Page 66 - The Art of Designing Embedded Systems

CHAPTER 4

Real Time Means

Right Now!

We’re taught to think of our code in the procedural domain: that of
actions and effects. IF statements and control loops create a logical flow to
implement algorithms and applications. There’s a not-so-subtle bias in
college toward viewing correctness as being nothing more than stringing
the right statements together.
Yet embedded systems are the realm of real time, where getting the
result on time is just as important as computing the correct answer.
A hard real-time task or system is one where an activity simply must
be completed-always-by a specified deadline. The deadline may be a
particular time or time interval, or may be the arrival of some event. Hard
real-time tasks fail, by definition, if they miss such a deadline.
Notice that this definition makes no assumptions about the frequency
or period of the tasks. A microsecond or a week-if missing the deadline
induces failure, then the task has hard real-time requirements.
“Soft” real time, though, has a definition as weak as its name. By
convention it’s those class of systems that are not hard real time, though
generally there is some sort of timeliness requirement. If missing a dead-
line won’t compromise the integrity of the system, if generally getting the
output in a timely manner is acceptable, then the application’s real-time re-
quirements are “soft.” Sometimes soft real-time systems are those where
multi-valued timeliness is acceptable: bad, better, and best responses are
all within the scope of possible system operation,

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