, &conn_sec_config);
        } break;

        case PM_EVT_STORAGE_FULL:
        {
            // Run garbage collection on the flash.
            err_code = fds_gc();
            if (err_code == FDS_ERR_BUSY || err_code == FDS_ERR_NO_SPACE_IN_QUEUES)
            {
                // Retry.
            }
            else
            {
                APP_ERROR_CHECK(err_code);
            }
        } break;

        case PM_EVT_PEERS_DELETE_SUCCEEDED:
        {
            advertising_start();
        } break;

        case PM_EVT_LOCAL_DB_CACHE_APPLY_FAILED:
        {
            // The local database has likely changed, send service changed indications.
            pm_local_database_has_changed();
        } break;

        case PM_EVT_PEER_DATA_UPDATE_FAILED:
        {
            // Assert.
            APP_ERROR_CHECK(p_evt->params.peer_data_update_failed.error);
        } break;

        case PM_EVT_PEER_DELETE_FAILED:
        {
            // Assert.
            APP_ERROR_CHECK(p_evt->params.peer_delete_failed.error);
        } break;

        case PM_EVT_PEERS_DELETE_FAILED:
        {
            // Assert.
            APP_ERROR_CHECK(p_evt->params.peers_delete_failed_evt.error);
        } break;

        case PM_EVT_ERROR_UNEXPECTED:
        {
            // Assert.
            APP_ERROR_CHECK(p_evt->params.error_unexpected.error);
        } break;

        case PM_EVT_CONN_SEC_START:
        case PM_EVT_PEER_DATA_UPDATE_SUCCEEDED:
        case PM_EVT_PEER_DELETE_SUCCEEDED:
        case PM_EVT_LOCAL_DB_CACHE_APPLIED:
        case PM_EVT_SERVICE_CHANGED_IND_SENT:
        case PM_EVT_SERVICE_CHANGED_IND_CONFIRMED:
        default:
            break;
    }
}


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

    // Initialize timer module.
    APP_TIMER_INIT(APP_TIMER_PRESCALER, APP_TIMER_OP_QUEUE_SIZE, false);

    // Create timers.

    /* YOUR_JOB: Create any timers to be used by the application.
                 Below is an example of how to create a timer.
                 For every new timer needed, increase the value of the macro APP_TIMER_MAX_TIMERS by
                 one.
       uint32_t err_code;
       err_code = app_timer_create(&m_app_timer_id, APP_TIMER_MODE_REPEATED, 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)
{
    uint32_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);

    /* YOUR_JOB: Use an appearance value matching the application's use case.
       err_code = sd_ble_gap_appearance_set(BLE_APPEARANCE_);
       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 the YYY Service events.
 * YOUR_JOB implement a service handler function depending on the event the service you are using can generate
 *
 * @details This function will be called for all YY Service events which are passed to
 *          the application.
 *
 * @param[in]   p_yy_service   YY Service structure.
 * @param[in]   p_evt          Event received from the YY Service.
 *
 *
   static void on_yys_evt(ble_yy_service_t     * p_yy_service,
                       ble_yy_service_evt_t * p_evt)
   {
    switch (p_evt->evt_type)
    {
        case BLE_YY_NAME_EVT_WRITE:
            APPL_LOG("[APPL]: charact written with value %s. \r\n", p_evt->params.char_xx.value.p_str);
            break;

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

/**@brief Function for initializing services that will be used by the application.
 */
static void services_init(void)
{
    /* YOUR_JOB: Add code to initialize the services used by the application.
       uint32_t                           err_code;
       ble_xxs_init_t                     xxs_init;
       ble_yys_init_t                     yys_init;

       // Initialize XXX Service.
       memset(&xxs_init, 0, sizeof(xxs_init));

       xxs_init.evt_handler                = NULL;
       xxs_init.is_xxx_notify_supported    = true;
       xxs_init.ble_xx_initial_value.level = 100;

       err_code = ble_bas_init(&m_xxs, &xxs_init);
       APP_ERROR_CHECK(err_code);

       // Initialize YYY Service.
       memset(&yys_init, 0, sizeof(yys_init));
       yys_init.evt_handler                  = on_yys_evt;
       yys_init.ble_yy_initial_value.counter = 0;

       err_code = ble_yy_service_init(&yys_init, &yy_init);
       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)
{
    uint32_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)
{
    uint32_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 starting timers.
 */
static void application_timers_start(void)
{
    /* YOUR_JOB: Start your timers. below is an example of how to start a timer.
       uint32_t err_code;
       err_code = app_timer_start(m_app_timer_id, TIMER_INTERVAL, NULL);
       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)
{
    uint32_t 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)
{
    uint32_t err_code;

    switch (ble_adv_evt)
    {
        case BLE_ADV_EVT_FAST:
            NRF_LOG_INFO("Fast advertising\r\n");
            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 the Application's BLE Stack events.
 *
 * @param[in] p_ble_evt  Bluetooth stack event.
 */
static void on_ble_evt(ble_evt_t * p_ble_evt)
{
    uint32_t err_code = NRF_SUCCESS;

    switch (p_ble_evt->header.evt_id)
    {
        case BLE_GAP_EVT_DISCONNECTED:
            NRF_LOG_INFO("Disconnected.\r\n");
            err_code = bsp_indication_set(BSP_INDICATE_IDLE);
            APP_ERROR_CHECK(err_code);
            break; // BLE_GAP_EVT_DISCONNECTED

        case BLE_GAP_EVT_CONNECTED:
            NRF_LOG_INFO("Connected.\r\n");
            err_code = bsp_indication_set(BSP_INDICATE_CONNECTED);
            APP_ERROR_CHECK(err_code);
            m_conn_handle = p_ble_evt->evt.gap_evt.conn_handle;
            break; // BLE_GAP_EVT_CONNECTED

        case BLE_GATTC_EVT_TIMEOUT:
            // Disconnect on GATT Client timeout event.
            NRF_LOG_DEBUG("GATT Client Timeout.\r\n");
            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; // BLE_GATTC_EVT_TIMEOUT

        case BLE_GATTS_EVT_TIMEOUT:
            // Disconnect on GATT Server timeout event.
            NRF_LOG_DEBUG("GATT Server Timeout.\r\n");
            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; // BLE_GATTS_EVT_TIMEOUT

        case BLE_EVT_USER_MEM_REQUEST:
            err_code = sd_ble_user_mem_reply(p_ble_evt->evt.gattc_evt.conn_handle, NULL);
            APP_ERROR_CHECK(err_code);
            break; // BLE_EVT_USER_MEM_REQUEST

        case BLE_GATTS_EVT_RW_AUTHORIZE_REQUEST:
        {
            ble_gatts_evt_rw_authorize_request_t  req;
            ble_gatts_rw_authorize_reply_params_t auth_reply;

            req = p_ble_evt->evt.gatts_evt.params.authorize_request;

            if (req.type != BLE_GATTS_AUTHORIZE_TYPE_INVALID)
            {
                if ((req.request.write.op == BLE_GATTS_OP_PREP_WRITE_REQ)     ||
                    (req.request.write.op == BLE_GATTS_OP_EXEC_WRITE_REQ_NOW) ||
                    (req.request.write.op == BLE_GATTS_OP_EXEC_WRITE_REQ_CANCEL))
                {
                    if (req.type == BLE_GATTS_AUTHORIZE_TYPE_WRITE)
                    {
                        auth_reply.type = BLE_GATTS_AUTHORIZE_TYPE_WRITE;
                    }
                    else
                    {
                        auth_reply.type = BLE_GATTS_AUTHORIZE_TYPE_READ;
                    }
                    auth_reply.params.write.gatt_status = APP_FEATURE_NOT_SUPPORTED;
                    err_code = sd_ble_gatts_rw_authorize_reply(p_ble_evt->evt.gatts_evt.conn_handle,
                                                               &auth_reply);
                    APP_ERROR_CHECK(err_code);
                }
            }
        } break; // BLE_GATTS_EVT_RW_AUTHORIZE_REQUEST

#if (NRF_SD_BLE_API_VERSION >= 3)
        case BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST:
            err_code = sd_ble_gatts_exchange_mtu_reply(p_ble_evt->evt.gatts_evt.conn_handle,
                                                       NRF_BLE_MAX_MTU_SIZE);
            APP_ERROR_CHECK(err_code);
            break; // BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST
#endif

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


/**@brief Function for dispatching a BLE stack event to all modules with a BLE stack event handler.
 *
 * @details This function is called from the BLE Stack event interrupt handler after a BLE stack
 *          event has been received.
 *
 * @param[in] p_ble_evt  Bluetooth stack event.
 */
static void ble_evt_dispatch(ble_evt_t * p_ble_evt)
{
    /** The Connection state module has to be fed BLE events in order to function correctly
     * Remember to call ble_conn_state_on_ble_evt before calling any ble_conns_state_* functions. */
    ble_conn_state_on_ble_evt(p_ble_evt);
    pm_on_ble_evt(p_ble_evt);
    ble_conn_params_on_ble_evt(p_ble_evt);
   // bsp_btn_ble_on_ble_evt(p_ble_evt);
    on_ble_evt(p_ble_evt);
    ble_advertising_on_ble_evt(p_ble_evt);
    /*YOUR_JOB add calls to _on_ble_evt functions from each service your application is using
       ble_xxs_on_ble_evt(&m_xxs, p_ble_evt);
       ble_yys_on_ble_evt(&m_yys, p_ble_evt);
     */
}


/**@brief Function for dispatching a system event to interested modules.
 *
 * @details This function is called from the System event interrupt handler after a system
 *          event has been received.
 *
 * @param[in] sys_evt  System stack event.
 */
static void sys_evt_dispatch(uint32_t sys_evt)
{
    // Dispatch the system event to the fstorage module, where it will be
    // dispatched to the Flash Data Storage (FDS) module.
    fs_sys_event_handler(sys_evt);

    // Dispatch to the Advertising module last, since it will check if there are any
    // pending flash operations in fstorage. Let fstorage process system events first,
    // so that it can report correctly to the Advertising module.
    ble_advertising_on_sys_evt(sys_evt);
}


/**@brief Function for initializing the BLE stack.
 *
 * @details Initializes the SoftDevice and the BLE event interrupt.
 */
static void ble_stack_init(void)
{
    uint32_t err_code;

    nrf_clock_lf_cfg_t clock_lf_cfg = NRF_CLOCK_LFCLKSRC;

    // Initialize the SoftDevice handler module.
    SOFTDEVICE_HANDLER_INIT(&clock_lf_cfg, NULL);

    ble_enable_params_t ble_enable_params;
    err_code = softdevice_enable_get_default_config(CENTRAL_LINK_COUNT,
                                                    PERIPHERAL_LINK_COUNT,
                                                    &ble_enable_params);
    APP_ERROR_CHECK(err_code);

    // Check the ram settings against the used number of links
    CHECK_RAM_START_ADDR(CENTRAL_LINK_COUNT, PERIPHERAL_LINK_COUNT);

    // Enable BLE stack.
#if (NRF_SD_BLE_API_VERSION >= 3)
    ble_enable_params.gatt_enable_params.att_mtu = NRF_BLE_MAX_MTU_SIZE;
#endif
    err_code = softdevice_enable(&ble_enable_params);
    APP_ERROR_CHECK(err_code);

    // Register with the SoftDevice handler module for BLE events.
    err_code = softdevice_ble_evt_handler_set(ble_evt_dispatch);
    APP_ERROR_CHECK(err_code);

    // Register with the SoftDevice handler module for BLE events.
    err_code = softdevice_sys_evt_handler_set(sys_evt_dispatch);
    APP_ERROR_CHECK(err_code);
}


/**@brief Function for the Peer Manager initialization.
 *
 * @param[in] erase_bonds  Indicates whether bonding information should be cleared from
 *                         persistent storage during initialization of the Peer Manager.
 */
static void peer_manager_init(bool erase_bonds)
{
    ble_gap_sec_params_t sec_param;
    ret_code_t           err_code;

    err_code = pm_init();
    APP_ERROR_CHECK(err_code);

    if (erase_bonds)
    {
        err_code = pm_peers_delete();
        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  = 1;
    sec_param.kdist_own.id   = 1;
    sec_param.kdist_peer.enc = 1;
    sec_param.kdist_peer.id  = 1;

    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 Function for handling events from the BSP module.
 *
 * @param[in]   event   Event generated when button is pressed.
 */
static void bsp_event_handler(bsp_event_t event)
{
    uint32_t err_code;

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

        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; // BSP_EVENT_DISCONNECT

        case BSP_EVENT_WHITELIST_OFF:
            if (m_conn_handle == BLE_CONN_HANDLE_INVALID)
            {
                err_code = ble_advertising_restart_without_whitelist();
                if (err_code != NRF_ERROR_INVALID_STATE)
                {
                    APP_ERROR_CHECK(err_code);
                }
            }
            break; // BSP_EVENT_KEY_0

        default:
            break;
    }
}


/**@brief Function for initializing the Advertising functionality.
 */
static void advertising_init(void)
{
    uint32_t               err_code;
    ble_advdata_t          advdata;
    ble_adv_modes_config_t options;

    // Build advertising data struct to pass into @ref ble_advertising_init.
    memset(&advdata, 0, sizeof(advdata));

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

    memset(&options, 0, sizeof(options));
    options.ble_adv_fast_enabled  = true;
    options.ble_adv_fast_interval = APP_ADV_INTERVAL;
    options.ble_adv_fast_timeout  = APP_ADV_TIMEOUT_IN_SECONDS;

    err_code = ble_advertising_init(&advdata, NULL, &options, on_adv_evt, NULL);
    APP_ERROR_CHECK(err_code);
}


/**@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)
{
    bsp_event_t startup_event;

    uint32_t err_code = bsp_init(BSP_INIT_LED ,
                                 APP_TIMER_TICKS(100, APP_TIMER_PRESCALER),
                                 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 the Power manager.
 */
static void power_manage(void)
{
    uint32_t err_code = sd_app_evt_wait();

    APP_ERROR_CHECK(err_code);
}


/**@brief Function for starting advertising.
 */
static void advertising_start(void)
{
    uint32_t err_code = ble_advertising_start(BLE_ADV_MODE_FAST);

    APP_ERROR_CHECK(err_code);
}


static void saadc_uninit(void)
{
	if(m_saadc_initialized)
	{	
		for(int i=0; i<NUM_OF_CHANNELS/2+1; i++)
		{
			NRF_SAADC->CH[i].PSELP = SAADC_CH_PSELP_PSELP_NC << SAADC_CH_PSELP_PSELP_Pos;
			NRF_SAADC->CH[i].PSELN = SAADC_CH_PSELN_PSELN_NC << SAADC_CH_PSELN_PSELN_Pos;
		}		
		 NRF_SAADC->ENABLE = SAADC_ENABLE_ENABLE_Disabled << SAADC_ENABLE_ENABLE_Pos;
		 nrf_saadc_int_disable(NRF_SAADC_INT_ALL);
		 nrf_drv_common_irq_disable(SAADC_IRQn);
		 NVIC_ClearPendingIRQ(SAADC_IRQn);	//Clear the SAADC interrupt if set
		 m_saadc_initialized = false;
	}
}

static void saadc_init(void)
{
	if(!m_saadc_initialized)
	{
		 // Configure SAADC singled-ended channel, Internal reference (0.6V) and 1/6 gain.
		NRF_SAADC->CH[0].CONFIG = (SAADC_CH_CONFIG_GAIN_Gain4    << SAADC_CH_CONFIG_GAIN_Pos) |
															(SAADC_CH_CONFIG_MODE_Diff       << SAADC_CH_CONFIG_MODE_Pos) |
															(SAADC_CH_CONFIG_REFSEL_Internal << SAADC_CH_CONFIG_REFSEL_Pos) |
															(SAADC_CH_CONFIG_RESN_Bypass    << SAADC_CH_CONFIG_RESN_Pos) |
															(SAADC_CH_CONFIG_RESP_Bypass     << SAADC_CH_CONFIG_RESP_Pos) |
															(SAADC_CH_CONFIG_TACQ_10us        << SAADC_CH_CONFIG_TACQ_Pos)	|
															(SAADC_OVERSAMPLE_OVERSAMPLE_Over4x << SAADC_OVERSAMPLE_OVERSAMPLE_Pos)|
															(SAADC_CH_CONFIG_BURST_Enabled   << SAADC_CH_CONFIG_BURST_Pos);
		// Configure the SAADC channel with VDD as positive input, no negative input(single ended).
		NRF_SAADC->CH[0].PSELP = ANALOG_CH0_PIN << SAADC_CH_PSELP_PSELP_Pos;
		NRF_SAADC->CH[0].PSELN = ANALOG_VREF_PIN << SAADC_CH_PSELN_PSELN_Pos;
		
		NRF_SAADC->CH[1].CONFIG = (SAADC_CH_CONFIG_GAIN_Gain4    << SAADC_CH_CONFIG_GAIN_Pos) |
															(SAADC_CH_CONFIG_MODE_Diff       << SAADC_CH_CONFIG_MODE_Pos) |
															(SAADC_CH_CONFIG_REFSEL_Internal << SAADC_CH_CONFIG_REFSEL_Pos) |
															(SAADC_CH_CONFIG_RESN_Bypass     << SAADC_CH_CONFIG_RESN_Pos) |
															(SAADC_CH_CONFIG_RESP_Bypass     << SAADC_CH_CONFIG_RESP_Pos) |
															(SAADC_CH_CONFIG_TACQ_10us        << SAADC_CH_CONFIG_TACQ_Pos)|
															(SAADC_OVERSAMPLE_OVERSAMPLE_Over4x << SAADC_OVERSAMPLE_OVERSAMPLE_Pos)|
															(SAADC_CH_CONFIG_BURST_Enabled   << SAADC_CH_CONFIG_BURST_Pos);

		// Configure the SAADC channel with VDD as positive input, no negative input(single ended).
		NRF_SAADC->CH[1].PSELP = ANALOG_CH1_PIN << SAADC_CH_PSELP_PSELP_Pos;
		NRF_SAADC->CH[1].PSELN = ANALOG_VREF_PIN2 << SAADC_CH_PSELN_PSELN_Pos;
			
		NRF_SAADC->CH[2].CONFIG = (SAADC_CH_CONFIG_GAIN_Gain1_6    << SAADC_CH_CONFIG_GAIN_Pos) |
															(SAADC_CH_CONFIG_MODE_SE        << SAADC_CH_CONFIG_MODE_Pos) |
															(SAADC_CH_CONFIG_REFSEL_Internal << SAADC_CH_CONFIG_REFSEL_Pos) |
															(SAADC_CH_CONFIG_RESN_Bypass     << SAADC_CH_CONFIG_RESN_Pos) |
															(SAADC_CH_CONFIG_RESP_Bypass     << SAADC_CH_CONFIG_RESP_Pos) |
															(SAADC_CH_CONFIG_TACQ_10us        << SAADC_CH_CONFIG_TACQ_Pos)|
															(SAADC_OVERSAMPLE_OVERSAMPLE_Over4x << SAADC_OVERSAMPLE_OVERSAMPLE_Pos)|
															(SAADC_CH_CONFIG_BURST_Enabled   << SAADC_CH_CONFIG_BURST_Pos);

	//  // Configure the SAADC channel with VDD as positive input, no negative input(single ended).
		NRF_SAADC->CH[2].PSELP = SAADC_CH_PSELP_PSELP_NC << SAADC_CH_PSELP_PSELP_Pos;
		NRF_SAADC->CH[2].PSELN = SAADC_CH_PSELN_PSELN_NC << SAADC_CH_PSELN_PSELN_Pos;

		// Configure the SAADC resolution.
		NRF_SAADC->RESOLUTION = SAADC_RESOLUTION_VAL_12bit << SAADC_RESOLUTION_VAL_Pos;

		// Configure result to be put in RAM at the location of "result" variable.
		NRF_SAADC->RESULT.MAXCNT = 2;
		NRF_SAADC->RESULT.PTR = (uint32_t)&result[0];
		
		// No automatic sampling, will trigger with TASKS_SAMPLE.
		NRF_SAADC->SAMPLERATE = SAADC_SAMPLERATE_MODE_Task << SAADC_SAMPLERATE_MODE_Pos;
		
		// Enable Interruot
		nrf_saadc_int_disable(NRF_SAADC_INT_ALL);
    nrf_saadc_event_clear(NRF_SAADC_EVENT_END);
		nrf_saadc_int_enable(NRF_SAADC_INT_STARTED);
		nrf_saadc_int_enable(NRF_SAADC_INT_END);
		nrf_saadc_int_enable(NRF_SAADC_INT_STOPPED);
    nrf_drv_common_irq_enable(SAADC_IRQn, SAADC_CONFIG_IRQ_PRIORITY);
		// Enable SAADC (would capture analog pins if they were used in CH[0].PSELP)
		NRF_SAADC->ENABLE = SAADC_ENABLE_ENABLE_Enabled << SAADC_ENABLE_ENABLE_Pos;	
		
		m_saadc_initialized = true;
	}
}

void SAADC_IRQHandler(void)
{
    uint32_t err_code;
		
		if(nrf_saadc_event_check(NRF_SAADC_EVENT_STARTED))
		{
			nrf_saadc_event_clear(NRF_SAADC_EVENT_STARTED);			
		}
	
//		if(nrf_saadc_event_check(NRF_SAADC_EVENT_RESULTDONE))
//		{
//			nrf_saadc_event_clear(NRF_SAADC_EVENT_RESULTDONE);	
//			nrf_gpio_pin_toggle(DOWNLOADER_DETECT);
//		}
		
	  if(nrf_saadc_event_check(NRF_SAADC_EVENT_END))
		{
			nrf_saadc_event_clear(NRF_SAADC_EVENT_END);			
		  // Implement later ( move data to array )
		}
		
		if(nrf_saadc_event_check(NRF_SAADC_EVENT_STOPPED))
		{
			nrf_saadc_event_clear(NRF_SAADC_EVENT_STOPPED);	
			saadc_uninit();
			*((volatile uint32_t *)0x40007FFC) = 0;
			*((volatile uint32_t *)0x40007FFC);
			*((volatile uint32_t *)0x40007FFC) = 1;
			saadc_init();
		}
}



static void ppi_enable()
{
	 uint32_t err_code;
	 err_code = nrf_drv_ppi_channel_enable(rtc_ppi_channel);
   APP_ERROR_CHECK(err_code);
	
	 err_code = nrf_drv_ppi_channel_enable(saadc_ppi_channel1);
   APP_ERROR_CHECK(err_code);
	
 	 err_code = nrf_drv_ppi_channel_enable(saadc_ppi_channel2);
   APP_ERROR_CHECK(err_code);	
}

static void ppi_disable()
{
	 uint32_t err_code;
	 err_code = nrf_drv_ppi_channel_disable(rtc_ppi_channel);
   APP_ERROR_CHECK(err_code);
	
	 err_code = nrf_drv_ppi_channel_disable(saadc_ppi_channel1);
   APP_ERROR_CHECK(err_code);
	
 	 err_code = nrf_drv_ppi_channel_disable(saadc_ppi_channel2);
   APP_ERROR_CHECK(err_code);	
}
	

static void rtc_handler(nrf_drv_rtc_int_type_t int_type)
{
    uint32_t err_code;
    if (int_type == NRF_DRV_RTC_INT_COMPARE0)
    {		
        err_code = nrf_drv_rtc_cc_set(&rtc_adc,0,RTC_CC_VALUE,true);       //Set RTC compare value. This needs to be done every time as the nrf_drv_rtc clears the compare register on every compare match
 //       nrf_drv_rtc_counter_clear(&rtc_adc);
			  APP_ERROR_CHECK(err_code);
    }
}

static void rtc_config(void)
{
  
    ret_code_t err_code;
    err_code = nrf_drv_ppi_init();
    APP_ERROR_CHECK(err_code);
    
    nrf_drv_rtc_config_t config = NRF_DRV_RTC_DEFAULT_CONFIG;
    err_code = nrf_drv_rtc_init(&rtc_adc, &config, rtc_handler);
    APP_ERROR_CHECK(err_code);

    //Set compare channel 0 to trigger at 4096 Hz
    err_code = nrf_drv_rtc_cc_set(&rtc_adc,0,RTC_CC_VALUE,true);
    APP_ERROR_CHECK(err_code);

    //Power on RTC instance
    nrf_drv_rtc_enable(&rtc_adc);
    
	  //Get Events
    uint32_t rtc_compare_event_addr   = nrf_drv_rtc_event_address_get(&rtc_adc, NRF_RTC_EVENT_COMPARE_0);
		uint32_t saddc_started_event_addr = nrf_saadc_event_address_get(NRF_SAADC_EVENT_STARTED);
		uint32_t saadc_done_event_addr    = nrf_saadc_event_address_get(NRF_SAADC_EVENT_END);

	  //Get Tasks
    uint32_t rtc_clear_task_addr		= nrf_drv_rtc_task_address_get(&rtc_adc, NRF_RTC_TASK_CLEAR);
    uint32_t saadc_start_task_addr  = nrf_saadc_task_address_get(NRF_SAADC_TASK_START);
		uint32_t saadc_sample_task_addr = nrf_saadc_task_address_get(NRF_SAADC_TASK_SAMPLE);
	  uint32_t saadc_stop_task_addr   = nrf_saadc_task_address_get(NRF_SAADC_TASK_STOP);

    /* setup ppi channel so that timer compare event is triggering sample task in SAADC */
    err_code = nrf_drv_ppi_channel_alloc(&rtc_ppi_channel);
    APP_ERROR_CHECK(err_code);
		
		err_code = nrf_drv_ppi_channel_alloc(&saadc_ppi_channel1);
    APP_ERROR_CHECK(err_code);
		
		err_code = nrf_drv_ppi_channel_alloc(&saadc_ppi_channel2);
    APP_ERROR_CHECK(err_code);
										
		
    err_code = nrf_drv_ppi_channel_assign(rtc_ppi_channel, rtc_compare_event_addr,rtc_clear_task_addr);
    APP_ERROR_CHECK(err_code);
    err_code = nrf_drv_ppi_channel_fork_assign(rtc_ppi_channel, saadc_start_task_addr);
    APP_ERROR_CHECK(err_code);   
				
		err_code = nrf_drv_ppi_channel_assign(saadc_ppi_channel1, saddc_started_event_addr, saadc_sample_task_addr);
    APP_ERROR_CHECK(err_code);
		
		err_code = nrf_drv_ppi_channel_assign(saadc_ppi_channel2, saadc_done_event_addr, saadc_stop_task_addr);
    APP_ERROR_CHECK(err_code);
		
		ppi_enable();
		//Enable RTC
}

static void lfclk_config(void)
{
    ret_code_t err_code = nrf_drv_clock_init();                        //Initialize the clock source specified in the nrf_drv_config.h file, i.e. the CLOCK_CONFIG_LF_SRC constant
    APP_ERROR_CHECK(err_code);
    nrf_drv_clock_lfclk_request(NULL);
}



/**@brief Function for application main entry.
 */
int main(void)
{
		NRF_POWER->DCDCEN = 1;
		
    uint32_t err_code;
    bool     erase_bonds;

    // Initialize.
    err_code = NRF_LOG_INIT(NULL);
    APP_ERROR_CHECK(err_code);
		NRF_LOG_INFO("Bonds erased!\r\n");
		NRF_LOG_FLUSH();
		bsp_board_leds_init();
	
	  APP_TIMER_INIT(APP_TIMER_PRESCALER, APP_TIMER_OP_QUEUE_SIZE, NULL);
	  
    ble_stack_init();
 
    gap_params_init();
    advertising_init();
    services_init();
    conn_params_init();

    // Start execution.
    NRF_LOG_INFO("Template started\r\n");
		NRF_LOG_FLUSH();
    err_code = ble_advertising_start(BLE_ADV_MODE_FAST);
    APP_ERROR_CHECK(err_code);
		
		NRF_LOG_INFO("Adverstising started\r\n");
		NRF_LOG_FLUSH();
		
		
		saadc_init();
//		lfclk_config();
		rtc_config();

		nrf_gpio_cfg_output(DOWNLOADER_DETECT);
		
		nrf_gpio_cfg_output(LED_CTRL_PIN);
		nrf_gpio_pin_clear(LED_CTRL_PIN);
		
		nrf_gpio_cfg_output(SPIM0_SS_NAND_PIN);
		nrf_gpio_pin_set(SPIM0_SS_NAND_PIN);
		
		nrf_gpio_cfg_output(SPIM0_SS_ACCEL_PIN);
		nrf_gpio_pin_set(SPIM0_SS_ACCEL_PIN);
    // Enter main loop.
		
		for (;;)
    {
      power_manage();
    }
}


/**
 * @}
 */

Thanks Jorgen.

The main thing I wanted to check was whether the compare flag would get retriggered once the counter of the RTC has been cleared via PPI, but looking at the hardware implementation of the RTC which is slightly different to other peripherals, it looks like event flag is latched by the register so has to be manually cleared by the CPU.  By disabling the interrupt wouldn't the RTC stop running after the first compare event?

My current implementation ( see attached file ) is this, I have 3 PPIs, one to trigger the ADC start task and forked to clear RTC counter.

Another PPI to trigger the start sample task on ADC started event.

And the final PPI to trigger the stop task on events end event.

The RTC interrupt calls cc_set which clears the compare flag.

The ADC interrupt clears each individual event and in the case of ADC stopped it uninits and restarts the ADC module before initializing it. 

For sanity check, with the above implementation would you agree that the ADC sampling would be accurate as long as the compare flag is cleared and ADC result is moved to an array before the next sampling period is up?

[Jørgen Holmefjord]

Hi,

bryanhsieh said:

The main thing I wanted to check was whether the compare flag would get retriggered once the counter of the RTC has been cleared via PPI, but looking at the hardware implementation of the RTC which is slightly different to other peripherals, it looks like event flag is latched by the register so has to be manually cleared by the CPU.

The event registers are usually latched when the event signal is generated by the HW peripheral, and needs to be cleared by the CPU. However, the peripheral can normally regenerate the signal event if the register is already set, see Events:

"Events can be generated by the peripheral even when the event register is set to '1'."

bryanhsieh said:

By disabling the interrupt wouldn't the RTC stop running after the first compare event?

The event register needs to be cleared by the CPU in order to trigger a new interrupt, see Interrupts. If interrupts are disabled, and the events are connected to tasks through PPI, there is no need to clear the event registers for a new event to be generated.

bryanhsieh said:

My current implementation ( see attached file ) is this, I have 3 PPIs, one to trigger the ADC start task and forked to clear RTC counter.

Another PPI to trigger the start sample task on ADC started event.

And the final PPI to trigger the stop task on events end event.

The RTC interrupt calls cc_set which clears the compare flag.

The ADC interrupt clears each individual event and in the case of ADC stopped it uninits and restarts the ADC module before initializing it. 

For sanity check, with the above implementation would you agree that the ADC sampling would be accurate as long as the compare flag is cleared and ADC result is moved to an array before the next sampling period is up?

Most likely it will work and give accurate sampling time, but there may occur a race condition/issue in case your interrupt handling is delayed long enough. The RTC events/interrupts may be disabled right before the RTC hits the CC value, and enabled again after the CC value is passed. Since you do not clear the RTC in the event handler, the RTC will run until it overflows and hit the CC value value a second time before the event is generated.

Best regards,
Jørgen

[bryanhsieh]

Thanks for the detailed answer. I was able to implement the above with the RTC interrupt disabled as clarified.