Greg Wilson, one of the editors of the book, (the other editor is Amy Brown) tweeted that they were looking for coverage for embedded software. Well, I thought, I’ve been wanting to learn about FreeRTOS, and that fits the embedded and open source bills, so here we go!

I learned a lot reading the current edition of the book, and I hope that others can learn something from my forthcoming chapter on FreeRTOS.

http://www.ember.com/company_careers.html

We develop the chips, software, and tools for wireless sensor networks.

I’ve worked at Ember since December, and it’s the best place I’ve ever worked (and I’ve worked at some pretty good places!) And, no joke, all of my coworkers say the same thing. So either we’re that great of a place to work, or there’s something in the water, but either way, it’s awesome.

But, as Levar Burton says, you don’t have to take my word for it: We were just voted one of the top places to work in Boston: http://www.bizjournals.com/boston/news/2011/05/03/bbj-announces-best-places-to-work.html

Our QA engineers are more “developer-y” than many QA roles, so definitely check it out even if you’re not in the market for a more traditional QA position.

Email me if you’re interested!

http://www.ganssle.com/tem/tem199.htm

I subscribe to very few newsletters, and Jack’s is one of them.  I read every issue, it’s that good.

If you work in embedded software, or software of any kind, you should subscribe!  (I don’t get anything if you subscribe, I just think it’s a worthwhile read.)

While I’m in fanboy mode, I’ll also recommend Jack’s other articles – click on a random one, you’ll probably learn something. My personal favorite is his guide to debouncing.  He does some good experiments and then shows hardware and software solutions to the pesky debouncing problems we embedded folks face.

1. Introduction to Rate Monotonic Scheduling, by David Stewart and Michael Barr
2. 3 Things Every Programmer Should Know About RMA, by Michael Barr

The first article lays out the basics: RMA is an algorithm that assigns static priorities to periodic tasks in order to maximize “schedulability.”  That’s a mouthful: 16 words, 37 syllables.  And one of the words isn’t real: “schedulability,” which means “able to be scheduled so that all tasks complete before their deadline every time.”

Let’s try again: RMA helps tasks get done in time.  That’s better: 7 words, 9 syllables.

The second article expands on the basics of RMA, giving some additional guidelines for when it should be used, especially as it applies to interrupts (which it does!).

I want to talk a bit more about the basics of RMA.  RMA is easy to describe: a task’s priority is based on how often it runs.  The task that runs the most often gets the highest priority, the task that runs the second most often gets the next highest priority, and so on.  Or, to quote the first article, “Assign the priority of each task according to its period, so that the shorter the period the higher the priority.”

RMA does not guarantee that your tasks will all complete in time; RMA does guarantee to find the optimal static prioritization assignment if one exists.  If RMA doesn’t produce a “schedulable” task set then no “schedulable” static priority scheme exists.

RMA’s greedy nature makes intuitive sense: starting at time zero, assume all tasks are ready to run.  Then run the task with the shortest period.  Since this task runs more often than any other task, it “feels” right to run it first to give it the best chance of finishing before it has to run again.  Next run the task with the next shortest period, and so on, until all tasks have been run.

Fortunately the intuition and “feel” that RMA works is backed by the fact that it does!

I’ve given a very basic overview of RMA here.  Please check out Stewart and Barr’s articles for more about RMA.

I came across this fantastic quote by Steve Wozniak in “Making it Big in Software,” by Sam Lightstone.  Woz is talking about designing the Apple II:

“And I did every piece of software from the ground up, through applications that you can’t pin down for any one of them.  The hardware was so interrelated that I can’t really divide it into software and hardware alone.  Those days were that way.  Today, if you work on embedded processors, you put a little microprocessor into a small product.  That’s the job in the world that I would love to this day! That’s what I did back then; it mixed both hardware and software.”

Sounds like Woz wants my job.  :-)