Keynote

Really Real Time

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32:32

EOC 2024

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Jack Ganssle

Jack Ganssle is an internationally-recognized embedded systems engineer, author and speaker. He designed some of the very first embedded systems. He started three high tech companies, built or managed the development of over 100 embedded systems, and now advises companies, lectures, and writes about embedded systems. He was the only embedded person on NASA's Super Problem Resolution Team, created by the Columbia Accident Investigation Board to harness the expertise of highly-respected people from outside the agency. He has worked on many classified government projects. Full bio here.

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Keynote

Really Real Time

Jack Ganssle

32:32 EOC 2024

Jack Ganssle

32:32

Real-time systems must respond to events in a timely manner. We view this as a software thing, when in fact it is a systems problem. The hardware and software should be tuned to achieve the response times we need.

In this talk you’ll learn how to profile software execution times, understand hardware design tradeoffs for working with high-speed systems, and we’ll look at some common myths about how real-time systems should be designed. And, as embedded people, we often have the ability to trade off hardware versus software to achieve the best performance.

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What did Jack identify as the primary practical obstacle to applying rate-monotonic analysis (RMA) to guarantee real-time behavior?

A Accurately determining each task's worst-case execution time is very difficult because compilers and toolchains hide time-domain behavior.

B RMA requires keeping CPU utilization below 1%, which is impossible in embedded systems.

C Preemptive multitasking is always deterministic, so RMA is unnecessary.

D RMA cannot be used if any task uses floating-point math.

E RMA fails because RTOSs never allow fixed task periods.

Which low-cost technique did Jack recommend to measure percent idle time on an embedded target without an expensive profiler?

A Toggle a GPIO in the RTOS idle task and connect it to a modified cheap analog voltmeter so the needle indicates percent idle.

B Use a handheld digital multimeter to directly measure the duty cycle on the idle GPIO pin.

C Place a standard 10 pF oscilloscope probe on the idle GPIO pin and read duty cycle from the scope.

D Implement a software logger that prints CPU idle percentage over serial and read it on the host.

E Rely solely on the RTOS built-in CPU usage report which always gives exact idle percent.

Why can attaching a typical oscilloscope probe to a logic gate output cause a previously-working board to fail, according to Jack?

A The probe's input capacitance presents a low impedance at relevant frequencies and draws more current than the gate can source, upsetting or crashing the circuit.

B Probes always short the signal node to ground at high frequencies, instantly destroying the gate.

C Oscilloscope probes convert digital signals to analog voltages that logic gates cannot tolerate.

D Probes emit electromagnetic fields that flip logic states on the board.

E Attaching a probe increases the system clock frequency by coupling energy back into the oscillator.

Why did Jack advise against using the non-maskable interrupt (NMI) for regularly-occurring interrupts?

A Because the NMI cannot be disabled and will break protections around non-reentrant code that rely on disabling interrupts.

B Because the NMI has lower priority than all other interrupts and will often be missed.

C Because the NMI is so slow that it introduces unacceptable latency for frequent events.

D Because most processors do not implement an NMI vector, making it unreliable.

E Because the NMI automatically cleans up global variables which can erase state unexpectedly.

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DS

Score: 0 | 2 years ago | no reply

I always love to hear Jack's thoughts on the industry, development practices, and design philosophy. Over the years, he has greatly enhanced my professional development - starting with his early books, "The Art of Designing Embedded Systems" and "The Art of Programming Embedded Systems", his prolific contributions to "Embedded Systems Programming" / "Embedded Systems Design" magazine, and his "Embedded Muse" newsletter. His website has lots of good, crunchy embedded systems articles (https://www.ganssle.com/articles-subj.htm) that are very topical and extremely relevant. Many thanks for all the contributions he makes to the field.

Andre.Hartmann

Score: 0 | 2 years ago | no reply

Jack, thank you very much for the presentation and especially the Q&A session.
Your experience is a treasure chest for the younger engineers. And while it's ok to let others take the lead, as long as you have something to say, please do. It's a great pleasure to hear this long expertise in the field, and especially anecdotes from the early days that we younger ones didn't get to experience.
Thank you once again!

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