FreeRTOS timers may fire earlier than expected when configUSE_TICKLESS_IDLE == 1

[KarRej]

Hi all,
I am using nrf52 with Softdevice 113 v7.0.1 and SDKv16 and FreeRTOS v10.0.1 and I am experiencing some issues with FreeRTOS timers expiring earlier than expected when configUSE_TICKLESS_IDLE == 1.

The problems started occurring when I applied fix in vTaskStepTick() proposed in the FreeRTOS forum: https://forums.freertos.org/t/assert-in-tasks-c-2611-on-xtickcount-wrap-around/9984
There were two solutions proposed, one in the Nordic port and the seconds in generic FreeRTOS code, but both of them cause the same problem.

I also created the corresponding post in the FreeRTOS forum: forums.freertos.org/.../13416

So is there any solution that fixes the original problem with assert on xTickCount wrap around that will not cause timers to expire too early?

[Susheel Nuguru]

Hi,

Sorry for the late reply, I was travelling and did not get a chance. It is interesting problem you have and I have seen the fix proposed and I like the idea but i think the fix needs a minor modification.

Current Code:

            if (diff > 0)
            {
                vTaskStepTick(diff);
            }

freertos forum Proposed Code:

            if (diff > 0)
            {
                vTaskStepTick(diff - 1);
                NVIC_SetPendingIRQ(portNRF_RTC_IRQn);
            }


New proposed code
            if (diff > 0)
            {
                BaseType_t switch_req = pdFALSE;

                vTaskStepTick(diff - 1);
                switch_req |= xTaskIncrementTick();

                /* Increment the RTOS tick as usual which checks if there is a need for rescheduling */
                if ( switch_req != pdFALSE )
                {
                    /* A context switch is required.  Context switching is performed in
                    the PendSV interrupt.  Pend the PendSV interrupt. */
                    SCB->ICSR = SCB_ICSR_PENDSVSET_Msk;
                    __SEV();
                }
            }

[KarRej]

Hi, thanks for the response and the proposed fix. 
Actually, there are two proposed fixes in the freeRTOS forum. 
The first one is in function vPortSuppressTicksAndSleep() and it looks exactly like you posted, but there is also the second fix placed in tasks.c in freeRTOS code. 
I once again tried to reproduce this bug on all fixes including yours, and here are the results:
(The tests I performed was to trigger pin state change every 20ms for 10 minutes and observe the minimum and maximum pulse time)
1. Original fix in vPortSuppressTicksAndSleep(): min = 17.5ms, max = 22.5ms
2. Original fix in tasks.c: min = 18.5ms, max = 21.5ms
3. FIx proposed by You: min = 18.5ms, max = 21.5ms
4. configUSE_TICKLESS_IDLE = 0, min = 19.4ms, max=20.65ms

So it looks like your proposition is better than the original fix in vPortSuppressTicksAndSleep() but I am not sure is it good enough.
Is it possible to achieve the timings exactly as for USE_TICKLESS_IDLE = 0? 

[KarRej]

Susheel Nuguru said:

xExpectedIdleTime in vPortSuppressTicksAndSleep is somehow handled wrongly, since we disable the RTC interrupts for the time based on this value, it might be possible that the device is sleeping more than it should even if the timers are supposed to be expired earlier.

and

Susheel Nuguru said:

The wakeup from the sleep in 

vPortSuppressTicksAndSleep is taking aweful lot of time. This seems less likely but still possible. Not sure what we could optimize if this is what it is happening.

as far as I understand it, both should cause the timer to expire later than expected. 
And for me, it is not such a big problem. 
I am more concerned about the timer expiring earlier than expected. 
But anyway, I will redo the tests with configUSE_TICKLESS_IDLE_SIMPLE_DEBUG = 0 and let you know about the results

[KarRej]

I just checked this fix combined with configUSE_TICKLESS_IDLE_SIMPLE_DEBUG = 0 and the results are the same: min = 18.5ms, max = 21.5ms

[Susheel Nuguru]

I will have to reproduce this. I will make a small project to test the accuracy of the timers and come back to you today

[Susheel Nuguru]

Hi, I have tried to reproduce the 17.5ms min and 22.5 ms max variation of toggles at my desk and I was not successful in that. Which makes me think that there could be probably something more in your application that might be masking the kernel for longer times during some operations. One thing that i can think of such long operations can be flash erase operations. Are you using FDS library actively in your application? if so that can explain some shadowing while using using sd_app_evt_wait. 

Do you see the same results when you use below version of the sleep instead of sd_app_evt_wait?

                do{
                    __WFE();
                } while (0 == (NVIC->ISPR[0] | NVIC->ISPR[1]));

[KarRej]

I used the code you suggested instead of 

uint32_t err_code = sd_app_evt_wait();
//                NRF_STATUS_CHECK(err_code);

but it didn't helped. 
I still observe the timings from range <18.5ms;21.5ms>

[Susheel Nuguru]

Hi KarRej,
I need to be able understand the underlying activity that is making this happen. If the critical section (disabling interrupts) was the problem, then i would have expected that the jitter is in only one direction, but you are seeing difference in both directions of the set delay. I have no theories anymore and need to reproduce this on my end to be able to crack it. 

I cannot reproduce it editing freertos examples on the DK, So it might be that i need your setup to be able to reproduce this. Can you help me get your setup? 
other way is that you try to recreate this on the Nordic DK and share me the project and i will debug to see the underlying activity that makes this happen.

[KarRej]

Hi, 
Sorry for the late reply but I was figuring out how can I share the setup with you, and unfortunately I can't do it. 

So in the meantime, I will try to reproduce it by myself on the freeRTOS examples and when I do it, I will share the code with you. 

I will get back to you when I reproduce this issue on the freeRTOS example.

[Susheel Nuguru]

Thanks for the update KarRej

[KatKam]

Hi
I think I have reproduced the issue with slightly modified nrf example.
As a base I've used ble_app_hrs_freertos example from sdk 16.
Additional RTOS timer was created with period 20ms. LED 2 pin is set on timer's callback entrance and cleared on exit.
Modified min.c file of the example:

/**
 * Copyright (c) 2014 - 2019, Nordic Semiconductor ASA
 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 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
 *    Semiconductor ASA integrated circuit in a product or a software update for
 *    such product, must reproduce the above copyright notice, this list of
 *    conditions and the following disclaimer in the documentation and/or other
 *    materials provided with the distribution.
 *
 * 3. Neither the name of Nordic Semiconductor ASA nor the names of its
 *    contributors may be used to endorse or promote products derived from this
 *    software without specific prior written permission.
 *
 * 4. This software, with or without modification, must only be used with a
 *    Nordic Semiconductor ASA integrated circuit.
 *
 * 5. Any software provided in binary form under this license must not be reverse
 *    engineered, decompiled, modified and/or disassembled.
 *
 * THIS SOFTWARE IS PROVIDED BY NORDIC SEMICONDUCTOR ASA "AS IS" AND ANY EXPRESS
 * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY, NONINFRINGEMENT, AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL NORDIC SEMICONDUCTOR ASA OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */
// Board/nrf6310/ble/ble_app_hrs_rtx/main.c
/**
 *
 * @brief Heart Rate Service Sample Application with RTX main file.
 *
 * This file contains the source code for a sample application using RTX and the
 * Heart Rate service (and also 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 "bsp.h"
#include "ble.h"
#include "ble_hci.h"
#include "ble_srv_common.h"
#include "ble_advdata.h"
#include "ble_advertising.h"
#include "ble_bas.h"
#include "ble_hrs.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 "nrf_sdh_freertos.h"
#include "app_timer.h"
#include "peer_manager.h"
#include "peer_manager_handler.h"
#include "bsp_btn_ble.h"
#include "FreeRTOS.h"
#include "task.h"
#include "timers.h"
#include "semphr.h"
#include "fds.h"
#include "ble_conn_state.h"
#include "nrf_drv_clock.h"
#include "nrf_ble_gatt.h"
#include "nrf_ble_qwr.h"

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


#define DEVICE_NAME                         "Nordic_HRM"                            /**< Name of device. Will be included in the advertising data. */
#define MANUFACTURER_NAME                   "NordicSemiconductor"                   /**< Manufacturer. 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                    300                                     /**< The advertising interval (in units of 0.625 ms. This value corresponds to 187.5 ms). */
#define APP_ADV_DURATION                    60000                                       /**< The advertising duration (180 seconds) in units of 10 milliseconds. */

#define BATTERY_LEVEL_MEAS_INTERVAL         200                                    /**< Battery level measurement interval (ms). */
#define MIN_BATTERY_LEVEL                   81                                      /**< Minimum simulated battery level. */
#define MAX_BATTERY_LEVEL                   100                                     /**< Maximum simulated battery level. */
#define BATTERY_LEVEL_INCREMENT             1                                       /**< Increment between each simulated battery level measurement. */

#define HEART_RATE_MEAS_INTERVAL            100                                    /**< Heart rate measurement interval (ms). */
#define MIN_HEART_RATE                      140                                     /**< Minimum heart rate as returned by the simulated measurement function. */
#define MAX_HEART_RATE                      300                                     /**< Maximum heart rate as returned by the simulated measurement function. */
#define HEART_RATE_INCREMENT                10                                      /**< Value by which the heart rate is incremented/decremented for each call to the simulated measurement function. */

#define RR_INTERVAL_INTERVAL                300                                     /**< RR interval interval (ms). */
#define MIN_RR_INTERVAL                     100                                     /**< Minimum RR interval as returned by the simulated measurement function. */
#define MAX_RR_INTERVAL                     500                                     /**< Maximum RR interval as returned by the simulated measurement function. */
#define RR_INTERVAL_INCREMENT               1                                       /**< Value by which the RR interval is incremented/decremented for each call to the simulated measurement function. */

#define SENSOR_CONTACT_DETECTED_INTERVAL    5000                                    /**< Sensor Contact Detected toggle interval (ms). */

#define MIN_CONN_INTERVAL                   MSEC_TO_UNITS(400, UNIT_1_25_MS)        /**< Minimum acceptable connection interval (0.4 seconds). */
#define MAX_CONN_INTERVAL                   MSEC_TO_UNITS(650, UNIT_1_25_MS)        /**< Maximum acceptable connection interval (0.65 second). */
#define SLAVE_LATENCY                       0                                       /**< Slave latency. */
#define CONN_SUP_TIMEOUT                    MSEC_TO_UNITS(4000, UNIT_10_MS)         /**< Connection supervisory time-out (4 seconds). */

#define FIRST_CONN_PARAMS_UPDATE_DELAY      5000                                    /**< 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       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                      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              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 OSTIMER_WAIT_FOR_QUEUE              2                                       /**< Number of ticks to wait for the timer queue to be ready */

TaskHandle_t  led_toggle_task_handle;   /**< Reference to LED0 toggling FreeRTOS task. */
TimerHandle_t led_toggle_timer_handle;  /**< Reference to LED1 toggling FreeRTOS timer. */
#define TASK_DELAY        10           /**< Task delay. Delays a LED0 task for 200 ms */
#define TIMER_PERIOD      20          /**< Timer period. LED1 timer will expire after 1000 ms */

BLE_BAS_DEF(m_bas);                                                 /**< Battery service instance. */
BLE_HRS_DEF(m_hrs);                                                 /**< Heart rate service instance. */
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 uint16_t m_conn_handle         = BLE_CONN_HANDLE_INVALID;    /**< Handle of the current connection. */
static bool     m_rr_interval_enabled = true;                       /**< Flag for enabling and disabling the registration of new RR interval measurements (the purpose of disabling this is just to test sending HRM without RR interval data. */

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_heart_rate_sim_cfg;                      /**< Heart Rate sensor simulator configuration. */
static sensorsim_state_t m_heart_rate_sim_state;                    /**< Heart Rate sensor simulator state. */
static sensorsim_cfg_t   m_rr_interval_sim_cfg;                     /**< RR Interval sensor simulator configuration. */
static sensorsim_state_t m_rr_interval_sim_state;                   /**< RR Interval sensor simulator state. */

static ble_uuid_t m_adv_uuids[] =                                   /**< Universally unique service identifiers. */
{
    {BLE_UUID_HEART_RATE_SERVICE, BLE_UUID_TYPE_BLE},
    {BLE_UUID_BATTERY_SERVICE, BLE_UUID_TYPE_BLE},
    {BLE_UUID_DEVICE_INFORMATION_SERVICE, BLE_UUID_TYPE_BLE}
};

static TimerHandle_t m_battery_timer;                               /**< Definition of battery timer. */
static TimerHandle_t m_heart_rate_timer;                            /**< Definition of heart rate timer. */
static TimerHandle_t m_rr_interval_timer;                           /**< Definition of RR interval timer. */
static TimerHandle_t m_sensor_contact_timer;                        /**< Definition of sensor contact detected timer. */

// #if NRF_LOG_ENABLED
// static TaskHandle_t m_logger_thread;                                /**< Definition of Logger thread. */
// #endif
static TaskHandle_t m_blinker_thread;                                /**< Definition of Logger thread. */

static void led_toggle_timer_callback (void * pvParameter)
{
    bsp_board_led_off(BSP_BOARD_LED_1);
    UNUSED_PARAMETER(pvParameter);
    UNUSED_VARIABLE(xTimerStart(led_toggle_timer_handle, 0));
    bsp_board_led_on(BSP_BOARD_LED_1);

    // bsp_board_led_invert(BSP_BOARD_LED_1);
}

static void advertising_start(void * p_erase_bonds);


/**@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)
{
    bool delete_bonds = false;

    pm_handler_on_pm_evt(p_evt);
    pm_handler_flash_clean(p_evt);

    switch (p_evt->evt_id)
    {
        case PM_EVT_PEERS_DELETE_SUCCEEDED:
            advertising_start(&delete_bonds);
            break;

        default:
            break;
    }
}


/**@brief Function for performing battery measurement and updating the Battery Level characteristic
 *        in Battery Service.
 */
static void battery_level_update(void)
{
    ret_code_t err_code;
    uint8_t  battery_level;

    battery_level = (uint8_t)sensorsim_measure(&m_battery_sim_state, &m_battery_sim_cfg);

    err_code = ble_bas_battery_level_update(&m_bas, battery_level, BLE_CONN_HANDLE_ALL);
    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);
    }
}


/**@brief Function for handling the Battery measurement timer time-out.
 *
 * @details This function will be called each time the battery level measurement timer expires.
 *
 * @param[in] xTimer Handler to the timer that called this function.
 *                   You may get identifier given to the function xTimerCreate using pvTimerGetTimerID.
 */
static void battery_level_meas_timeout_handler(TimerHandle_t xTimer)
{
    UNUSED_PARAMETER(xTimer);
    battery_level_update();
}


/**@brief Function for handling the Heart rate measurement timer time-out.
 *
 * @details This function will be called each time the heart rate measurement timer expires.
 *          It will exclude RR Interval data from every third measurement.
 *
 * @param[in] xTimer Handler to the timer that called this function.
 *                   You may get identifier given to the function xTimerCreate using pvTimerGetTimerID.
 */
static void heart_rate_meas_timeout_handler(TimerHandle_t xTimer)
{
    static uint32_t cnt = 0;
    ret_code_t      err_code;
    uint16_t        heart_rate;

    UNUSED_PARAMETER(xTimer);

    heart_rate = (uint16_t)sensorsim_measure(&m_heart_rate_sim_state, &m_heart_rate_sim_cfg);

    cnt++;
    err_code = ble_hrs_heart_rate_measurement_send(&m_hrs, heart_rate);
    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);
    }

    // Disable RR Interval recording every third heart rate measurement.
    // NOTE: An application will normally not do this. It is done here just for testing generation
    // of messages without RR Interval measurements.
    m_rr_interval_enabled = ((cnt % 3) != 0);
}


/**@brief Function for handling the RR interval timer time-out.
 *
 * @details This function will be called each time the RR interval timer expires.
 *
 * @param[in] xTimer Handler to the timer that called this function.
 *                   You may get identifier given to the function xTimerCreate using pvTimerGetTimerID.
 */
static void rr_interval_timeout_handler(TimerHandle_t xTimer)
{
    UNUSED_PARAMETER(xTimer);

    if (m_rr_interval_enabled)
    {
        uint16_t rr_interval;

        rr_interval = (uint16_t)sensorsim_measure(&m_rr_interval_sim_state,
                                                  &m_rr_interval_sim_cfg);
        ble_hrs_rr_interval_add(&m_hrs, rr_interval);
    }
}


/**@brief Function for handling the Sensor Contact Detected timer time-out.
 *
 * @details This function will be called each time the Sensor Contact Detected timer expires.
 *
 * @param[in] xTimer Handler to the timer that called this function.
 *                   You may get identifier given to the function xTimerCreate using pvTimerGetTimerID.
 */
static void sensor_contact_detected_timeout_handler(TimerHandle_t xTimer)
{
    static bool sensor_contact_detected = false;

    UNUSED_PARAMETER(xTimer);

    sensor_contact_detected = !sensor_contact_detected;
    ble_hrs_sensor_contact_detected_update(&m_hrs, sensor_contact_detected);
}


/**@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.
    ret_code_t err_code = app_timer_init();
    APP_ERROR_CHECK(err_code);

    // Create timers.
    m_battery_timer = xTimerCreate("BATT",
                                   BATTERY_LEVEL_MEAS_INTERVAL,
                                   pdTRUE,
                                   NULL,
                                   battery_level_meas_timeout_handler);
    m_heart_rate_timer = xTimerCreate("HRT",
                                      HEART_RATE_MEAS_INTERVAL,
                                      pdTRUE,
                                      NULL,
                                      heart_rate_meas_timeout_handler);
    m_rr_interval_timer = xTimerCreate("RRT",
                                       RR_INTERVAL_INTERVAL,
                                       pdTRUE,
                                       NULL,
                                       rr_interval_timeout_handler);
    m_sensor_contact_timer = xTimerCreate("SCT",
                                          SENSOR_CONTACT_DETECTED_INTERVAL,
                                          pdTRUE,
                                          NULL,
                                          sensor_contact_detected_timeout_handler);

    /* Error checking */
    if ( (NULL == m_battery_timer)
         || (NULL == m_heart_rate_timer)
         || (NULL == m_rr_interval_timer)
         || (NULL == m_sensor_contact_timer) )
    {
        APP_ERROR_HANDLER(NRF_ERROR_NO_MEM);
    }
}


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


/**@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 initializing services that will be used by the application.
 *
 * @details Initialize the Heart Rate, Battery and Device Information services.
 */
static void services_init(void)
{
    ret_code_t         err_code;
    ble_hrs_init_t     hrs_init;
    ble_bas_init_t     bas_init;
    ble_dis_init_t     dis_init;
    nrf_ble_qwr_init_t qwr_init = {0};
    uint8_t            body_sensor_location;

    // 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 Heart Rate Service.
    body_sensor_location = BLE_HRS_BODY_SENSOR_LOCATION_FINGER;

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

    hrs_init.evt_handler                 = NULL;
    hrs_init.is_sensor_contact_supported = true;
    hrs_init.p_body_sensor_location      = &body_sensor_location;

    // Here the sec level for the Heart Rate Service can be changed/increased.
    hrs_init.hrm_cccd_wr_sec = SEC_OPEN;
    hrs_init.bsl_rd_sec      = SEC_OPEN;

    err_code = ble_hrs_init(&m_hrs, &hrs_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, (char *)MANUFACTURER_NAME);

    dis_init.dis_char_rd_sec = SEC_OPEN;

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


/**@brief Function for initializing the sensor simulators. */
static void sensor_simulator_init(void)
{
    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);

    m_heart_rate_sim_cfg.min          = MIN_HEART_RATE;
    m_heart_rate_sim_cfg.max          = MAX_HEART_RATE;
    m_heart_rate_sim_cfg.incr         = HEART_RATE_INCREMENT;
    m_heart_rate_sim_cfg.start_at_max = false;

    sensorsim_init(&m_heart_rate_sim_state, &m_heart_rate_sim_cfg);

    m_rr_interval_sim_cfg.min          = MIN_RR_INTERVAL;
    m_rr_interval_sim_cfg.max          = MAX_RR_INTERVAL;
    m_rr_interval_sim_cfg.incr         = RR_INTERVAL_INCREMENT;
    m_rr_interval_sim_cfg.start_at_max = false;

    sensorsim_init(&m_rr_interval_sim_state, &m_rr_interval_sim_cfg);
}


/**@brief   Function for starting application timers.
 * @details Timers are run after the scheduler has started.
 */
static void application_timers_start(void)
{
    // Start application timers.
    if (pdPASS != xTimerStart(m_battery_timer, OSTIMER_WAIT_FOR_QUEUE))
    {
        APP_ERROR_HANDLER(NRF_ERROR_NO_MEM);
    }
    if (pdPASS != xTimerStart(m_heart_rate_timer, OSTIMER_WAIT_FOR_QUEUE))
    {
        APP_ERROR_HANDLER(NRF_ERROR_NO_MEM);
    }
    if (pdPASS != xTimerStart(m_rr_interval_timer, OSTIMER_WAIT_FOR_QUEUE))
    {
        APP_ERROR_HANDLER(NRF_ERROR_NO_MEM);
    }
    if (pdPASS != xTimerStart(m_sensor_contact_timer, OSTIMER_WAIT_FOR_QUEUE))
    {
        APP_ERROR_HANDLER(NRF_ERROR_NO_MEM);
    }
}


/**@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    = m_hrs.hrm_handles.cccd_handle;
    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)
{
    uint32_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;

    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);
            break;

        case BLE_GAP_EVT_DISCONNECTED:
            NRF_LOG_INFO("Disconnected");
            m_conn_handle = BLE_CONN_HANDLE_INVALID;
            break;

        case BLE_GAP_EVT_PHY_UPDATE_REQUEST:
        {
            NRF_LOG_DEBUG("PHY update request.");
            ble_gap_phys_t const phys =
            {
                .rx_phys = BLE_GAP_PHY_AUTO,
                .tx_phys = BLE_GAP_PHY_AUTO,
            };
            err_code = sd_ble_gap_phy_update(p_ble_evt->evt.gap_evt.conn_handle, &phys);
            APP_ERROR_CHECK(err_code);
        } break;

        case BLE_GATTC_EVT_TIMEOUT:
            // Disconnect on GATT Client timeout event.
            NRF_LOG_DEBUG("GATT Client Timeout.");
            err_code = sd_ble_gap_disconnect(p_ble_evt->evt.gattc_evt.conn_handle,
                                             BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);
            APP_ERROR_CHECK(err_code);
            break;

        case BLE_GATTS_EVT_TIMEOUT:
            // Disconnect on GATT Server timeout event.
            NRF_LOG_DEBUG("GATT Server Timeout.");
            err_code = sd_ble_gap_disconnect(p_ble_evt->evt.gatts_evt.conn_handle,
                                             BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);
            APP_ERROR_CHECK(err_code);
            break;

        default:
            // 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;

        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  = 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 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. */
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 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 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 starting advertising. */
static void advertising_start(void * p_erase_bonds)
{
    bool erase_bonds = *(bool*)p_erase_bonds;

    if (erase_bonds)
    {
        delete_bonds();
        // Advertising is started by PM_EVT_PEERS_DELETE_SUCCEEDED event.
    }
    else
    {
        ret_code_t err_code = ble_advertising_start(&m_advertising, BLE_ADV_MODE_FAST);
        APP_ERROR_CHECK(err_code);
    }
}


/**@brief A function which is hooked to idle task.
 * @note Idle hook must be enabled in FreeRTOS configuration (configUSE_IDLE_HOOK).
 */
void vApplicationIdleHook( void )
{
// #if NRF_LOG_ENABLED
//      vTaskResume(m_logger_thread);
// #endif
}

void vApplicationTickHook( void )
{
    bsp_board_led_invert(BSP_BOARD_LED_0);
}

/**@brief Function for initializing the clock.
 */
static void clock_init(void)
{
    ret_code_t err_code = nrf_drv_clock_init();
    APP_ERROR_CHECK(err_code);
}

static void blinker_thread (void * pvParameter)
{
    UNUSED_PARAMETER(pvParameter);
    while (true)
    {
        bsp_board_led_invert(BSP_BOARD_LED_3);

        /* Delay a task for a given number of ticks */
        vTaskDelay(TASK_DELAY);

        /* Tasks must be implemented to never return... */
    }
}

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

    // Initialize modules.
    log_init();
    clock_init();

    // Do not start any interrupt that uses system functions before system initialisation.
    // The best solution is to start the OS before any other initalisation.


    // Activate deep sleep mode.
    SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk;

    // Configure and initialize the BLE stack.
    ble_stack_init();

    // Initialize modules.
    timers_init();
    buttons_leds_init(&erase_bonds);
    gap_params_init();
    gatt_init();
    advertising_init();
    services_init();
    sensor_simulator_init();
    conn_params_init();
    peer_manager_init();
    application_timers_start();

    // Create a FreeRTOS task for the BLE stack.
    // The task will run advertising_start() before entering its loop.
    nrf_sdh_freertos_init(advertising_start, &erase_bonds);

    NRF_LOG_INFO("HRS FreeRTOS example started.");

    /* Create task for LED0 blinking with priority set to 2 */
    UNUSED_VARIABLE(xTaskCreate(blinker_thread, "blinker", 256, NULL, 1, &m_blinker_thread));

    /* Start timer for LED1 blinking */
    led_toggle_timer_handle = xTimerCreate( "LED1", TIMER_PERIOD, pdFALSE, NULL, led_toggle_timer_callback);
    UNUSED_VARIABLE(xTimerStart(led_toggle_timer_handle, 0));

    // Start FreeRTOS scheduler.
    vTaskStartScheduler();

    for (;;)
    {
        APP_ERROR_HANDLER(NRF_ERROR_FORBIDDEN);
    }
}

I've connected to the example device using NRFconnect and I've turned on heart rate and battery service notifications.
Then I've observed LED2 pin for around 10 minutes with:
1. Original fix in vPortSuppressTicksAndSleep()
2. Original fix in tasks.c
3. FIx proposed by Susheel Nuguru in one of the previous posts
4. configUSE_TICKLESS_IDLE = 0
5  configUSE_TICKLESS_IDLE = 1 no fix applied

When any of fixes present (1,2,3) I got period in range 18 - 21ms
When tickless idle disabled or without any fix (4,5) I got period in range 19,5 - 20,5ms