Page 214 - Making PIC Microcontroller Instruments and Controllers

PRqTECT 2 2ll

we will use each of the tbree timers (we do not have the ability to use the watchdog
timer) in the PIC 16F877A to create instruments that provide us with accurate pre-
dictable timed intervals we can select with a potentiometer. Two ofthe thee timers can
also be used as counters, and that will be unde(aken in ihe next chaptel where we
count marbles in some of the many different ways available to us. CounteN are essen-
tially timers that get their count pulse liom an external source.
On the mehonomes created, (he counts per minute will be controlled by a poten-
tiometer, they will be displayed on the display, and they will bo annunciated on a speaker
(or LED). Since both the tachometer hardware and the LAB-XI hardware rvill have these
accessodes on them, either one can be programmed for the purpose we have in mind.
Though the output needed ftom a metronome is rather limited (40 to 208 ticks per
minute is the standard), our project will be able to provide any count dosired, from a
count every few seconds to a few thousand counts per second, Al1 we will need to do
is change a few lines of code ifl the software. That, in a nutshell, is the power of instru-
ments that you yourself make.
The three timers in the PIC 16F8?7A have the folowing salient features, and the char-
acteristics of the timers in most other PlCs are similar. Onco you get confortable with
the thr€e timers in this PIC, you should have no difliculty with any timer in any PIC.
The hard part will be getting familiar with the new datasheet for the PIC you select.

IIIUERO
t This is an 8-bit timer
: Official nameTMR0.
I Register used TMRo.
t This register can be written to and rcad ftom at will.
a Rur.J all the time, cantot be stopped or started.
I Uses a prescalar,
I Does not use a postscalar.
I Can run from an irtemal clock (at Fosc/ ).
I Can be used with an extemal sigral for counting.
I Generates a progranmable intenupt on overflow from 255 to 0.

TtiitERt
t This is a 16'bit timer
I Official name: TMRI.
I Registers used: TMR1H and TMR1L for the high and low bytes of the l6-bit word.
I The two r€gisteN mentioned in the previous bullet point can be \tritten to and Iead ftom.
I The timer can be tumed on and off.
I Uses a prescalar.
I Do€s not use a postscalar,
I Can run from an intemal clock (at Fosc/4).
I Can also run from an extemal clock for timing drd counting.
I Generates a programmable interrupt.