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Timer Manager

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.. index:: timers

Introduction

The timer manager provides support for timer facilities. The directives provided by the timer manager are:

  • rtems.timer_create - Create a timer
  • rtems.timer_ident - Get ID of a timer
  • rtems.timer_cancel - Cancel a timer
  • rtems.timer_delete - Delete a timer
  • rtems.timer_fire_after - Fire timer after interval
  • rtems.timer_fire_when - Fire timer when specified
  • rtems.timer_initiate_server - Initiate server for task-based timers
  • rtems.timer_server_fire_after - Fire task-based timer after interval
  • rtems.timer_server_fire_when - Fire task-based timer when specified
  • rtems.timer_reset - Reset an interval timer

Background

Required Support

A clock tick is required to support the functionality provided by this manager.

Timers

A timer is an RTEMS object which allows the application to schedule operations to occur at specific times in the future. User supplied timer service routines are invoked by either the rtems.clock_tick directive or a special Timer Server task when the timer fires. Timer service routines may perform any operations or directives which normally would be performed by the application code which invoked the``rtems.clock_tick`` directive.

The timer can be used to implement watchdog routines which only fire to denote that an application error has occurred. The timer is reset at specific points in the application to ensure that the watchdog does not fire. Thus, if the application does not reset the watchdog timer, then the timer service routine will fire to indicate that the application has failed to reach a reset point. This use of a timer is sometimes referred to as a "keep alive" or a "deadman" timer.

Timer Server

The Timer Server task is responsible for executing the timer service routines associated with all task-based timers. This task executes at a priority higher than any RTEMS application task, and is created non-preemptible, and thus can be viewed logically as the lowest priority interrupt.

By providing a mechanism where timer service routines execute in task rather than interrupt space, the application is allowed a bit more flexibility in what operations a timer service routine can perform. For example, the Timer Server can be configured to have a floating point context in which case it would be safe to perform floating point operations from a task-based timer. Most of the time, executing floating point instructions from an interrupt service routine is not considered safe. However, since the Timer Server task is non-preemptible, only directives allowed from an ISR can be called in the timer service routine.

The Timer Server is designed to remain blocked until a task-based timer fires. This reduces the execution overhead of the Timer Server.

Timer Service Routines

The timer service routine should adhere to Ada calling conventions and have a prototype similar to the following:

procedure User_Routine(
Timer_ID  : in     RTEMS.ID;
User_Data : in     System.Address
);

Where the timer_id parameter is the RTEMS object ID of the timer which is being fired and user_data is a pointer to user-defined information which may be utilized by the timer service routine. The argument user_data may be NULL.

Operations

Creating a Timer

The rtems.timer_create directive creates a timer by allocating a Timer Control Block (TMCB), assigning the timer a user-specified name, and assigning it a timer ID. Newly created timers do not have a timer service routine associated with them and are not active.

Obtaining Timer IDs

When a timer is created, RTEMS generates a unique timer ID and assigns it to the created timer until it is deleted. The timer ID may be obtained by either of two methods. First, as the result of an invocation of the``rtems.timer_create`` directive, the timer ID is stored in a user provided location. Second, the timer ID may be obtained later using the``rtems.timer_ident`` directive. The timer ID is used by other directives to manipulate this timer.

Initiating an Interval Timer

The rtems.timer_fire_after and rtems.timer_server_fire_after directives initiate a timer to fire a user provided timer service routine after the specified number of clock ticks have elapsed. When the interval has elapsed, the timer service routine will be invoked from the``rtems.clock_tick`` directive if it was initiated by the rtems.timer_fire_after directive and from the Timer Server task if initiated by the``rtems.timer_server_fire_after`` directive.

Initiating a Time of Day Timer

The rtems.timer_fire_when and rtems.timer_server_fire_when directive initiate a timer to fire a user provided timer service routine when the specified time of day has been reached. When the interval has elapsed, the timer service routine will be invoked from the``rtems.clock_tick`` directive by the rtems.timer_fire_when directive and from the Timer Server task if initiated by the``rtems.timer_server_fire_when`` directive.

Canceling a Timer

The rtems.timer_cancel directive is used to halt the specified timer. Once canceled, the timer service routine will not fire unless the timer is reinitiated. The timer can be reinitiated using the rtems.timer_reset,``rtems.timer_fire_after``, and``rtems.timer_fire_when`` directives.

Resetting a Timer

The rtems.timer_reset directive is used to restore an interval timer initiated by a previous invocation of``rtems.timer_fire_after`` or``rtems.timer_server_fire_after`` to its original interval length. If the timer has not been used or the last usage of this timer was by the rtems.timer_fire_when or rtems.timer_server_fire_when directive, then an error is returned. The timer service routine is not changed or fired by this directive.

Initiating the Timer Server

The rtems.timer_initiate_server directive is used to allocate and start the execution of the Timer Server task. The application can specify both the stack size and attributes of the Timer Server. The Timer Server executes at a priority higher than any application task and thus the user can expect to be preempted as the result of executing the rtems.timer_initiate_server directive.

Deleting a Timer

The rtems.timer_delete directive is used to delete a timer. If the timer is running and has not expired, the timer is automatically canceled. The timer’s control block is returned to the TMCB free list when it is deleted. A timer can be deleted by a task other than the task which created the timer. Any subsequent references to the timer’s name and ID are invalid.

Directives

This section details the timer manager’s directives. A subsection is dedicated to each of this manager’s directives and describes the calling sequence, related constants, usage, and status codes.

TIMER_CREATE - Create a timer

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.. index:: create a timer

CALLING SEQUENCE:

procedure Timer_Create (
Name   : in     RTEMS.Name;
ID     :    out RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - timer created successfully RTEMS.INVALID_ADDRESS - id is NULL RTEMS.INVALID_NAME - invalid timer name RTEMS.TOO_MANY - too many timers created

DESCRIPTION:

This directive creates a timer. The assigned timer id is returned in id. This id is used to access the timer with other timer manager directives. For control and maintenance of the timer, RTEMS allocates a TMCB from the local TMCB free pool and initializes it.

NOTES:

This directive will not cause the calling task to be preempted.

TIMER_IDENT - Get ID of a timer

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.. index:: obtain the ID of a timer

CALLING SEQUENCE:

procedure Timer_Ident (
Name   : in     RTEMS.Name;
ID     :    out RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - timer identified successfully RTEMS.INVALID_ADDRESS - id is NULL RTEMS.INVALID_NAME - timer name not found

DESCRIPTION:

This directive obtains the timer id associated with the timer name to be acquired. If the timer name is not unique, then the timer id will match one of the timers with that name. However, this timer id is not guaranteed to correspond to the desired timer. The timer id is used to access this timer in other timer related directives.

NOTES:

This directive will not cause the running task to be preempted.

TIMER_CANCEL - Cancel a timer

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.. index:: cancel a timer

CALLING SEQUENCE:

procedure Timer_Cancel (
ID     : in     RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - timer canceled successfully RTEMS.INVALID_ID - invalid timer id

DESCRIPTION:

This directive cancels the timer id. This timer will be reinitiated by the next invocation of rtems.timer_reset,``rtems.timer_fire_after``, or``rtems.timer_fire_when`` with this id.

NOTES:

This directive will not cause the running task to be preempted.

TIMER_DELETE - Delete a timer

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.. index:: delete a timer

CALLING SEQUENCE:

procedure Timer_Delete (
ID     : in     RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - timer deleted successfully RTEMS.INVALID_ID - invalid timer id

DESCRIPTION:

This directive deletes the timer specified by id. If the timer is running, it is automatically canceled. The TMCB for the deleted timer is reclaimed by RTEMS.

NOTES:

This directive will not cause the running task to be preempted.

A timer can be deleted by a task other than the task which created the timer.

TIMER_FIRE_AFTER - Fire timer after interval

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.. index:: fire a timer after an interval

CALLING SEQUENCE:

procedure Timer_Fire_After (
ID        : in     RTEMS.ID;
Ticks     : in     RTEMS.Interval;
Routine   : in     RTEMS.Timer_Service_Routine;
User_Data : in     RTEMS.Address;
Result    :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - timer initiated successfully RTEMS.INVALID_ADDRESS - routine is NULL RTEMS.INVALID_ID - invalid timer id RTEMS.INVALID_NUMBER - invalid interval

DESCRIPTION:

This directive initiates the timer specified by id. If the timer is running, it is automatically canceled before being initiated. The timer is scheduled to fire after an interval ticks clock ticks has passed. When the timer fires, the timer service routine routine will be invoked with the argument user_data.

NOTES:

This directive will not cause the running task to be preempted.

TIMER_FIRE_WHEN - Fire timer when specified

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.. index:: fire a timer at wall time

CALLING SEQUENCE:

procedure Timer_Fire_When (
ID        : in     RTEMS.ID;
Wall_Time : in     RTEMS.Time_Of_Day;
Routine   : in     RTEMS.Timer_Service_Routine;
User_Data : in     RTEMS.Address;
Result    :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - timer initiated successfully RTEMS.INVALID_ADDRESS - routine is NULL RTEMS.INVALID_ADDRESS - wall_time is NULL RTEMS.INVALID_ID - invalid timer id RTEMS.NOT_DEFINED - system date and time is not set RTEMS.INVALID_CLOCK - invalid time of day

DESCRIPTION:

This directive initiates the timer specified by id. If the timer is running, it is automatically canceled before being initiated. The timer is scheduled to fire at the time of day specified by wall_time. When the timer fires, the timer service routine routine will be invoked with the argument user_data.

NOTES:

This directive will not cause the running task to be preempted.

TIMER_INITIATE_SERVER - Initiate server for task-based timers

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.. index:: initiate the Timer Server

CALLING SEQUENCE:

procedure Timer_Initiate_Server (
Server_Priority : in     RTEMS.Task_Priority;
Stack_Size      : in     RTEMS.Unsigned32;
Attribute_Set   : in     RTEMS.Attribute;
Result          :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - Timer Server initiated successfully RTEMS.TOO_MANY - too many tasks created

DESCRIPTION:

This directive initiates the Timer Server task. This task is responsible for executing all timers initiated via the``rtems.timer_server_fire_after`` or``rtems.timer_server_fire_when`` directives.

NOTES:

This directive could cause the calling task to be preempted.

The Timer Server task is created using the``rtems.task_create`` service and must be accounted for when configuring the system.

Even through this directive invokes the rtems.task_create and rtems.task_start directives, it should only fail due to resource allocation problems.

TIMER_SERVER_FIRE_AFTER - Fire task-based timer after interval

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.. index:: fire task-based a timer after an interval

CALLING SEQUENCE:

procedure Timer_Fire_Server_After (
ID        : in     RTEMS.ID;
Ticks     : in     RTEMS.Interval;
Routine   : in     RTEMS.Timer_Service_Routine;
User_Data : in     RTEMS.Address;
Result    :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - timer initiated successfully RTEMS.INVALID_ADDRESS - routine is NULL RTEMS.INVALID_ID - invalid timer id RTEMS.INVALID_NUMBER - invalid interval RTEMS.INCORRECT_STATE - Timer Server not initiated

DESCRIPTION:

This directive initiates the timer specified by id and specifies that when it fires it will be executed by the Timer Server.

If the timer is running, it is automatically canceled before being initiated. The timer is scheduled to fire after an interval ticks clock ticks has passed. When the timer fires, the timer service routine routine will be invoked with the argument user_data.

NOTES:

This directive will not cause the running task to be preempted.

TIMER_SERVER_FIRE_WHEN - Fire task-based timer when specified

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.. index:: fire a task-based timer at wall time

CALLING SEQUENCE:

procedure Timer_Fire_Server_When (
ID        : in     RTEMS.ID;
Wall_Time : in     RTEMS.Time_Of_Day;
Routine   : in     RTEMS.Timer_Service_Routine;
User_Data : in     RTEMS.Address;
Result    :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - timer initiated successfully RTEMS.INVALID_ADDRESS - routine is NULL RTEMS.INVALID_ADDRESS - wall_time is NULL RTEMS.INVALID_ID - invalid timer id RTEMS.NOT_DEFINED - system date and time is not set RTEMS.INVALID_CLOCK - invalid time of day RTEMS.INCORRECT_STATE - Timer Server not initiated

DESCRIPTION:

This directive initiates the timer specified by id and specifies that when it fires it will be executed by the Timer Server.

If the timer is running, it is automatically canceled before being initiated. The timer is scheduled to fire at the time of day specified by wall_time. When the timer fires, the timer service routine routine will be invoked with the argument user_data.

NOTES:

This directive will not cause the running task to be preempted.

TIMER_RESET - Reset an interval timer

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.. index:: reset a timer

CALLING SEQUENCE:

procedure Timer_Reset (
ID     : in     RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - timer reset successfully RTEMS.INVALID_ID - invalid timer id RTEMS.NOT_DEFINED - attempted to reset a when or newly created timer

DESCRIPTION:

This directive resets the timer associated with id. This timer must have been previously initiated with either the``rtems.timer_fire_after`` or``rtems.timer_server_fire_after`` directive. If active the timer is canceled, after which the timer is reinitiated using the same interval and timer service routine which the original``rtems.timer_fire_after````rtems.timer_server_fire_after`` directive used.

NOTES:

If the timer has not been used or the last usage of this timer was by a rtems.timer_fire_when or``rtems.timer_server_fire_when`` directive, then the RTEMS.NOT_DEFINED error is returned.

Restarting a cancelled after timer results in the timer being reinitiated with its previous timer service routine and interval.

This directive will not cause the running task to be preempted.

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