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c-user: Update key concepts time

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Sebastian Huber 2017-01-30 11:31:45 +01:00
parent 73ad65dff0
commit e23f46c3b9
2 changed files with 14 additions and 11 deletions
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c-user/configuring_a_system.rst
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@ -1650,7 +1650,7 @@ NOTES:
.. COMMENT: === CONFIGURE_MICROSECONDS_PER_TICK ===
.. _Length of Each Clock Tick:
.. _CONFIGURE_MICROSECONDS_PER_TICK:
Length of Each Clock Tick
-------------------------

23
c-user/key_concepts.rst
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@ -245,9 +245,16 @@ Time
The development of responsive real-time applications requires an understanding
of how RTEMS maintains and supports time-related operations. The basic unit of
time in RTEMS is known as a tick. The frequency of clock ticks is completely
application dependent and determines the granularity and accuracy of all
interval and calendar time operations.
time in RTEMS is known as a `clock tick` or simply `tick`. The tick interval
is defined by the application configuration option
:ref:`CONFIGURE_MICROSECONDS_PER_TICK <CONFIGURE_MICROSECONDS_PER_TICK>`. The
tick interval defines the basic resolution of all interval and calendar time
operations. Obviously, the directives which use intervals or wall time cannot
operate without some external mechanism which provides a periodic clock tick.
This clock tick is provided by the clock driver. The tick precision and
stability depends on the clock driver and interrupt latency. Most clock
drivers provide a timecounter to measure the time with a higher resolution than
the tick.
.. index:: rtems_interval
@ -257,13 +264,13 @@ execution of timer service routines, and the rate monotonic scheduling of
tasks. An interval is defined as a number of ticks relative to the current
time. For example, when a task delays for an interval of ten ticks, it is
implied that the task will not execute until ten clock ticks have occurred.
All intervals are specified using data type ``rtems_interval``.
All intervals are specified using data type :c:type:`rtems_interval`.
A characteristic of interval timing is that the actual interval period may be a
fraction of a tick less than the interval requested. This occurs because the
time at which the delay timer is set up occurs at some time between two clock
ticks. Therefore, the first countdown tick occurs in less than the complete
time interval for a tick. This can be a problem if the clock granularity is
time interval for a tick. This can be a problem if the tick resolution is
large.
The rate monotonic scheduling algorithm is a hard real-time scheduling
@ -277,15 +284,11 @@ time be kept in wall time or true calendar form. Consequently, RTEMS maintains
the current date and time. This allows selected time operations to be
scheduled at an actual calendar date and time. For example, a task could
request to delay until midnight on New Year's Eve before lowering the ball at
Times Square. The data type ``rtems_time_of_day`` is used to specify calendar
Times Square. The data type :c:type:`rtems_time_of_day` is used to specify calendar
time in RTEMS services. See :ref:`Time and Date Data Structures`.
.. index:: rtems_time_of_day
Obviously, the directives which use intervals or wall time cannot operate
without some external mechanism which provides a periodic clock tick. This
clock tick is typically provided by a real time clock or counter/timer device.
Memory Management
=================
.. index:: memory management