Page 95 - The Art of Designing Embedded Systems

82 THE ART OF DESIGNING EMBEDDED SYSTEMS

FIGURE 4-10 An idle loop quickly toggles the debug bit. . . until there’s
something to do!

ready to ship. Wrong. Hardware engineers stress their creations by run-
ning them over a temperature range. We should do the same, instrument-
ing our code or otherwise using performance-measuring tools, to be quite
sure the system has sufficient margins. It’s trivial to take quite accurate
performance data.

The RTOS
Whenever an application manages multiple processes and devices,
whenever one handles a variety of activities, an RTOS is a logical tool that
lets us simplify the code and help it run better.
Consider the difficulty of building, say, a printer. Without an RTOS,
one monolithic hunk of code would have to manage the door switches and
paper feeding and communications and the print engine-all at the same
time. Add an RTOS, and individual tasks each manage one of these activ-
ities; except for some status information, no task needs to know much
about what any other one is doing. In this case the RTOS allows us to par-
tition our code in the time domain (each of these activities is running con-
currently) and procedurally (each task handles one thing).
An important truism of software engineering is that code complex-
ity-and thus development time-grows much faster than program size.
Any mechanism that segments the code into many small independent
pieces reduces the complexity; after all, this is why we write with lots of
functions and not one huge main() program. Clever partitioning yields bet-