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+@c
+@c  COPYRIGHT (c) 1988-2002.
+@c  On-Line Applications Research Corporation (OAR).
+@c  All rights reserved.
+@c
+@c  $Id$
+@c
+
+@chapter Clock Manager
+
+@cindex clock
+
+@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{@value{DIRPREFIX}clock_set} - Set system date and time
+@item @code{@value{DIRPREFIX}clock_get} - Get system date and time information
+@item @code{@value{DIRPREFIX}clock_tick} - Announce a clock tick
+@end itemize
+
+@section Background
+
+@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 @code{@value{DIRPREFIX}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.
+
+@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 @value{STRUCTURE} for the native time and date format:
+
+
+@ifset is-C
+@findex rtems_time_of_day
+@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
+@end ifset
+
+@ifset is-Ada
+@example
+type Time_Of_Day is
+   record
+      Year    : RTEMS.Unsigned32; -- year, A.D.
+      Month   : RTEMS.Unsigned32; -- month, 1 .. 12
+      Day     : RTEMS.Unsigned32; -- day, 1 .. 31
+      Hour    : RTEMS.Unsigned32; -- hour, 0 .. 23
+      Minute  : RTEMS.Unsigned32; -- minute, 0 .. 59
+      Second  : RTEMS.Unsigned32; -- second, 0 .. 59
+      Ticks   : RTEMS.Unsigned32; -- elapsed ticks between seconds
+   end record;
+@end example
+@end ifset
+
+
+The native date and time format is the only format
+supported when setting the system date and time using the
+@code{@value{DIRPREFIX}clock_get} directive.  Some applications
+expect to operate on a "UNIX-style" date and time data structure.  The 
+@code{@value{DIRPREFIX}clock_get} directive can optionally return
+the current date and time in the
+following @value{STRUCTURE}:
+
+@ifset is-C
+@example
+@group
+typedef struct @{
+  rtems_unsigned32 seconds;       /* seconds since RTEMS epoch*/
+  rtems_unsigned32 microseconds;  /* since last second        */
+@} rtems_clock_time_value;
+@end group
+@end example
+@end ifset
+
+@ifset is-Ada
+@example
+type Clock_Time_Value is
+   record
+      Seconds      : Unsigned32;
+      Microseconds : Unsigned32;
+   end record;
+@end example
+@end ifset
+
+The seconds field in this @value{STRUCTURE} is the number of
+seconds since the RTEMS epoch of January 1, 1988.
+
+@subsection Clock Tick and Timeslicing
+
+@cindex 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 @code{@value{DIRPREFIX}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.
+
+@subsection Delays
+
+@cindex 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
+@code{@value{DIRPREFIX}task_wake_after}
+and @code{@value{DIRPREFIX}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.
+
+@subsection Timeouts
+
+@cindex timeouts
+
+Timeouts are a special type of timer automatically
+created when the timeout option is used on the
+@code{@value{DIRPREFIX}message_queue_receive},
+@code{@value{DIRPREFIX}event_receive},
+@code{@value{DIRPREFIX}semaphore_obtain} and
+@code{@value{DIRPREFIX}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.
+
+@section Operations
+
+@subsection Announcing a Tick
+
+RTEMS provides the @code{@value{DIRPREFIX}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
+@code{@value{DIRPREFIX}clock_tick} directive per second.  The
+frequency of @code{@value{DIRPREFIX}clock_tick}
+calls determines the resolution
+(granularity) for all time dependent RTEMS actions.  For
+example, calling @code{@value{DIRPREFIX}clock_tick}
+ten times per second yields a higher
+resolution than calling @code{@value{DIRPREFIX}clock_tick}
+two times per second.  The @code{@value{DIRPREFIX}clock_tick}
+directive is responsible for maintaining both
+calendar time and the dynamic set of timers.
+
+@subsection Setting the Time
+
+The @code{@value{DIRPREFIX}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 @code{@value{DIRPREFIX}clock_set} directive.
+
+@subsection Obtaining the Time
+
+The @code{@value{DIRPREFIX}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
+@code{@value{DIRPREFIX}clock_get} directive is
+dependent on the option selected by the caller.  This
+is specified using one of the following constants
+associated with the enumerated type
+@code{@value{DIRPREFIX}clock_get_options}:
+
+@findex rtems_clock_get_options 
+
+@itemize @bullet
+@item @code{@value{RPREFIX}CLOCK_GET_TOD} - obtain native style date and time
+
+@item @code{@value{RPREFIX}CLOCK_GET_TIME_VALUE} - obtain UNIX-style
+date and time
+
+@item @code{@value{RPREFIX}CLOCK_GET_TICKS_SINCE_BOOT} - obtain number of ticks
+since RTEMS was initialized
+
+@item @code{@value{RPREFIX}CLOCK_GET_SECONDS_SINCE_EPOCH} - obtain number
+of seconds since RTEMS epoch
+
+@item @code{@value{RPREFIX}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.
+
+@section Directives
+
+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.
+
+@c
+@c
+@c
+@page
+@subsection CLOCK_SET - Set system date and time
+
+@subheading CALLING SEQUENCE:
+
+@cindex set the time of day
+
+@ifset is-C
+@findex rtems_clock_set
+@example
+rtems_status_code rtems_clock_set(
+  rtems_time_of_day *time_buffer
+);
+@end example
+@end ifset
+
+@ifset is-Ada
+@example
+procedure Clock_Set (
+   Time_Buffer : in     RTEMS.Time_Of_Day;
+   Result      :    out RTEMS.Status_Codes
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}SUCCESSFUL} - date and time set successfully@*
+@code{@value{RPREFIX}INVALID_ADDRESS} - @code{time_buffer} is NULL@*
+@code{@value{RPREFIX}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 @value{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
+@code{@value{DIRPREFIX}clock_set} is required to re-initialize
+the system date and time to application specific specifications.
+
+@c
+@c
+@c
+@page
+@subsection CLOCK_GET - Get system date and time information
+
+@cindex obtain the time of day
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_clock_get
+@example
+rtems_status_code rtems_clock_get(
+  rtems_clock_get_options  option,
+  void                    *time_buffer
+);
+@end example
+@end ifset
+
+@ifset is-Ada
+@example
+procedure Clock_Get (
+   Option      : in     RTEMS.Clock_Get_Options;
+   Time_Buffer : in     RTEMS.Address;
+   Result      :    out RTEMS.Status_Codes
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}SUCCESSFUL} - current time obtained successfully@*
+@code{@value{RPREFIX}NOT_DEFINED} - system date and time is not set@*
+@code{@value{RPREFIX}INVALID_ADDRESS} - @code{time_buffer} is NULL
+
+@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 @code{@value{RPREFIX}CLOCK_GET_SECONDS_SINCE_EPOCH},
+@code{@value{RPREFIX}CLOCK_GET_TOD}, or
+@code{@value{RPREFIX}CLOCK_GET_TIME_VALUE}) and the date and time
+has not been set with a previous call to
+@code{@value{DIRPREFIX}clock_set}, then the
+@code{@value{RPREFIX}NOT_DEFINED} status code is returned. 
+The caller can always obtain the number of ticks per second (option is
+@code{@value{RPREFIX}CLOCK_GET_TICKS_PER_SECOND}) and the number of
+ticks since the executive was initialized option is
+@code{@value{RPREFIX}CLOCK_GET_TICKS_SINCE_BOOT}).
+
+The @code{option} argument may taken on any value of the enumerated
+type @code{rtems_clock_get_options}.  The data type expected for
+@code{time_buffer} is based on the value of @code{option} as
+indicated below:
+
+@findex rtems_clock_get_options 
+@ifset is-C
+@itemize @bullet
+@item @code{@value{RPREFIX}CLOCK_GET_TOD} - (rtems_time_of_day *)
+
+@item @code{@value{RPREFIX}CLOCK_GET_TIME_VALUE} - (rtems_clock_time_value *)
+
+@item @code{@value{RPREFIX}CLOCK_GET_TICKS_SINCE_BOOT} - (rtems_interval *)
+
+@item @code{@value{RPREFIX}CLOCK_GET_SECONDS_SINCE_EPOCH} - (rtems_interval *)
+
+@item @code{@value{RPREFIX}CLOCK_GET_TICKS_PER_SECOND} - (rtems_interval *)
+
+@end itemize
+@end ifset
+
+@ifset is-Ada
+@itemize @bullet
+@item @code{@value{RPREFIX}CLOCK_GET_TOD} - Address of an variable of
+type RTEMS.Time_Of_Day
+
+@item @code{@value{RPREFIX}CLOCK_GET_TIME_VALUE} - Address of an variable of
+type RTEMS.Clock_Time_Value
+
+@item @code{@value{RPREFIX}CLOCK_GET_TICKS_SINCE_BOOT} - Address of an
+variable of type RTEMS.Interval
+
+@item @code{@value{RPREFIX}CLOCK_GET_SECONDS_SINCE_EPOCH} - Address of an
+variable of type RTEMS.Interval
+
+@item @code{@value{RPREFIX}CLOCK_GET_TICKS_PER_SECOND} - Address of an
+variable of type RTEMS.Interval
+
+@end itemize
+@end ifset
+
+@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
+@code{@value{DIRPREFIX}clock_set} is required to re-initialize the
+system date and time to application specific specifications.
+
+@c
+@c
+@c
+@page
+@subsection CLOCK_TICK - Announce a clock tick
+
+@cindex clock tick
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_clock_tick
+@example
+rtems_status_code rtems_clock_tick( void );
+@end example
+@end ifset
+
+@ifset is-Ada
+@example
+procedure Clock_Tick (
+   Result :    out RTEMS.Status_Codes
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}SUCCESSFUL} - clock tick processed 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.
+