RTEMS C User's Guide
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.
By tracking time in units of ticks, RTEMS is capable
of supporting interval timing functions such as task delays,
timeouts, timeslicing, the delayed 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.
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 large.
The rate monotonic scheduling algorithm is a hard real-time scheduling methodology. This methodology provides rules which allows one to guarantee that a set of independent periodic tasks will always meet their deadlines -- even under transient overload conditions. The rate monotonic manager provides directives built upon the Clock Manager's interval timer support routines.
Interval timing is not sufficient for the many
applications which require that 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 time in RTEMS services.
See Clock Manager Time and Date Data Structures.
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.
RTEMS C User's Guide
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