mirror of
https://git.rtems.org/rtems-docs/
synced 2025-06-04 00:05:11 +08:00
86c39ef2c4
This makes it easier to automatically generate parts of the manager documentation in the future. Update #3993.
97 lines
3.7 KiB
ReStructuredText
97 lines
3.7 KiB
ReStructuredText
.. SPDX-License-Identifier: CC-BY-SA-4.0
|
|
|
|
.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
|
|
|
|
Background
|
|
==========
|
|
|
|
Required Support
|
|
----------------
|
|
|
|
For the features provided by the clock manager to be utilized, periodic timer
|
|
interrupts are required. Therefore, a real-time clock or hardware timer is
|
|
necessary to create the timer interrupts. The clock tick directive
|
|
is normally called by the timer ISR to announce to RTEMS that a system clock
|
|
tick has occurred. Elapsed time is measured in ticks. A tick is defined to be
|
|
an integral number of microseconds which is specified by the user in the
|
|
Configuration Table.
|
|
|
|
.. _Time and Date Data Structures:
|
|
|
|
Time and Date Data Structures
|
|
-----------------------------
|
|
|
|
The clock facilities of the clock manager operate upon calendar time. These
|
|
directives utilize the following date and time structure for the native time
|
|
and date format:
|
|
|
|
.. index:: rtems_time_of_day
|
|
|
|
.. code-block:: c
|
|
|
|
struct rtems_tod_control {
|
|
uint32_t year; /* greater than 1987 */
|
|
uint32_t month; /* 1 - 12 */
|
|
uint32_t day; /* 1 - 31 */
|
|
uint32_t hour; /* 0 - 23 */
|
|
uint32_t minute; /* 0 - 59 */
|
|
uint32_t second; /* 0 - 59 */
|
|
uint32_t ticks; /* elapsed between seconds */
|
|
};
|
|
typedef struct rtems_tod_control rtems_time_of_day;
|
|
|
|
The native date and time format is the only format supported when setting the
|
|
system date and time using the ``rtems_clock_set`` directive. Some
|
|
applications expect to operate on a *UNIX-style* date and time data structure.
|
|
The ``rtems_clock_get_tod_timeval`` always returns the date and time in
|
|
``struct timeval`` format.
|
|
|
|
The ``struct timeval`` data structure has two fields: ``tv_sec`` and
|
|
``tv_usec`` which are seconds and microseconds, respectively. The ``tv_sec``
|
|
field in this data structure is the number of seconds since the POSIX epoch of
|
|
*January 1, 1970* but will never be prior to the RTEMS epoch of *January 1,
|
|
1988*.
|
|
|
|
.. index:: timeslicing
|
|
|
|
Clock Tick and Timeslicing
|
|
--------------------------
|
|
|
|
Timeslicing is a task scheduling discipline in which tasks of equal priority
|
|
are executed for a specific period of time before control of the CPU is passed
|
|
to another task. It is also sometimes referred to as the automatic round-robin
|
|
scheduling algorithm. The length of time allocated to each task is known as
|
|
the quantum or timeslice.
|
|
|
|
The system's timeslice is defined as an integral number of ticks, and is
|
|
specified in the Configuration Table. The timeslice is defined for the entire
|
|
system of tasks, but timeslicing is enabled and disabled on a per task basis.
|
|
|
|
The clock tick directives implement timeslicing by decrementing the
|
|
running task's time-remaining counter when both timeslicing and preemption are
|
|
enabled. If the task's timeslice has expired, then that task will be preempted
|
|
if there exists a ready task of equal priority.
|
|
|
|
.. index:: delays
|
|
|
|
Delays
|
|
------
|
|
|
|
A sleep timer allows a task to delay for a given interval or up until a given
|
|
time, and then wake and continue execution. This type of timer is created
|
|
automatically by the ``rtems_task_wake_after`` and ``rtems_task_wake_when``
|
|
directives and, as a result, does not have an RTEMS ID. Once activated, a
|
|
sleep timer cannot be explicitly deleted. Each task may activate one and only
|
|
one sleep timer at a time.
|
|
|
|
.. index:: timeouts
|
|
|
|
Timeouts
|
|
--------
|
|
|
|
Timeouts are a special type of timer automatically created when the timeout
|
|
option is used on the ``rtems_message_queue_receive``, ``rtems_event_receive``,
|
|
``rtems_semaphore_obtain`` and ``rtems_region_get_segment`` directives. Each
|
|
task may have one and only one timeout active at a time. When a timeout
|
|
expires, it unblocks the task with a timeout status code.
|