Page 93 - Making PIC Microcontroller Instruments and Controllers

TIMERS AND COUNTERS

General

If you have no knowledge about timers, you should read this chapter carefully before
taking on the chapters that follow. Those chapters provide a much morc advanced dis-
cussion of the devices based on their usage, as opposed to the introductory approach
prcvided here. However, there is some repetition, so as to allow each part of this book
to stand independendy.
j
Most users will frnd that using time$ and counters s the hardest part of leaming how
to use PIC microcontrolle$. With this in mind, we will Foceed in a step-by-step manner
and build up the programs in pieces that are easier to digest. Once you get comfoftable
with their setup procedures, you will find that timels and counters arc not so intimidating.
We will cover timers and counters separately- Counters are essentially timers that get
their clock input ftom an outside so rce. Therc are two counters in the 16F877A, and
they are associated with Timero and Timerl . Timet cannot be used as a counter because
it has no way to rcad an extemal signal into this timer.

No|€ The clockfre.quercy utilizd by the timers is o e'foul'th ofthe os,:illator
frequencj.
This is the frequencr of the instuction clock. This means that the counte^ are allected
by every founh count of the mdin oscillator. The Irequenct is rcfeffed to as Fosc/4 in
the literaturc . When rcsponding to an extemal clock sigwl, the response is to the actual
frequencr ofthe input.

Caulion The PICBASIC PRO Compiler generutes code that does not respond to inter
rupts \thile a conpiled instruction is being executed. Therefore, lone PAUSEs (meanins
long enough to los. ar interrupt sig/al, dependiB on how the tirur is set up) cdn l.ad to
lost interrupts if more than one interrupt occws during the paute. Since interrupts are used
for the express purpose oJ handling critical response/timing neeAr, this is most undesirable .
PAUSE comnands should be usedwith care unAer these co ditions.The prcgrum samples
ptorided nen give emmples oI how theJ can be wrixen to generate shorter pauses.