Idling along, (or what to do in the idle task)

If you are using an RTOS in your latest design then no doubt you have an idle task. (Most of the time, the idle task is explicit and is the user task with the lowest priority; sometimes it’s built into the RTOS). It’s been my experience that the idle task is an interesting beast. On the one hand it is what gets executed when there’s nothing else to do, and thus inherently contains nothing directly related the product. On the other hand it’s this wonderful resource that you can exploit to do all sorts of interesting things to improve your product without having to worry too much about it impacting the running of your product.

With that being said, here are some suggestions for what your idle task can do, starting with one thing it shouldn’t do.

Do Nothing

If your idle task consists of a do-nothing loop, then you are almost certainly missing an opportunity. Hopefully the suggestions below will serve to spark your creative juices.

Watchdog

In any RTOS based design, the idle task should play a role in the overall watchdog supervisor. Without going into a treatise on watchdog design, suffice it to say if the idle task isn’t being run frequently then you’ve got a problem. Thus if the idle task doesn’t feature in your watchdog supervisor then you are doing something wrong. Note that if the idle task is featured in your watchdog, then it doesn’t necessarily mean you are doing it right either!

Power Save

For power constrained systems, the idle task is usually the place to put the microprocessor to sleep and / or indulge in other power saving strategies. I often find that my idle tasks consist of some of the features described here, plus power save. In other words, the idle task takes care of some housekeeping and then takes a nap.

Load calculation

Used in conjunction with hardware timers and the task switch hook function, it’s normally possible to construct a system that gives a decent indication of both overall CPU load and also the CPU utilization of each of the tasks. The idle task isn’t a bad place to do all the calculations. I find this a very useful diagnostic aid as I’m developing a system. Once you are done using it as a development aid, with a bit more work it can be modified to be part of your overall watchdog strategy, in that it can provide useful information about how tasks are (mis)behaving.

Flash Check

Just about every embedded system I’ve looked at in the last decade or two performs a CRC check on program memory on start up. However, if you are designing a system that is safety critical and / or expected to run a long time between power cycles, then you should seriously consider running a Flash CRC check in the idle task. Because no writing of memory is involved and one is instead reading memory that is supposed to be constant, there are are no real race-conditions to worry about and thus there’s no need to be entering critical sections to perform the reads. Of course if you are using an MMU or MPU then things might get a little more challenging. Naturally such a challenge is nothing for a reader of your ability! [As an aside, one of my electrical engineering professors used to say to me, “Nigel, this is nothing for a man of your ability!” One is of course simultaneously flattered, irritated and motivated. I’ve never forgotten it.]

RAM Check

This is of the course the evil twin to the Flash check. However this time you need to perform both reads and writes from locations that are being used by higher priority tasks and interrupts. You can of course only do this safely by executing a suitable lock / unlock procedure on each RAM location. Now doing this in the idle task could seriously change your system’s response time, so you need to think very seriously about how to structure such a test. A good starting point is to do just one locked read / write per idle task invocation. Of course if that results in a 10 year RAM test on your system, then you’ll need to rethink the strategy.

If you have other good ideas for idle task work then please leave them in the comments.

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