15.4: Memory and Hardware

Most embedded applications just run one piece of code. Therefore, you can think of a program as “owning” everything. There’s no sharing of resources. This makes life easy in many regards. For example, there’s not much need for an operating system. Also, the system is “known” in that your code will be running on fixed hardware. Execution times are very predictable. Of course, the computational power of the processors tends to be much less than in the desktop world. Still, you can do things that are not practical in the desktop world due to hardware variation. A classic example is a “timing loop”. Sometimes you need to create a time delay or to “waste” a certain amount of time, perhaps for synchronization to some external hardware. You can do this with a simple loop that iterates a specific number of times:

for( c=0; c<1000; c++ );

This loop does nothing but count, but each count will require a certain number of clock cycles from the microcontroller, and thus a specific time. These are usually determined experimentally. You could sit down with the processor manuals and figure out how long a loop will take, but it’s usually easier to just write the thing and try a few values. The result will depend on the specific microcontroller used as well as its clock frequency.

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