Archive for January, 2010

Is Toyota’s Accelerator Problem Caused by Embedded Software Bugs?

Thursday, January 28th, 2010 Michael Barr

Last month I received an interesting e-mail in response to a column I wrote for Embedded Systems Design called The Lawyers are Coming! My column was partly about the poor state of embedded software quality across all industries, and my correspondent was writing to say my observations were accurate from his perch within the automotive industry. Included in his e-mail was this interesting tidbit:

I read something about the big Toyota recall being related to floor mats interfering with the accelerator, but I was told that the problem appears to be software (firmware) for the control-by-wire pedal.  Me thinks somebody probably forgot to check ranges, overflows, or stability properly when implementing the “algorithm”.

As background for those of you who have been working in SCIFs or other labs, the “big Toyota recall” was first announced in September 2009. It was said to concern removable floor mats causing the accelerator pedal to be pressed down. Some 3.8 million Toyota and Lexus vehicles were involved and owners were told to remove floor mats immediately.

This week several related major news events have transpired, including:

But none of the articles I’ve read have talked about software being a cause. And it’s not clear if the affected models are drive-by-wire. However, at least one article I read yesterday suggested that one fix being worked on is a software interlock to ensure that if both the brake and the gas pedal are depressed, the brake will override the accelerator. On the one hand, that seems to mean that software is already in the middle; on the other, I would be extremely surprised to learn that such an interlock wasn’t already present in a drive-by-wire system.

So what’s the story? Are embedded software bugs to blame for this massive recall? Do you know? Have you found any helpful articles pointing at software problems? Please share what you know in the comments below, or e-mail me privately.

Firmware Update – A Free Newsletter for Firmware Engineers

Tuesday, January 26th, 2010 Michael Barr

I’ve been writing about the practice of embedded software development–in the form of books, articles, columns, conference papers, and blog posts–for nearly 15 years.  (How time flies…)  I also served as editor-in-chief of Embedded Systems Design magazine for about 3-1/2 years in the middle.  But it wasn’t until August of last year that it occurred to me to write an e-mail newsletter.

My newsletter is called Firmware Update, and it is published about every 3 weeks.  The stated mission of Firmware Update is to spread the word about the firmware development best practices I have learned in my career as an engineer, consultant, and trainer.  In addition to connecting my past, present, and future writings into a coherent storyline, I am using the newsletter to link to articles and papers by others who influence my thinking.

In less than six months, the newsletter is up to more than 11,000 subscribers.  We’ve placed a helpful archive of all past issues at  And I’m working hard to make the format as easy and fun to read as it is informative.  If you develop embedded software, I’m certain you will find it valuable.   If you’re not already a subscriber, you can join the mailing list at

Note that each issue of Firmware Update is Copyright 2009-2010 by Netrino, LLC.  However, you may reprint the newsletter for non-commercial purposes. Indeed, I encourage you to forward it to colleagues who may benefit from the information.

Rate Monotonic Analysis and Round Robin Scheduling

Friday, January 22nd, 2010 Michael Barr

Rate Monotonic Analysis (RMA) is a way of proving a priori via mathematics (rather than post-implementation via testing) that a set of tasks and interrupt service routines (ISRs) will always meet their deadlines–even under worst-case timing.  In this blog, I address the issue of what to do if two or more tasks or ISRs have equal priority and whether round robin scheduling is necessary in an RTOS to deal with that special case.

First a little background.  In order for the schedulability analysis portion of the RMA mathematics to provide meaningful results, the following assumptions must hold:

Under RMA, the relative priorities are assigned according to a simple rule: “The more often a task or ISR runs (in the worst-case), the higher its priority.” Put another way, the task or ISR with the longest period between iterations (interarrival time, if you prefer) is least important. This is because an infrequent but high-priority task could prevent a more frequent task from missing an entire iteration.

So what happens if you are using RMA to assign priorities and you wind up with two (or more) tasks or ISRs assigned equal priority? (Translation: they have the same worst-case interarrival times). Must they be assigned equal priority in the real system? What if the RTOS (in the case of tasks) or hardware (in the case of interrupts) doesn’t support round-robin scheduling–or even equal priorities with run-to-completion?

Interestingly, it turns out not to matter a bit whether you:

  1. Merge the two tasks into one (i.e., executed code for Task A then Task B).
  2. Give them equal priority, either with round robin or run-to-completion behavior.
  3. Give them adjacent unequal priorities (in either relative order).

If you run through the timing diagrams for each of the above scenarios, you’ll see that all three are equivalent. Except that the equal priority with round robin potentially suffers a performance impact from unnecessary additional context switches.

Firmware Wall of Shame: Kenmore Elite Electric Range

Monday, January 11th, 2010 Michael Barr

A couple of years back, my wife and I remodeled our kitchen. In the process, we replaced our oven and range with a Kenmore Elite slide-in unit, similar to the one in the picture below. My wife was pretty nervous about buying an oven with a display and a keyboard–because she understood that meant embedded software with its all-too-frequent crashes and upgrades. At the time, I assured her that oven controller firmware was the sort of thing anyone who could spell ‘C’ could write.

But now my day of reckoning has come. Our 3-year old oven isn’t working properly. It even failed my wife on Christmas Eve, as she prepared a meal for about 20 family and friends. (Praise be for a full tank of gas and a 3-burner outdoor grill!) But still I felt vindicated. Our oven problem was with the electronics not the firmware, I assured her–as if that were some great thing in itself! The problem only occurred when the oven was hot. And a power-cycle didn’t cure it. We have learned that the buttons and display will work again, eventually, after the heat has dissipated.

Today the repairman is here. (I didn’t dare void the warranty by peeking at the electronics inside before he came.) “What error code does it give when it fails?,” he wants to know. “F-1-?,” I reported quickly. “We can’t read the last digit, because that’s a part of the display that doesn’t work when the oven fails in this way.” “Hmm.”, he muttered, turning to his repair manual, “the fix for F10 is as different from the fix for F19 as for every error code in between.” “Can’t you hook up your laptop to the oven’s diagnostic serial port?,” I wanted to know. “Nope,” he replied, “The display is the diagnostic port.”

Crap. My wife was right. Writing the embedded software for an oven controller is harder than I thought. The designers of the Kenmore Elite slide-in electric range’s firmware forgot to account for the fact that they only had one diagnostic port and that it itself might break. Or they knew it and cheated their customers (including us), to reduce the BOM cost, out of a serial port we wouldn’t know we didn’t have until it was too late. Either way, shame on them.

Worst-Case Context Switch Times by RTOS

Wednesday, January 6th, 2010 Michael Barr

This morning I received an e-mail from an embedded software developer. It read in part:

We are trying to find the best case, average, and worst-case context switch times for the ThreadX and eCOS real-time operating systems. I have searched the Internet extensively. I found one source stating that the ThreadX context switch time can be under 1 microsecond, but it was unclear if that was the best-case, average, or worst-case timing. Can you help us?

As questions like this keep coming, I shall respond via this blog.

None of the timings sought (even the 1 microsecond timing found online) can be calculated without knowledge of the specific processor family, clock speed, and memory architecture. Context switch code is generally written in assembly language and mostly consists of pushing a number of CPU register contents to RAM and popping older data from RAM into registers. The primary factors in context switch timing are the number of opcodes involved, the speed of their execution, and RAM access speeds.

Note though that even for a given hardware platform, I am unaware of any analytical use of any but the worst-case context switch timings for an RTOS. ThreadX purveyor ExpressLogic should, like any RTOS vendor, be willing and able to provide a prospective customer an estimate of the worst-case context switch timing on their planned hardware. But you will want to validate that number on your final hardware before performing Rate Monotonic Analysis (RMA) to prove that all critical deadlines will be met.

Related Article: How to Choose a Real-Time Operating System.