Книга: Real-Time Concepts for Embedded Systems

1.2.2 Characteristics of Real-Time Systems

1.2.2 Characteristics of Real-Time Systems

The C&D system in the weapons defense system must calculate the anticipated flight path of the incoming missile quickly and guide the firing system to shoot the missile down before it reaches the destroyer. Assume T1 is the time the missile takes to reach the ship and is a function of the missile's distance and velocity. Assume T2 is the time the C&D system takes to activate the weapons firing control system and includes transmitting the firing coordinates plus the firing delay. The difference between T1 and T2 is how long the computation may take. The missile would reach its intended target if the C&D system took too long in computing the flight path. The missile would still reach its target if the computation produced by the C&D system was inaccurate. The navigation system in the cruise missile must respond to the changing terrain fast enough so that it can re-compute coordinates and guide the altitude control system to a new flight path. The missile might collide with a mountain if the navigation system cannot compute new flight coordinates fast enough, or if the new coordinates do not steer the missile out of the collision course.

Therefore, we can extract two essential characteristics of real-time systems from the examples given earlier. These characteristics are that real-time systems must produce correct computational results, called logical or functional correctness, and that these computations must conclude within a predefined period, called timing correctness.

It is important to note that we said the timing correctness is at least as important as the functional correctness. In some real-time systems, functional correctness is sometimes sacrificed for timing correctness. We address this point shortly after we introduce the classifications of real-time systems.

Similar to embedded systems, real-time systems also have substantial knowledge of the environment of the controlled system and the applications running on it. This reason is one why many real-time systems are said to be deterministic, because in those real-time systems, the response time to a detected event is bounded. The action (or actions) taken in response to an event is known a priori. A deterministic real-time system implies that each component of the system must have a deterministic behavior that contributes to the overall determinism of the system. As can be seen, a deterministic real-time system can be less adaptable to the changing environment. The lack of adaptability can result in a less robust system. The levels of determinism and of robustness must be balanced. The method of balancing between the two is system- and application-specific. This discussion, however, is beyond the scope of this book. Consult the reference material for additional coverage on this topic.

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