Page 108 - Making PIC Microcontroller Instruments and Controllers

Trr/tERs llto cout{tERs

limer operalion that depends value ol
iiB!!6iii!U$!i: Timerl usage (Rudimentary on
POr-l) (Continued)
IF PORIB.s = 0 THEI STOPCI,OCK ;
TMRIB=AIRESE
PIRl=0
RESttltE ; go back to the nain routine
ENABIJE

DISABIJE
sraRtct ocx:
rNacoN = %10100011 ; enable T'MR1 interrupts
TICRS = 0
Gcfro uarNt ooP

sroPct ocK:
rNEcoN - %10000011 ; disable TMR1 interrupts
PAUSE 2
TICKS = 0
GOITO !4AINI.OOP

CUEIICLOCK.
rMtcoN = %10000011 ; disable TMR1 interrupts
UINS = 0
SECS = 0
fEMEBS = 0
TICI(g = 0
GdTO !4AINI,OOP
ENAEI.E
END

Run this Fogram to see how the setting of the potentiometer affects the operation of
the stopwatch. It becomes clear that choosing how the intempt will serve our purposes
is very impoitant, and a bad choice can pretty much compromise the operation of the
program.
We can read the limer and the intenxpts at our discretion either befol€ or after an irter-
rupt has occufed, and the interupt flag can be cleafed whenever we lvish, if it has been
set. lf it has not been set, fhere is no need to clear it.
Figure 6.1 provides a diagrammatic rcpresentation of how an interrupt routine is
implemented in a typical program.
Even the 16 bit Timerl on the l6F877A cannot time a long interval. Repeated inter-
vals have to be put together to create long time pedods. The longest possible time
between interupts for Timerl (with a 4-MHz clock) is 0.524288 seconds. The maxr-
mum prescale value is 1:8. The postscalar is only available on Timer2 (which in any
case is a shorter s-bit timer). This results in a maximum time and is determined by multi-
plying the instruction clock cycle (1 tlsec @ 4 MHz) by the prescale (8) by the number
* *
of counts from one overfiow to the next (65536). 1 psec 8 655 36 = 0.524288 seconds
On a 20-MHz machine, the interval would be one-fifth of this.