Page 250 - Making PIC Microcontroller Instruments and Controllers

24e UI{DERST NDIiIG tHE GOUITERS: COUt{tl G MAEBIES

ialligliffr7!si Demonstration ofthe use ofTMRo as a Counter (Contitued)
IIfECON-0 reset INTCON register
ELSE
ENDIF
GOTO MAII{

END
In Program 17.4, we press SWI again and again a see what happens in the LCD.
For each key press, you need to both release and hold the button down for a good part
of a second to actually observe the key-down and key-up effect so that the 750 pause
that allows us to look at the LCD can be discounted.

Usins Timerl in Counter Mode

We will use ?lr?ert as the counter for the marbles because it is the only timer that can
count to more than 255 without having to count fhe interrupts at every overliow of the
counter and nake the related calculations. It will count to the 1000 marbles designated
jn Mr Marbles' specification.
Actually, the microprocessor is much faster than the marbles coming out the chute,
but we will pretend that we need to really hurry in our counting prccess for some other
prccess we mjght encounter in the future.
If the marbles are coming so fast we do not have time to update the registers, we
have to set up an inlernal counter and feed the signals that the intenupt provides
directly into that counter. The signals will have to be debounced before they can be
connected to t}Ie intemal counter because these counten arc extremely fast and will
pick up every litdejitter as a countable event.

THE BASICS OF COUI{TII{G DIRECTLY II{TO A TITUER BEING
USED AS A COUIITER
In order to use a timer as a cou er, $Ie must set up the timer as usual and then change
the input to the timer from the intemal clock to the external signal and set the appro-
priate pammeters for ihe incoming signal (rising edge, and so on).
For Timerl. the 16-bit counter. this is done as follows:
Setiins Timerl uD as a countef.
T1COriI.4 antt tlCON 5 = 0 for no pre scale
P I E 1 . 0 = 1 to enable intelrupl
I&aCON.6=1
T1CON.0=1 ro start the counter. We have to

lncomlng s1gnal
E1CON.l=1 selects PORTC.0 for input
l
t1CON.3=3 " l s o . - - d - d o . e e c c D O o l ^ . 0 o - - n p , L