text: /* Copyright (c) 2012 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.
   *
   */
  
  #include "app_timer.h"
  #include <stdlib.h>
  #include "nrf.h"
  #include "nrf_soc.h"
  #include "app_error.h"
  #include "nrf_delay.h"
  #include "app_util_platform.h"
  #include "sdk_common.h"
  
  #define RTC1_IRQ_PRI            APP_IRQ_PRIORITY_LOW                        /**< Priority of the RTC1 interrupt (used for checking for timeouts and executing timeout handlers). */
  #define SWI_IRQ_PRI             APP_IRQ_PRIORITY_LOW                        /**< Priority of the SWI  interrupt (used for updating the timer list). */
  
  // The current design assumes that both interrupt handlers run at the same interrupt level.
  // If this is to be changed, protection must be added to prevent them from interrupting each other
  // (e.g. by using guard/trigger flags).
  STATIC_ASSERT(RTC1_IRQ_PRI == SWI_IRQ_PRI);
  
  #define MAX_RTC_COUNTER_VAL     0x00FFFFFF                                  /**< Maximum value of the RTC counter. */
  
  #define APP_HIGH_USER_ID        0                                           /**< User Id for the Application High "user". */
  #define APP_LOW_USER_ID         1                                           /**< User Id for the Application Low "user". */
  #define THREAD_MODE_USER_ID     2                                           /**< User Id for the Thread Mode "user". */
  
  #define RTC_COMPARE_OFFSET_MIN  3                                           /**< Minimum offset between the current RTC counter value and the Capture Compare register. Although the nRF51 Series User Specification recommends this value to be 2, we use 3 to be safer.*/
  
  #define MAX_RTC_TASKS_DELAY     47                                          /**< Maximum delay until an RTC task is executed. */
  
  #ifdef NRF51
  #define SWI_IRQn SWI0_IRQn
  #define SWI_IRQHandler SWI0_IRQHandler
  #elif defined NRF52
  #define SWI_IRQn SWI0_EGU0_IRQn
  #define SWI_IRQHandler SWI0_EGU0_IRQHandler
  #endif
  
  /**@brief Timer node type. The nodes will be used form a linked list of running timers. */
  typedef struct
  {
      uint32_t                    ticks_to_expire;                            /**< Number of ticks from previous timer interrupt to timer expiry. */
      uint32_t                    ticks_at_start;                             /**< Current RTC counter value when the timer was started. */
      uint32_t                    ticks_first_interval;                       /**< Number of ticks in the first timer interval. */
      uint32_t                    ticks_periodic_interval;                    /**< Timer period (for repeating timers). */
      bool                        is_running;                                 /**< True if timer is running, False otherwise. */
      app_timer_mode_t            mode;                                       /**< Timer mode. */
      app_timer_timeout_handler_t p_timeout_handler;                          /**< Pointer to function to be executed when the timer expires. */
      void *                      p_context;                                  /**< General purpose pointer. Will be passed to the timeout handler when the timer expires. */
      void *                      next;                                       /**< Pointer to the next node. */
  } timer_node_t;
  
  STATIC_ASSERT(sizeof(timer_node_t) == APP_TIMER_NODE_SIZE);
  
  /**@brief Set of available timer operation types. */
  typedef enum
  {
      TIMER_USER_OP_TYPE_NONE,                                                /**< Invalid timer operation type. */
      TIMER_USER_OP_TYPE_START,                                               /**< Timer operation type Start. */
      TIMER_USER_OP_TYPE_STOP,                                                /**< Timer operation type Stop. */
      TIMER_USER_OP_TYPE_STOP_ALL                                             /**< Timer operation type Stop All. */
  } timer_user_op_type_t;
  
  /**@brief Structure describing a timer start operation. */
  typedef struct
  {
      uint32_t ticks_at_start;                                                /**< Current RTC counter value when the timer was started. */
      uint32_t ticks_first_interval;                                          /**< Number of ticks in the first timer interval. */
      uint32_t ticks_periodic_interval;                                       /**< Timer period (for repeating timers). */
      void *   p_context;                                                     /**< General purpose pointer. Will be passed to the timeout handler when the timer expires. */
  } timer_user_op_start_t;
  
  /**@brief Structure describing a timer operation. */
  typedef struct
  {
      timer_user_op_type_t op_type;                                             /**< Id of timer on which the operation is to be performed. */
      timer_node_t *       p_node;
      union
      {
          timer_user_op_start_t start;                                        /**< Structure describing a timer start operation. */
      } params;
  } timer_user_op_t;
  
  STATIC_ASSERT(sizeof(timer_user_op_t) <= APP_TIMER_USER_OP_SIZE);
  STATIC_ASSERT(sizeof(timer_user_op_t) % 4 == 0);
  
  /**@brief Structure describing a timer user.
   *
   * @details For each user of the timer module, there will be a timer operations queue. This queue
   *          will hold timer operations issued by this user until the timer interrupt handler
   *          processes these operations. For the current implementation, there will be one user for
   *          each interrupt level available to the application (APP_HIGH, APP_LOW and THREAD_MODE),
   *          but the module can easily be modified to e.g. have one queue per process when using an
   *          RTOS. The purpose of the queues is to be able to have a completely lockless timer
   *          implementation.
   */
  typedef struct
  {
      uint8_t           first;                                                    /**< Index of first entry to have been inserted in the queue (i.e. the next entry to be executed). */
      uint8_t           last;                                                     /**< Index of last entry to have been inserted in the queue. */
      uint8_t           user_op_queue_size;                                       /**< Queue size. */
      timer_user_op_t * p_user_op_queue;                                          /**< Queue buffer. */
  } timer_user_t;
  
  STATIC_ASSERT(sizeof(timer_user_t) == APP_TIMER_USER_SIZE);
  STATIC_ASSERT(sizeof(timer_user_t) % 4 == 0);
  
  /**@brief User id type.
   *
   * @details In the current implementation, this will automatically be generated from the current
   *          interrupt level.
   */
  typedef uint32_t timer_user_id_t;
  
  #define CONTEXT_QUEUE_SIZE_MAX      (2)                 
  
  static uint8_t                       m_user_array_size;                         /**< Size of timer user array. */
  static timer_user_t *                mp_users = NULL;                           /**< Array of timer users. */
  static timer_node_t *                mp_timer_id_head;                          /**< First timer in list of running timers. */
  static uint32_t                      m_ticks_latest;                            /**< Last known RTC counter value. */
  static uint32_t                      m_ticks_elapsed[CONTEXT_QUEUE_SIZE_MAX];   /**< Timer internal elapsed ticks queue. */
  static uint8_t                       m_ticks_elapsed_q_read_ind;                /**< Timer internal elapsed ticks queue read index. */
  static uint8_t                       m_ticks_elapsed_q_write_ind;               /**< Timer internal elapsed ticks queue write index. */
  static app_timer_evt_schedule_func_t m_evt_schedule_func;                       /**< Pointer to function for propagating timeout events to the scheduler. */
  static bool                          m_rtc1_running;                            /**< Boolean indicating if RTC1 is running. */
  static bool                          m_rtc1_reset;                              /**< Boolean indicating if RTC1 counter has been reset due to last timer removed from timer list during the timer list handling. */
  
  #ifdef APP_TIMER_WITH_PROFILER
  static uint8_t                      m_max_user_op_queue_utilization;                  /**< Maximum observed timer user operations queue utilization. */
  #endif
  
  #define MODULE_INITIALIZED (mp_users != NULL)
  #include "sdk_macros.h"
  
  /**@brief Function for initializing the RTC1 counter.
   *
   * @param[in] prescaler   Value of the RTC1 PRESCALER register. Set to 0 for no prescaling.
   */
  static void rtc1_init(uint32_t prescaler)
  {
      NRF_RTC1->PRESCALER = prescaler;
      NVIC_SetPriority(RTC1_IRQn, RTC1_IRQ_PRI);
  }
  
  
  /**@brief Function for starting the RTC1 timer.
   */
  static void rtc1_start(void)
  {
      NRF_RTC1->EVTENSET = RTC_EVTEN_COMPARE0_Msk;
      NRF_RTC1->INTENSET = RTC_INTENSET_COMPARE0_Msk;
  
      NVIC_ClearPendingIRQ(RTC1_IRQn);
      NVIC_EnableIRQ(RTC1_IRQn);
  
      NRF_RTC1->TASKS_START = 1;
      nrf_delay_us(MAX_RTC_TASKS_DELAY);
  
      m_rtc1_running = true;
  }
  
  
  /**@brief Function for stopping the RTC1 timer.
   */
  static void rtc1_stop(void)
  {
      NVIC_DisableIRQ(RTC1_IRQn);
  
      NRF_RTC1->EVTENCLR = RTC_EVTEN_COMPARE0_Msk;
      NRF_RTC1->INTENCLR = RTC_INTENSET_COMPARE0_Msk;
  
      NRF_RTC1->TASKS_STOP = 1;
      nrf_delay_us(MAX_RTC_TASKS_DELAY);
  
      NRF_RTC1->TASKS_CLEAR = 1;
      m_ticks_latest        = 0;
      nrf_delay_us(MAX_RTC_TASKS_DELAY);
  
      m_rtc1_running = false;
  }
  
  
  /**@brief Function for returning the current value of the RTC1 counter.
   *
   * @return     Current value of the RTC1 counter.
   */
  static __INLINE uint32_t rtc1_counter_get(void)
  {
      return NRF_RTC1->COUNTER;
  }
  
  
  /**@brief Function for computing the difference between two RTC1 counter values.
   *
   * @return     Number of ticks elapsed from ticks_old to ticks_now.
   */
  static __INLINE uint32_t ticks_diff_get(uint32_t ticks_now, uint32_t ticks_old)
  {
      return ((ticks_now - ticks_old) & MAX_RTC_COUNTER_VAL);
  }
  
  
  /**@brief Function for setting the RTC1 Capture Compare register 0, and enabling the corresponding
   *        event.
   *
   * @param[in] value   New value of Capture Compare register 0.
   */
  static __INLINE void rtc1_compare0_set(uint32_t value)
  {
      NRF_RTC1->CC[0] = value;
  }
  
  
  /**@brief Function for inserting a timer in the timer list.
   *
   * @param[in]  timer_id   Id of timer to insert.
   */
  static void timer_list_insert(timer_node_t * p_timer)
  {
      if (mp_timer_id_head == NULL)
      {
          mp_timer_id_head = p_timer;
      }
      else
      {
          if (p_timer->ticks_to_expire <= mp_timer_id_head->ticks_to_expire)
          {
              mp_timer_id_head->ticks_to_expire -= p_timer->ticks_to_expire;
              
              p_timer->next   = mp_timer_id_head;
              mp_timer_id_head = p_timer;
          }
          else
          {
              timer_node_t * p_previous;
              timer_node_t * p_current;
              uint32_t       ticks_to_expire;
  
              ticks_to_expire   = p_timer->ticks_to_expire;
              p_previous        = mp_timer_id_head;
              p_current         = mp_timer_id_head;
              
              while ((p_current != NULL) && (ticks_to_expire > p_current->ticks_to_expire))
              {
                  ticks_to_expire   -= p_current->ticks_to_expire;
                  p_previous         = p_current;
                  p_current          = p_current->next;
              }
  
              if (p_current != NULL)
              {
                  p_current->ticks_to_expire -= ticks_to_expire;
              }
  
              p_timer->ticks_to_expire = ticks_to_expire;
              p_timer->next            = p_current;
              p_previous->next         = p_timer;
          }
      }
  }
  
  
  /**@brief Function for removing a timer from the timer queue.
   *
   * @param[in]  timer_id   Id of timer to remove.
   */
  static void timer_list_remove(timer_node_t * p_timer)
  {
      timer_node_t * p_previous;
      timer_node_t * p_current;
      uint32_t       timeout;
  
      // Find the timer's position in timer list.
      p_previous = mp_timer_id_head;
      p_current  = p_previous;
      
      while (p_current != NULL)
      {
          if (p_current == p_timer)
          {
              break;
          }
          p_previous = p_current;
          p_current  = p_current->next;
      }
  
      // Timer not in active list.
      if (p_current == NULL)
      {
          return;
      }
  
      // Timer is the first in the list
      if (p_previous == p_current)
      {
          mp_timer_id_head = mp_timer_id_head->next;
  
          // No more timers in the list. Reset RTC1 in case Start timer operations are present in the queue.
          if (mp_timer_id_head == NULL)
          {
              NRF_RTC1->TASKS_CLEAR = 1;
              m_ticks_latest        = 0;
              m_rtc1_reset          = true;
          }
      }
  
      // Remaining timeout between next timeout.
      timeout = p_current->ticks_to_expire;
  
      // Link previous timer with next of this timer, i.e. removing the timer from list.
      p_previous->next = p_current->next;
  
      // If this is not the last timer, increment the next timer by this timer timeout.
      p_current = p_previous->next;
      if (p_current != NULL)
      {
          p_current->ticks_to_expire += timeout;
      }
  }
  
  
  /**@brief Function for scheduling a check for timeouts by generating a RTC1 interrupt.
   */
  static void timer_timeouts_check_sched(void)
  {
      NVIC_SetPendingIRQ(RTC1_IRQn);
  }
  
  
  /**@brief Function for scheduling a timer list update by generating a SWI interrupt.
   */
  static void timer_list_handler_sched(void)
  {
      NVIC_SetPendingIRQ(SWI_IRQn);
  }
  
  
  /**@brief Function for executing an application timeout handler, either by calling it directly, or
   *        by passing an event to the @ref app_scheduler.
   *
   * @param[in]  p_timer   Pointer to expired timer.
   */
  static void timeout_handler_exec(timer_node_t * p_timer)
  {
      if (m_evt_schedule_func != NULL)
      {
          uint32_t err_code = m_evt_schedule_func(p_timer->p_timeout_handler, p_timer->p_context);
          APP_ERROR_CHECK(err_code);
      }
      else
      {
          p_timer->p_timeout_handler(p_timer->p_context);
      }
  }
  
  
  /**@brief Function for checking for expired timers.
   */
  static void timer_timeouts_check(void)
  {
      // Handle expired of timer 
      if (mp_timer_id_head != NULL)
      {
          timer_node_t *  p_timer;
          timer_node_t *  p_previous_timer;
          uint32_t        ticks_elapsed;
          uint32_t        ticks_expired;
  
          // Initialize actual elapsed ticks being consumed to 0.
          ticks_expired = 0;
  
          // ticks_elapsed is collected here, job will use it.
          ticks_elapsed = ticks_diff_get(rtc1_counter_get(), m_ticks_latest);
  
          // Auto variable containing the head of timers expiring.
          p_timer = mp_timer_id_head;
  
          // Expire all timers within ticks_elapsed and collect ticks_expired.
          while (p_timer != NULL)
          {
              // Do nothing if timer did not expire.
              if (ticks_elapsed < p_timer->ticks_to_expire)
              {
                  break;
              }
  
              // Decrement ticks_elapsed and collect expired ticks.
              ticks_elapsed -= p_timer->ticks_to_expire;
              ticks_expired += p_timer->ticks_to_expire;
  
              // Move to next timer.
              p_previous_timer = p_timer;
              p_timer = p_timer->next;
  
              // Execute Task.
              if (p_previous_timer->is_running)
              {
                  p_previous_timer->is_running = false;
                  timeout_handler_exec(p_previous_timer);
              }
          }
  
          // Prepare to queue the ticks expired in the m_ticks_elapsed queue.
          if (m_ticks_elapsed_q_read_ind == m_ticks_elapsed_q_write_ind)
          {
              // The read index of the queue is equal to the write index. This means the new
              // value of ticks_expired should be stored at a new location in the m_ticks_elapsed
              // queue (which is implemented as a double buffer).
  
              // Check if there will be a queue overflow.
              if (++m_ticks_elapsed_q_write_ind == CONTEXT_QUEUE_SIZE_MAX)
              {
                  // There will be a queue overflow. Hence the write index should point to the start
                  // of the queue.
                  m_ticks_elapsed_q_write_ind = 0;
              }
          }
  
          // Queue the ticks expired.
          m_ticks_elapsed[m_ticks_elapsed_q_write_ind] = ticks_expired;
  
          timer_list_handler_sched();
      }
  }
  
  
  /**@brief Function for acquiring the number of ticks elapsed.
   *
   * @param[out] p_ticks_elapsed   Number of ticks elapsed.
   *
   * @return     TRUE if elapsed ticks was read from queue, FALSE otherwise.
   */
  static bool elapsed_ticks_acquire(uint32_t * p_ticks_elapsed)
  {
      // Pick the elapsed value from queue.
      if (m_ticks_elapsed_q_read_ind != m_ticks_elapsed_q_write_ind)
      {
          // Dequeue elapsed value.
          m_ticks_elapsed_q_read_ind++;
          if (m_ticks_elapsed_q_read_ind == CONTEXT_QUEUE_SIZE_MAX)
          {
              m_ticks_elapsed_q_read_ind = 0;
          }
  
          *p_ticks_elapsed = m_ticks_elapsed[m_ticks_elapsed_q_read_ind];
  
          m_ticks_latest += *p_ticks_elapsed;
          m_ticks_latest &= MAX_RTC_COUNTER_VAL;
  
          return true;
      }
      else
      {
          // No elapsed value in queue.
          *p_ticks_elapsed = 0;
          return false;
      }
  }
  
  
  /**@brief Function for handling the timer list deletions.
   *
   * @return     TRUE if Capture Compare register must be updated, FALSE otherwise.
   */
  static bool list_deletions_handler(void)
  {
      timer_node_t * p_timer_old_head;
      uint8_t        user_id;
  
      // Remember the old head, so as to decide if new compare needs to be set.
      p_timer_old_head = mp_timer_id_head;
  
      user_id = m_user_array_size;
      while (user_id--)
      {
          timer_user_t * p_user         = &mp_users[user_id];
          uint8_t        user_ops_first = p_user->first;
          
          while (user_ops_first != p_user->last)
          {
              timer_user_op_t * p_user_op = &p_user->p_user_op_queue[user_ops_first];
  
              // Traverse to next operation in queue.
              user_ops_first++;
              if (user_ops_first == p_user->user_op_queue_size)
              {
                  user_ops_first = 0;
              }
  
              switch (p_user_op->op_type)
              {
                  case TIMER_USER_OP_TYPE_STOP:
                      // Delete node if timer is running.
                      timer_list_remove(p_user_op->p_node);
                      break;
                      
                  case TIMER_USER_OP_TYPE_STOP_ALL:
                      // Delete list of running timers, and mark all timers as not running.
                      while (mp_timer_id_head != NULL)
                      {
                          timer_node_t * p_head = mp_timer_id_head;
  
                          p_head->is_running = false;
                          mp_timer_id_head    = p_head->next;
                      }
                      break;
                      
                  default:
                      // No implementation needed.
                      break;
              }
          }
      }
  
      // Detect change in head of the list.
      return (mp_timer_id_head != p_timer_old_head);
  }
  
  
  /**@brief Function for updating the timer list for expired timers.
   *
   * @param[in]  ticks_elapsed         Number of elapsed ticks.
   * @param[in]  ticks_previous        Previous known value of the RTC counter.
   * @param[out] p_restart_list_head   List of repeating timers to be restarted.
   */
  static void expired_timers_handler(uint32_t         ticks_elapsed,
                                     uint32_t         ticks_previous,
                                     timer_node_t **  p_restart_list_head)
  {
      uint32_t ticks_expired = 0;
  
      while (mp_timer_id_head != NULL)
      {
          timer_node_t * p_timer;
          timer_node_t * p_timer_expired;
  
          // Auto variable for current timer node.
          p_timer = mp_timer_id_head;
  
          // Do nothing if timer did not expire 
          if (ticks_elapsed < p_timer->ticks_to_expire)
          {
              p_timer->ticks_to_expire -= ticks_elapsed;
              break;
          }
  
          // Decrement ticks_elapsed and collect expired ticks.
          ticks_elapsed -= p_timer->ticks_to_expire;
          ticks_expired += p_timer->ticks_to_expire;
  
          // Timer expired, set ticks_to_expire zero.
          p_timer->ticks_to_expire = 0;
  
          // Remove the expired timer from head.
          p_timer_expired  = mp_timer_id_head;
          mp_timer_id_head = p_timer->next;
  
          // Timer will be restarted if periodic.
          if (p_timer->ticks_periodic_interval != 0)
          {
              p_timer->ticks_at_start       = (ticks_previous + ticks_expired) & MAX_RTC_COUNTER_VAL;
              p_timer->ticks_first_interval = p_timer->ticks_periodic_interval;
              p_timer->next                 = *p_restart_list_head;
              *p_restart_list_head          = p_timer_expired;
          }
      }
  }
  
  
  /**@brief Function for handling timer list insertions.
   *
   * @param[in]  p_restart_list_head   List of repeating timers to be restarted.
   *
   * @return     TRUE if Capture Compare register must be updated, FALSE otherwise.
   */
  static bool list_insertions_handler(timer_node_t * p_restart_list_head)
  {
      timer_node_t * p_timer_id_old_head;
      uint8_t        user_id;
  
      // Remember the old head, so as to decide if new compare needs to be set.
      p_timer_id_old_head = mp_timer_id_head;
  
      user_id = m_user_array_size;
      while (user_id--)
      {
          timer_user_t * p_user = &mp_users[user_id];
  
          // Handle insertions of timers.
          while ((p_restart_list_head != NULL) || (p_user->first != p_user->last))
          {
              timer_node_t * p_timer;
  
              if (p_restart_list_head != NULL)
              {
                  p_timer           = p_restart_list_head;
                  p_restart_list_head = p_timer->next;
              }
              else
              {
                  timer_user_op_t * p_user_op = &p_user->p_user_op_queue[p_user->first];
  
                  p_user->first++;
                  if (p_user->first == p_user->user_op_queue_size)
                  {
                      p_user->first = 0;
                  }
  
                  p_timer = p_user_op->p_node;
  
                  if ((p_user_op->op_type != TIMER_USER_OP_TYPE_START) || p_timer->is_running)
                  {
                      continue;
                  }
  
                  p_timer->ticks_at_start          = p_user_op->params.start.ticks_at_start;
                  p_timer->ticks_first_interval    = p_user_op->params.start.ticks_first_interval;
                  p_timer->ticks_periodic_interval = p_user_op->params.start.ticks_periodic_interval;
                  p_timer->p_context               = p_user_op->params.start.p_context;
  
                  if (m_rtc1_reset)
                  {
                      p_timer->ticks_at_start = 0;
                  }
              }
  
              // Prepare the node to be inserted.
              if (
                   ((p_timer->ticks_at_start - m_ticks_latest) & MAX_RTC_COUNTER_VAL)
                   <
                   (MAX_RTC_COUNTER_VAL / 2)
                  )
              {
                  p_timer->ticks_to_expire = ticks_diff_get(p_timer->ticks_at_start, m_ticks_latest) + 
                                             p_timer->ticks_first_interval;
              }
              else
              {
                  uint32_t delta_current_start;
  
                  delta_current_start = ticks_diff_get(m_ticks_latest, p_timer->ticks_at_start);
                  if (p_timer->ticks_first_interval > delta_current_start)
                  {
                      p_timer->ticks_to_expire = p_timer->ticks_first_interval - delta_current_start;
                  }
                  else
                  {
                      p_timer->ticks_to_expire = 0;
                  }
              }
  
              p_timer->ticks_at_start       = 0;
              p_timer->ticks_first_interval = 0;
              p_timer->is_running           = true;
              p_timer->next                 = NULL;
  
              // Insert into list 
              timer_list_insert(p_timer);
          }
      }
      
      return (mp_timer_id_head != p_timer_id_old_head);
  }
  
  
  /**@brief Function for updating the Capture Compare register.
   */
  static void compare_reg_update(timer_node_t * p_timer_id_head_old)
  {
      // Setup the timeout for timers on the head of the list 
      if (mp_timer_id_head != NULL)
      {
          uint32_t ticks_to_expire = mp_timer_id_head->ticks_to_expire;
          uint32_t pre_counter_val = rtc1_counter_get();
          uint32_t cc              = m_ticks_latest;
          uint32_t ticks_elapsed   = ticks_diff_get(pre_counter_val, cc) + RTC_COMPARE_OFFSET_MIN;
  
          if (!m_rtc1_running)
          {
              // No timers were already running, start RTC
              rtc1_start();
          }
  
          cc += (ticks_elapsed < ticks_to_expire) ? ticks_to_expire : ticks_elapsed;
          cc &= MAX_RTC_COUNTER_VAL;
          
          rtc1_compare0_set(cc);
  
          uint32_t post_counter_val = rtc1_counter_get();
  
          if (
              (ticks_diff_get(post_counter_val, pre_counter_val) + RTC_COMPARE_OFFSET_MIN)
              >
              ticks_diff_get(cc, pre_counter_val)
             )
          {
              // When this happens the COMPARE event may not be triggered by the RTC.
              // The nRF51 Series User Specification states that if the COUNTER value is N
              // (i.e post_counter_val = N), writing N or N+1 to a CC register may not trigger a
              // COMPARE event. Hence the RTC interrupt is forcefully pended by calling the following
              // function.
              rtc1_compare0_set(rtc1_counter_get());  // this should prevent CC to fire again in the background while the code is in RTC-ISR
              nrf_delay_us(MAX_RTC_TASKS_DELAY);
              timer_timeouts_check_sched();
          }
      }
      else
      {
          // No timers are running, stop RTC
          rtc1_stop();
      }
  }
  
  
  /**@brief Function for handling changes to the timer list.
   */
  static void timer_list_handler(void)
  {
      timer_node_t * p_restart_list_head = NULL;
  
      uint32_t       ticks_elapsed;
      uint32_t       ticks_previous;
      bool           ticks_have_elapsed;
      bool           compare_update;
      timer_node_t * p_timer_id_head_old;
      
  #ifdef APP_TIMER_WITH_PROFILER
      {
          unsigned int i;
  
          for (i = 0; i < APP_TIMER_INT_LEVELS; i++)
          {
              timer_user_t *p_user = &mp_users[i];
              uint8_t size = p_user->user_op_queue_size;
              uint8_t first = p_user->first;
              uint8_t last = p_user->last;
              uint8_t utilization = (first <= last) ? (last - first) : (size + 1 - first + last);
  
              if (utilization > m_max_user_op_queue_utilization)
              {
                  m_max_user_op_queue_utilization = utilization;
              }
          }
      }
  #endif
  
      // Back up the previous known tick and previous list head
      ticks_previous    = m_ticks_latest;
      p_timer_id_head_old = mp_timer_id_head;
      
      // Get number of elapsed ticks
      ticks_have_elapsed = elapsed_ticks_acquire(&ticks_elapsed);
  
      // Handle list deletions
      compare_update = list_deletions_handler();
      
      // Handle expired timers
      if (ticks_have_elapsed)
      {
          expired_timers_handler(ticks_elapsed, ticks_previous, &p_restart_list_head);
          compare_update = true;
      }
      
      // Handle list insertions
      if (list_insertions_handler(p_restart_list_head))
      {
          compare_update = true;
      }
  
      // Update compare register if necessary
      if (compare_update)
      {
          compare_reg_update(p_timer_id_head_old);
      }
      m_rtc1_reset = false;
  }
  
  
  /**@brief Function for enqueueing a new operations queue entry.
   *
   * @param[in]  p_user     User that the entry is to be enqueued for.
   * @param[in]  last_index Index of the next last index to be enqueued.
   */
  static void user_op_enque(timer_user_t * p_user, uint8_t last_index)
  {
      p_user->last = last_index;
  }
  
  
  /**@brief Function for allocating a new operations queue entry.
   *
   * @param[in]  p_user       User that the entry is to be allocated for.
   * @param[out] p_last_index Index of the next last index to be enqueued.
   *
   * @return     Pointer to allocated queue entry, or NULL if queue is full.
   */
  static timer_user_op_t * user_op_alloc(timer_user_t * p_user, uint8_t * p_last_index)
  {        
      uint8_t           last;
      timer_user_op_t * p_user_op;
      
      last = p_user->last + 1;
      if (last == p_user->user_op_queue_size)
      {
          // Overflow case.
          last = 0;
      }
      if (last == p_user->first)
      {
          // Queue is full.
          return NULL;
      }
      
      *p_last_index = last;    
      p_user_op     = &p_user->p_user_op_queue[p_user->last];
          
      return p_user_op;
  }
  
  
  /**@brief Function for scheduling a Timer Start operation.
   *
   * @param[in]  user_id           Id of user calling this function.
   * @param[in]  timer_id          Id of timer to start.
   * @param[in]  timeout_initial   Time (in ticks) to first timer expiry.
   * @param[in]  timeout_periodic  Time (in ticks) between periodic expiries.
   * @param[in]  p_context         General purpose pointer. Will be passed to the timeout handler when
   *                               the timer expires.
   * @return     NRF_SUCCESS on success, otherwise an error code.
   */
  static uint32_t timer_start_op_schedule(timer_user_id_t user_id,
                                          timer_node_t * p_node,
                                          uint32_t        timeout_initial,
                                          uint32_t        timeout_periodic,
                                          void *          p_context)
  {
      uint8_t last_index;
      
      timer_user_op_t * p_user_op = user_op_alloc(&mp_users[user_id], &last_index);
      if (p_user_op == NULL)
      {
          return NRF_ERROR_NO_MEM;
      }
      
      p_user_op->op_type                              = TIMER_USER_OP_TYPE_START;
      p_user_op->p_node                               = p_node;
      p_user_op->params.start.ticks_at_start          = rtc1_counter_get();
      p_user_op->params.start.ticks_first_interval    = timeout_initial;
      p_user_op->params.start.ticks_periodic_interval = timeout_periodic;
      p_user_op->params.start.p_context               = p_context;
      
      user_op_enque(&mp_users[user_id], last_index);    
  
      timer_list_handler_sched();
  
      return NRF_SUCCESS;
  }
  
  
  /**@brief Function for scheduling a Timer Stop operation.
   *
   * @param[in]  user_id    Id of user calling this function.
   * @param[in]  timer_id   Id of timer to stop.
   *
   * @return NRF_SUCCESS on successful scheduling a timer stop operation. NRF_ERROR_NO_MEM when there
   *         is no memory left to schedule the timer stop operation.
   */
  static uint32_t timer_stop_op_schedule(timer_user_id_t user_id, timer_node_t * p_node)
  {
      uint8_t last_index;
      
      timer_user_op_t * p_user_op = user_op_alloc(&mp_users[user_id], &last_index);
      if (p_user_op == NULL)
      {
          return NRF_ERROR_NO_MEM;
      }
      
      p_user_op->op_type  = TIMER_USER_OP_TYPE_STOP;
      p_user_op->p_node = p_node;
      
      user_op_enque(&mp_users[user_id], last_index);        
  
      timer_list_handler_sched();
  
      return NRF_SUCCESS;
  }
  
  
  /**@brief Function for scheduling a Timer Stop All operation.
   *
   * @param[in]  user_id    Id of user calling this function.
   */
  static uint32_t timer_stop_all_op_schedule(timer_user_id_t user_id)
  {
      uint8_t last_index;
      
      timer_user_op_t * p_user_op = user_op_alloc(&mp_users[user_id], &last_index);
      if (p_user_op == NULL)
      {
          return NRF_ERROR_NO_MEM;
      }
      
      p_user_op->op_type  = TIMER_USER_OP_TYPE_STOP_ALL;
      p_user_op->p_node = NULL;
      
      user_op_enque(&mp_users[user_id], last_index);        
  
      timer_list_handler_sched();
  
      return NRF_SUCCESS;
  }
  
  
  /**@brief Function for handling the RTC1 interrupt.
   *
   * @details Checks for timeouts, and executes timeout handlers for expired timers.
   */
  void RTC1_IRQHandler(void)
  {
      // Clear all events (also unexpected ones)
      NRF_RTC1->EVENTS_COMPARE[0] = 0;
      NRF_RTC1->EVENTS_COMPARE[1] = 0;
      NRF_RTC1->EVENTS_COMPARE[2] = 0;
      NRF_RTC1->EVENTS_COMPARE[3] = 0;
      NRF_RTC1->EVENTS_TICK       = 0;
      NRF_RTC1->EVENTS_OVRFLW     = 0;
  
      // Check for expired timers
      timer_timeouts_check();
  }
  
  
  /**@brief Function for handling the SWI interrupt.
   *
   * @details Performs all updates to the timer list.
   */
  void SWI_IRQHandler(void)
  {
      timer_list_handler();
  }
  
  
  uint32_t app_timer_init(uint32_t                      prescaler,
                          uint8_t                       op_queues_size,
                          void *                        p_buffer,
                          app_timer_evt_schedule_func_t evt_schedule_func)
  {
      int i;
  
      // Check that buffer is correctly aligned
      if (!is_word_aligned(p_buffer))
      {
          return NRF_ERROR_INVALID_PARAM;
      }
      // Check for NULL buffer
      if (p_buffer == NULL)
      {
          mp_users = NULL;
          return NRF_ERROR_INVALID_PARAM;
      }
      
      // Stop RTC to prevent any running timers from expiring (in case of reinitialization)
      rtc1_stop();
      
      m_evt_schedule_func = evt_schedule_func;
      
      // Initialize users array
      m_user_array_size = APP_TIMER_INT_LEVELS;
      mp_users          = p_buffer;
      
      // Skip user array
      p_buffer = &((uint8_t *)p_buffer)[APP_TIMER_INT_LEVELS * sizeof(timer_user_t)];
  
      // Initialize operation queues
      for (i = 0; i < APP_TIMER_INT_LEVELS; i++)
      {
          timer_user_t * p_user = &mp_users[i];
          
          p_user->first              = 0;
          p_user->last               = 0;
          p_user->user_op_queue_size = op_queues_size;
          p_user->p_user_op_queue    = p_buffer;
      
          // Skip operation queue
          p_buffer = &((uint8_t *)p_buffer)[op_queues_size * sizeof(timer_user_op_t)];
      }
  
      mp_timer_id_head             = NULL;
      m_ticks_elapsed_q_read_ind  = 0;
      m_ticks_elapsed_q_write_ind = 0;
  
  #ifdef APP_TIMER_WITH_PROFILER
      m_max_user_op_queue_utilization   = 0;
  #endif
  
      NVIC_ClearPendingIRQ(SWI_IRQn);
      NVIC_SetPriority(SWI_IRQn, SWI_IRQ_PRI);
      NVIC_EnableIRQ(SWI_IRQn);
  
      rtc1_init(prescaler);
  
      m_ticks_latest = rtc1_counter_get();
      
      return NRF_SUCCESS;
  }
  
  
  uint32_t app_timer_create(app_timer_id_t const *      p_timer_id,
                            app_timer_mode_t            mode,
                            app_timer_timeout_handler_t timeout_handler)
  {
      // Check state and parameters
      VERIFY_MODULE_INITIALIZED();
  
      if (timeout_handler == NULL)
      {
          return NRF_ERROR_INVALID_PARAM;
      }
      if (p_timer_id == NULL)
      {
          return NRF_ERROR_INVALID_PARAM;
      }
      if (((timer_node_t*)*p_timer_id)->is_running)
      {
          return NRF_ERROR_INVALID_STATE;
      }
      
      timer_node_t * p_node     = (timer_node_t *)*p_timer_id;
      p_node->is_running        = false;
      p_node->mode              = mode;
      p_node->p_timeout_handler = timeout_handler;
      return NRF_SUCCESS;
  }
  
  
  /**@brief Function for creating a timer user id from the current interrupt level.
   *
   * @return     Timer user id.
  */
  static timer_user_id_t user_id_get(void)
  {
      timer_user_id_t ret;
  
      STATIC_ASSERT(APP_TIMER_INT_LEVELS == 3);
      
      switch (current_int_priority_get())
      {
          case APP_IRQ_PRIORITY_HIGH:
              ret = APP_HIGH_USER_ID;
              break;
              
          case APP_IRQ_PRIORITY_LOW:
              ret = APP_LOW_USER_ID;
              break;
              
          default:
              ret = THREAD_MODE_USER_ID;
              break;
      }
      
      return ret;
  }
  
  
  uint32_t app_timer_start(app_timer_id_t timer_id, uint32_t timeout_ticks, void * p_context)
  {
      uint32_t timeout_periodic;
      timer_node_t * p_node = (timer_node_t*)timer_id;
      
      // Check state and parameters
      VERIFY_MODULE_INITIALIZED();
  
      if (timer_id == 0)
      {
          return NRF_ERROR_INVALID_STATE;
      }
      if (timeout_ticks < APP_TIMER_MIN_TIMEOUT_TICKS)
      {
          return NRF_ERROR_INVALID_PARAM;
      }
      if (p_node->p_timeout_handler == NULL)
      {
          return NRF_ERROR_INVALID_STATE;
      }
      
      // Schedule timer start operation
      timeout_periodic = (p_node->mode == APP_TIMER_MODE_REPEATED) ? timeout_ticks : 0;
  
      return timer_start_op_schedule(user_id_get(),
                                     p_node,
                                     timeout_ticks,
                                     timeout_periodic,
                                     p_context);
  }
  
  
  uint32_t app_timer_stop(app_timer_id_t timer_id)
  {
      timer_node_t * p_node = (timer_node_t*)timer_id;
      // Check state and parameters
      VERIFY_MODULE_INITIALIZED();
  
      if ((timer_id == NULL) || (p_node->p_timeout_handler == NULL))
      {
          return NRF_ERROR_INVALID_STATE;
      }
      
      p_node->is_running = false;
      // Schedule timer stop operation
      return timer_stop_op_schedule(user_id_get(), p_node);
  }
  
  
  uint32_t app_timer_stop_all(void)
  {
      // Check state
      VERIFY_MODULE_INITIALIZED();
  
      return timer_stop_all_op_schedule(user_id_get());
  }
  
  
  uint32_t app_timer_cnt_get(uint32_t * p_ticks)
  {
      *p_ticks = rtc1_counter_get();
      return NRF_SUCCESS;
  }
  
  
  uint32_t app_timer_cnt_diff_compute(uint32_t   ticks_to,
                                      uint32_t   ticks_from,
                                      uint32_t * p_ticks_diff)
  {
      *p_ticks_diff = ticks_diff_get(ticks_to, ticks_from);
      return NRF_SUCCESS;
  }
  
  #ifdef APP_TIMER_WITH_PROFILER
  uint8_t app_timer_op_queue_utilization_get(void)
  {
      return m_max_user_op_queue_utilization;
  }
  #endif
  