EC-20.1 Real Time as a Requirement: Deadlines, Latency and Jitter
You can state a timing requirement as a deadline with a consequence attached, measure the latency and jitter a system actually delivers, and say whether a superloop can meet the requirement or whether it needs a scheduler.
Before:EC-11. Data Communications and Computer NetworksEC-16. Ethics, Safety, Sustainability and Standards
Fast and on time are different properties, and most embedded trouble comes from confusing them. A system that usually responds in one millisecond and occasionally takes forty has a jitter problem, not a speed problem, and no amount of a faster processor fixes it. This topic makes timing a written requirement with a number and a consequence beside it, so that everything after it has something to be checked against.
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Hard, firm and soft deadlines, and what each one costs when it is missed
A deadline is only meaningful when the cost of missing it is written down beside it. A missed frame in a display is a shrug, a missed sample in a motor controller is a burnt winding, and the same code is correct in one system and dangerous in the other.
NPTEL: Real Time Operating System (IIT Kharagpur) · CourseLatency, response time and jitter, measured rather than estimated
Latency is how long a response takes, and jitter is how much that time varies from one occasion to the next. Both are measured with a pin and an oscilloscope before they are argued about, because a worst case that has never been observed is a guess.
NPTEL: Real-Time Systems (IIT Kharagpur) · CoursePeriodic, aperiodic and sporadic work, and why the difference matters
Work that arrives on a fixed period can be planned for exactly, work that arrives at unpredictable moments cannot, and work with a guaranteed minimum gap between arrivals sits between the two. Naming which kind you have decides which scheduling argument applies.
NPTEL: Real Time Operating System (IIT Kharagpur) · CourseWorst-case execution time, and why the average is the wrong number
A timing argument rests on the longest a piece of code can take, not the usual. Caches, interrupts and data-dependent loops all make the worst case much larger than the average, and a system sized on the average fails rarely and unpredictably.
Where a superloop stops working, and the symptoms it shows first
A single loop calling everything in turn is adequate until one piece of work becomes long or one deadline becomes short. The first symptom is usually a response time that grows whenever an unrelated feature is added, which is the point at which the structure rather than the code is wrong.
NPTEL: Real-Time Systems (IIT Kharagpur) · CourseWriting a timing requirement somebody else can test
A requirement such as fast enough cannot be verified or refused. A requirement giving the event, the deadline in milliseconds, the fraction of occasions it must hold and the consequence of missing it can be tested by a person who was not in the room when it was written.
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