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This makes it easier to automatically generate parts of the manager documentation in the future. Update #3993.
72 lines
2.9 KiB
ReStructuredText
72 lines
2.9 KiB
ReStructuredText
.. SPDX-License-Identifier: CC-BY-SA-4.0
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.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
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Background
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==========
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Required Support
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----------------
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A clock tick is required to support the functionality provided by this manager.
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Timers
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------
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A timer is an RTEMS object which allows the application to schedule operations
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to occur at specific times in the future. User supplied timer service routines
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are invoked by either a clock tick directive or a special Timer
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Server task when the timer fires. Timer service routines may perform any
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operations or directives which normally would be performed by the application
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code which invoked a clock tick directive.
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The timer can be used to implement watchdog routines which only fire to denote
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that an application error has occurred. The timer is reset at specific points
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in the application to ensure that the watchdog does not fire. Thus, if the
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application does not reset the watchdog timer, then the timer service routine
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will fire to indicate that the application has failed to reach a reset point.
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This use of a timer is sometimes referred to as a "keep alive" or a "deadman"
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timer.
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Timer Server
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------------
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The Timer Server task is responsible for executing the timer service routines
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associated with all task-based timers. This task executes at a priority
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specified by :ref:`rtems_timer_initiate_server() <rtems_timer_initiate_server>`
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and it may have a priority of zero (the highest priority). In uniprocessor
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configurations, it is created non-preemptible.
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By providing a mechanism where timer service routines execute in task rather
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than interrupt space, the application is allowed a bit more flexibility in what
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operations a timer service routine can perform. For example, the Timer Server
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can be configured to have a floating point context in which case it would be
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safe to perform floating point operations from a task-based timer. Most of the
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time, executing floating point instructions from an interrupt service routine
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is not considered safe. The timer service routines invoked by the Timer Server
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may block, however, since this blocks the Timer Server itself, other timer
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service routines that are already pending do not run until the blocked timer
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service routine finished its work.
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The Timer Server is designed to remain blocked until a task-based timer fires.
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This reduces the execution overhead of the Timer Server.
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.. index:: rtems_timer_service_routine
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Timer Service Routines
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----------------------
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The timer service routine should adhere to C calling conventions and have a
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prototype similar to the following:
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.. code-block:: c
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rtems_timer_service_routine user_routine(
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rtems_id timer_id,
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void *user_data
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);
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Where the timer_id parameter is the RTEMS object ID of the timer which is being
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fired and user_data is a pointer to user-defined information which may be
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utilized by the timer service routine. The argument user_data may be NULL.
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