os_init.h and os_init.c for RTX on Nordic NRF51822

os_tick.h
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#ifndef __OS_TICK_H
#define __OS_TICK_H

#include "global.h"

int os_tick_init (void);
uint32_t os_tick_val(void);
uint32_t os_tick_ovf(void);
void os_tick_irqack (void);
uint32_t os_tick_get(void);

#endif // __OS_TICK_H
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os_tick.c
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#include "global.h"
#include "nrf.h"
#include "nrf_gpio.h"

// System tick count.
static volatile uint32_t os_tick_count = 0;

// Starting the internal LFCLK XTAL oscillator.
static void lfclk_config(void)
{
  NRF_CLOCK->LFCLKSRC = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos);
  NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
  NRF_CLOCK->TASKS_LFCLKSTART = 1;
  while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0);
  NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
}

// Initialize alternative hardware timer as RTX kernel timer
// Return: IRQ number of the alternative hardware timer
int os_tick_init (void)
{
  // Start the internal LFCLK XTAL oscillator.
  lfclk_config();

  // Set prescaler to a TICK of ~1000Hz.
  NRF_RTC1->PRESCALER = 32;

  // Enable TICK event and TICK interrupt:
  NRF_RTC1->EVTENSET = RTC_EVTENSET_TICK_Msk;
  NRF_RTC1->INTENSET = RTC_INTENSET_TICK_Msk;

  // Start the clock.
  NRF_RTC1->TASKS_START = 1;

  // Return the IRQ number.
  return RTC1_IRQn;
}

// Get alternative hardware timer current value (0 .. OS_TRV)
uint32_t os_tick_val(void)
{
  return 0;
}

// Get alternative hardware timer overflow flag
// Return: 1 - overflow, 0 - no overflow
uint32_t os_tick_ovf(void)
{
  return 0;
}

// Acknowledge alternative hardware timer interrupt
void os_tick_irqack (void)
{
  // Is this a tick condition?
  if ((NRF_RTC1->EVENTS_TICK != 0) && ((NRF_RTC1->INTENSET & RTC_INTENSET_TICK_Msk) != 0))
  {
    // Increment the system tick count.
    ++os_tick_count;

    // Clear the interrupt?
    NRF_RTC1->EVENTS_TICK = 0;
  }
}

// Return the current OS tick count.
uint32_t os_tick_get(void)
{
  return os_tick_count;
}

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