Page 412 - Programming Microcontrollers in C

Delay Routine 397

This rather clumsy loop reads the value contained in ITADR into the
variable now. This register is then read into the location next until
the value in ITADR is changed by the clock. That should occur 122
microseconds later. At this time, count is decremented. When count
becomes 0 the time has expired.
The variables now and next are both declared to be volatile. This
declaration is necessary to avoid loss of the whole loop when the code is
optimized. The variable ITADR is declared to be volatile in the header
file. That declaration does not guarantee to the compiler that the variables
will ever change, so the optimizer could well remove the code

now = ITADR; /* and don’t care how fast your test is */
do
{
next = ITADR;
} while(now == next); /* wait until next tick */
during the optimization phase. That would make the whole function
rather pointless.

Semaphore

This delay function is used in later code to implement debounce
routines. A more practical delay routine can be implemented with
the use of a semaphore. Here, we are going to develop a semaphore
3
that follows that developed in Reusable Software Components . We
will not create a semaphore object as done in that text, but the program
will follow the items shown there.
A semaphore is merely a flag that is attached to a process like a
program or a function. The semaphore is set and the calling program is
not able to proceed until the called program resets the semaphore. This
reset is usually done in an interrupt service routine implemented in the
called program. Let’s look at the elements of a semaphore first.
When a semaphore is set, it usually marks a resource as busy. It is
interesting, but you will find that many semaphores are used around
interrupt service routines and while interrupts are being used. Often a
strange race condition can occur that will cause a semaphore to be
improperly set. Suppose that I want to set a flag, semaphore, to indicate
that a resource is busy. The process of setting the semaphore is to first

3 Reusable Software Components, Ted Van Sickle, Prentice Hall, Upper Saddle River, NJ, 1997