Page 94 - Making PIC Microcontroller Instruments and Controllers

A4 TIIIIERSAI{DCOUIITERS

Timers

Timedl will be covered in more detail as a protollpical timer, and discussion and exam
ples for the use ofTimerl and Tirner2 will be providcd.
is a bit more complicated than what we
The use oftimers internal to microprocessors
amount ofsetup required before
have been doing so far because therc is a considerable
the limercan be used, rnd also because the options for setting up the timers are exten-
sivc. We will cover lhe timers one at a time in an intoductory manner, but be warned
that there is an entirc man\al (Plcnicro Micl Range MCU Faniry Rekrence Manual
lDS33023l) available liom Microchip Technology Inc- that covers nothing but timers'
so our coverage here will, olnccessity, be rudimentary.
how to tum them ON and OFF
Understanding timers has to do with understanding
and how to read and set the various bits and bytes rhat relatc to them. Essentially, in ihe
typical timer applicatiot, you turn a tiner ON by turning on its enable bit. The timer
then counts a certain number ofclock cycles and sets all interrupt bit, thus causing an
intemrpt. Youlll progmm responds to the intenupt by execnting a specifc inkrrupt han'
dling routine ard iJ|'et\ clearing the intenupt bit. ]lhe progra"n then relurns to wherever
it was when the inteffupt occurred. The pre/postscalars have to do with modifying the
time it takes for an inteffupt to take place. The hard pail is finding ou! which bit does
what and where it is located , so reading and understarding the datasheet chaptcr on the
tiner you are using is imperative. There is no escaping ihis horrorl
Timers allow the micrcontmller to cr€aF and rcact to chronological cvents These include:

: Timing events
I The creation of clocks lbr various purposes
I Generating timed interrupts
I Controling PWM generation
to do work (and back to sleep)
! Waking the PIC from its sleep mode at jntervals
! Special use oflhe watchdog timer

The PIC 16F877Ahas thrce intern.rl timers. There is also the watchdog timer,which
is discussed after the standard timers.

t Timero An 8-bit free-running timer/counter with an optioDal prescalar. It's the
simplest of the timers to use.
! Timerl A 16 bi! tiner that can be used as a timer or as a counter- It is the only I6-bit
timer that can be used as a counter.It is also the most complicaled of the timers.
r Timer2 An 8-bit timer with a prescalar and postscalar and cannot be used as a
There i. n,J Inpul lrne for rhi. lim(r.
counrer.
Each timer has a timer control register that sels the options and properties that the timer
will exhibit.All the time$ are similar and each of them has special features that givc it
special propertics. It is imperative you refer to your datasheet for the PIC l6F877A as
you experiment with the timer functions. Once you start to understand what ihe PIC
designers are up to wilh lhe timerfunctions. it will start to come together in yourmind