keeping simple

You can find an article on writing real-time control loops in RTLinux inside the giant Hristu-Varsakelis, Levine Handbook of Networked and Embedded Control. The article is by Edgar Hilton, Matt Sherer and myself and covers simple loops, loops with data and control going to the non-real-time “client” and loops using the Interface Kit (XML/RPC).

One of the longstanding problems with operating systems is that there is no way to validate their correctness in the same way that engineers can calculate the ability of a beam to carry a weight or a wire to carry a current. Well, the problem is worse than that, because we don’t even have a precise way of stating exactly what we want the OS to do. We have APIs, a good thing, but suppose that you want to show that an implementation of pthread_create is correct. The first question is correct according to what specification and then you start trying to figure out a precise way to state the requirements implied by the man page. Not easy. At least for me. This paper is a start.

I wrote a chapter for an academic book recently and the publisher sent me a “copyright assignment” form ( don’t even start) that asked me, among other things, to provide a warranty that it was perfectly safe to use the methods I described. Well, to be plain – it’s not. Real-time software gives you a plethora of opportunities for causing damage (there was that programmer in Jersey, not using RTLinux, who discovered his mistake when the placer robot started tossing 50Kg blocks of steel at him). So please let’s not have a “real-time programming for dummies” anytime soon. But it’s a lot easier to write correct real-time software, even for smart and well educated people, if the operating system is not making things unnecessarily complex. In fact, that’s a good description of our mission at FSMLabs: rooting out and destroying unnecessary complexity in real-time systems. The PSDD system by Barabanov is pretty impressive at that, but here’s how to write a simple data acquisition loop in vanilla RTLinux courtesy of the “main” system mostly due to Cort Dougan. This is a quantum jump over the original “Linux module” stuff we had in the old days and it opens up many new improvements. Note that standard out even works! Proof that “simple” and “raw” are different.

The basic loop looks like”while(running){ input data; forward data; wait until next period;}. A RTLinuxPro application to implement this consists of a Linux/BSD executable dot-rtl program and some Linux or BSD post processing. The dot-rtl program usually has the form ” thread; main(){}” where the main routine initializes data and starts the thread. A simple version would output to standard out. Then you could use a command line “myprogram.rtl > logfile” or “myprogram.rtl | postprocess”to log or postprocess the data. Here is an example running at a period of 200 microseconds with data consisting of a single 32bit value and a count that can be used as a kind of timestamp. Just add some error checking and go.

#include
#include
#include

}


I was at some trade show wandering around with a cynic who was pointing out that every booth advertised productivity improvements. How many improvements we’ve made to productivity, and this has gone on for years” he said,“By now, we must be able to make incredible things with no effort.” In the embedded world, productivity is, of course, not incredible – or at least not incredibly good – but this can change. Using Linux or BSD as platforms in embedded development is a huge step although the Legacy RTOS (LRTOS) vendors and the 16bitters are fighting all the way. Our customers who come from the LRTOS world are often amazed to discover that many complex problems (like: display a graphical window on a remote machine) are trivial on a modern POSIX operating system. And the productivity gains of hardware platform independence are also big: You mean I can start software development now on a PC or reference board and just run the same software on our hardware when (or if) our hardware is ready? You mean we don’t have to start six months late?

A standard data acquisition program on RTLinuxPro is so easy it’s hard to believe.
See the post on control loops