fixup! fixup! xtimer: timer & target overflow, hang resolved.

Author: Josar

sys/include/xtimer/implementation.h

@@ -65,7 +65,21 @@ static inline uint32_t _xtimer_lltimer_mask(uint32_t val)
  * @brief xtimer internal stuff
  * @internal
  */
+
 uint64_t _xtimer_now64(void);
+
+/**
+ * @brief Sets the timer to the appropriate timer_list or list_head.
+ *
+ * @note    The target to set the timer to has to be at least bigger then then
+ *          ticks needed to jump into the function and calculate '_xtimer_now()'.
+ *          So that 'now' did not pass the target.
+ *          This is crucial when using low CPU frequencies and/or when the
+ *          '_xtimer_now()' call needs multiple xtimer ticks to evaluate.
+ *
+ * @param[in] timer   pointer to xtimer_t which is added to the list.
+ * @param[in] target  Absolut target value in ticks. 
+ */
 int _xtimer_set_absolute(xtimer_t *timer, uint32_t target);
 void _xtimer_set(xtimer_t *timer, uint32_t offset);
 void _xtimer_set64(xtimer_t *timer, uint32_t offset, uint32_t long_offset);