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+@c
+@c  COPYRIGHT (c) 1996.
+@c  On-Line Applications Research Corporation (OAR).
+@c  All rights reserved.
+@c
+
+@ifinfo
+@node Clock Manager, Clock Manager Introduction, INTERRUPT_CATCH - Establish an ISR, Top
+@end ifinfo
+@chapter Clock Manager
+@ifinfo
+@menu
+* Clock Manager Introduction::
+* Clock Manager Background::
+* Clock Manager Operations::
+* Clock Manager Directives::
+@end menu
+@end ifinfo
+
+@ifinfo
+@node Clock Manager Introduction, Clock Manager Background, Clock Manager, Clock Manager
+@end ifinfo
+@section Introduction
+
+The clock manager provides support for time of day
+and other time related capabilities.  The directives provided by
+the clock manager are:
+
+@itemize @bullet
+@item @code{clock_set} - Set system date and time
+@item @code{clock_get} - Get system date and time information
+@item @code{clock_tick} - Announce a clock tick
+@end itemize
+
+@ifinfo
+@node Clock Manager Background, Required Support, Clock Manager Introduction, Clock Manager
+@end ifinfo
+@section Background
+@ifinfo
+@menu
+* Required Support::
+* Time and Date Data Structures::
+* Clock Tick and Timeslicing::
+* Delays::
+* Timeouts::
+@end menu
+@end ifinfo
+
+@ifinfo
+@node Required Support, Time and Date Data Structures, Clock Manager Background, Clock Manager Background
+@end ifinfo
+@subsection 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.
+
+@ifinfo
+@node Time and Date Data Structures, Clock Tick and Timeslicing, Required Support, Clock Manager Background
+@end ifinfo
+@subsection 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:
+
+@example
+struct rtems_tod_control @{
+  rtems_unsigned32 year;   /* greater than 1987 */
+  rtems_unsigned32 month;  /* 1 - 12 */
+  rtems_unsigned32 day;    /* 1 - 31 */
+  rtems_unsigned32 hour;   /* 0 - 23 */
+  rtems_unsigned32 minute; /* 0 - 59 */
+  rtems_unsigned32 second; /* 0 - 59 */
+  rtems_unsigned32 ticks;  /* elapsed between seconds */
+@};
+
+typedef struct rtems_tod_control rtems_time_of_day;
+@end example
+
+
+The native date and time format is the only format
+supported when setting the system date and time using the
+clock_get directive.  Some applications expect to operate on a
+"UNIX-style" date and time data structure.  The clock_get
+directive can optionally return the current date and time in the
+following structure:
+
+@example
+@group
+typedef struct @{
+  rtems_unsigned32 seconds;       /* seconds since RTEMS epoch*/
+  rtems_unsigned32 microseconds;  /* since last second        */
+@} rtems_clock_time_value_control;
+@end group
+@end example
+
+
+The seconds field in this structure is the number of
+seconds since the RTEMS epoch of January 1, 1988.
+
+@ifinfo
+@node Clock Tick and Timeslicing, Delays, Time and Date Data Structures, Clock Manager Background
+@end ifinfo
+@subsection 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 directive implements 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.
+
+@ifinfo
+@node Delays, Timeouts, Clock Tick and Timeslicing, Clock Manager Background
+@end ifinfo
+@subsection 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
+task_wake_after and 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.
+
+@ifinfo
+@node Timeouts, Clock Manager Operations, Delays, Clock Manager Background
+@end ifinfo
+@subsection Timeouts
+
+Timeouts are a special type of timer automatically
+created when the timeout option is used on the
+message_queue_receive, event_receive, semaphore_obtain and
+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.
+
+@ifinfo
+@node Clock Manager Operations, Announcing a Tick, Timeouts, Clock Manager
+@end ifinfo
+@section Operations
+@ifinfo
+@menu
+* Announcing a Tick::
+* Setting the Time::
+* Obtaining the Time::
+@end menu
+@end ifinfo
+
+@ifinfo
+@node Announcing a Tick, Setting the Time, Clock Manager Operations, Clock Manager Operations
+@end ifinfo
+@subsection Announcing a Tick
+
+RTEMS provides the clock_tick directive which is
+called from the user's real-time clock ISR to inform RTEMS that
+a tick has elapsed.  The tick frequency value, defined in
+microseconds, is a configuration parameter found in the
+Configuration Table.  RTEMS divides one million microseconds
+(one second) by the number of microseconds per tick to determine
+the number of calls to the clock_tick directive per second.  The
+frequency of clock_tick calls determines the resolution
+(granularity) for all time dependent RTEMS actions.  For
+example, calling clock_tick ten times per second yields a higher
+resolution than calling clock_tick two times per second.  The
+clock_tick directive is responsible for maintaining both
+calendar time and the dynamic set of timers.
+
+@ifinfo
+@node Setting the Time, Obtaining the Time, Announcing a Tick, Clock Manager Operations
+@end ifinfo
+@subsection Setting the Time
+
+The clock_set directive allows a task or an ISR to
+set the date and time maintained by RTEMS.  If setting the date
+and time causes any outstanding timers to pass their deadline,
+then the expired timers will be fired during the invocation of
+the clock_set directive.
+
+@ifinfo
+@node Obtaining the Time, Clock Manager Directives, Setting the Time, Clock Manager Operations
+@end ifinfo
+@subsection Obtaining the Time
+
+The clock_get directive allows a task or an ISR to
+obtain the current date and time or date and time related
+information.  The current date and time can be returned in
+either native or UNIX-style format.  Additionally, the
+application can obtain date and time related information such as
+the number of seconds since the RTEMS epoch, the number of ticks
+since the executive was initialized, and the number of ticks per
+second.  The information returned by the clock_get directive is
+dependent on the option selected by the caller.  The following
+options are available:
+
+@itemize @bullet
+@item CLOCK_GET_TOD - obtain native style date and time
+@item CLOCK_GET_TIME_VALUE - obtain UNIX-style date and time
+@item CLOCK_GET_TICKS_SINCE_BOOT - obtain number of ticks since RTEMS was initialized
+@item CLOCK_GET_SECONDS_SINCE_EPOCH - obtain number of seconds since RTEMS epoch
+@item CLOCK_GET_TICKS_PER_SECOND - obtain number of clock ticks per second
+@end itemize
+
+Calendar time operations will return an error code if
+invoked before the date and time have been set.
+
+@ifinfo
+@node Clock Manager Directives, CLOCK_SET - Set system date and time, Obtaining the Time, Clock Manager
+@end ifinfo
+@section Directives
+@ifinfo
+@menu
+* CLOCK_SET - Set system date and time::
+* CLOCK_GET - Get system date and time information::
+* CLOCK_TICK - Announce a clock tick::
+@end menu
+@end ifinfo
+
+This section details the clock 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.
+
+@page
+@ifinfo
+@node CLOCK_SET - Set system date and time, CLOCK_GET - Get system date and time information, Clock Manager Directives, Clock Manager Directives
+@end ifinfo
+@subsection CLOCK_SET - Set system date and time
+
+@subheading CALLING SEQUENCE:
+
+@example
+rtems_status_code rtems_clock_set(
+  rtems_time_of_day *time_buffer
+);
+@end example
+
+@subheading DIRECTIVE STATUS CODES:
+@code{SUCCESSFUL} - date and time set successfully@*
+@code{INVALID_TIME_OF_DAY} - invalid time of day
+
+@subheading DESCRIPTION:
+
+This directive sets the system date and time.  The
+date, time, and ticks in the time_buffer structure are all
+range-checked, and an error is returned if any one is out of its
+valid range.
+
+@subheading NOTES:
+
+Years before 1988 are invalid.
+
+The system date and time are based on the configured
+tick rate (number of microseconds in a tick).
+
+Setting the time forward may cause a higher priority
+task, blocked waiting on a specific time, to be made ready.  In
+this case, the calling task will be preempted after the next
+clock tick.
+
+Re-initializing RTEMS causes the system date and time
+to be reset to an uninitialized state.  Another call to
+clock_set is required to re-initialize the system date and time
+to application specific specifications.
+
+@page
+@ifinfo
+@node CLOCK_GET - Get system date and time information, CLOCK_TICK - Announce a clock tick, CLOCK_SET - Set system date and time, Clock Manager Directives
+@end ifinfo
+@subsection CLOCK_GET - Get system date and time information
+
+@subheading CALLING SEQUENCE:
+
+@example
+rtems_status_code rtems_clock_get(
+  rtems_clock_get_options  option,
+  void                    *time_buffer
+);
+@end example
+
+@subheading DIRECTIVE STATUS CODES:
+@code{SUCCESSFUL} - current time obtained successfully@*
+@code{NOT_DEFINED} - system date and time is not set
+
+@subheading DESCRIPTION:
+
+This directive obtains the system date and time.  If
+the caller is attempting to obtain the date and time (i.e.
+option is set to either CLOCK_GET_SECONDS_SINCE_EPOCH,
+CLOCK_GET_TOD, or CLOCK_GET_TIME_VALUE) and the date and time
+has not been set with a previous call to clock_set, then the
+NOT_DEFINED status code is returned.  The caller can always
+obtain the number of ticks per second (option is
+CLOCK_GET_TICKS_PER_SECOND) and the number of ticks since the
+executive was initialized option is CLOCK_GET_TICKS_SINCE_BOOT).
+
+@subheading NOTES:
+
+This directive is callable from an ISR.
+
+This directive will not cause the running task to be
+preempted.  Re-initializing RTEMS causes the system date and
+time to be reset to an uninitialized state.  Another call to
+clock_set is required to re-initialize the system date and time
+to application specific specifications.
+
+@page
+@ifinfo
+@node CLOCK_TICK - Announce a clock tick, Timer Manager, CLOCK_GET - Get system date and time information, Clock Manager Directives
+@end ifinfo
+@subsection CLOCK_TICK - Announce a clock tick
+
+@subheading CALLING SEQUENCE:
+
+@example
+rtems_status_code rtems_clock_tick( void );
+@end example
+
+@subheading DIRECTIVE STATUS CODES:
+@code{SUCCESSFUL} - current time obtained successfully
+
+@subheading DESCRIPTION:
+
+This directive announces to RTEMS that a system clock
+tick has occurred.  The directive is usually called from the
+timer interrupt ISR of the local processor.  This directive
+maintains the system date and time, decrements timers for
+delayed tasks, timeouts, rate monotonic periods, and implements
+timeslicing.
+
+@subheading NOTES:
+
+This directive is typically called from an ISR.
+
+The microseconds_per_tick and ticks_per_timeslice
+parameters in the Configuration Table contain the number of
+microseconds per tick and number of ticks per timeslice,
+respectively.
+