Page 216 - Making PIC Microcontroller Instruments and Controllers

PnoJECt 2 2ta

The compiler software for the system is designed to be able tojump to a designated sub-
routine when this happens. We specjfy the target destination for this with the ON
INTERRUPI GOTO command.

tnterrupt Fleg An intenupt flag is a bit that is set in one oI the registers associated
with a timer when the limer overflows and afler all the pre- and postscalar effects havc
been taken into account. It indicates that the prcgam is ready to be given attention by
the main part of the processor. Aftcr the interrupt has been taken care oI, the intenxpt
bit must be set to 0 (cleared) by the intenupt service rouline to allow it to be set to 1
again when the next interrupt takes place. The bit is osually cleared at the end of the
i nteffupt-handling routine. The interupt hand] ing routine must be shot enough to com-
plete before the next intenlpt adves. Ii this is not the case, an interrupt will bc losl.
(This is considered fatal and is unacceptable.)

Inff,'ftupt Enable Bit An inteffupt enable bit is the bit that has to be set to l, within
one of the registers that controls the timer, before an inteffupt can be generated. This
bitcan us ally be tumed on and off by lhe program. Before a specilic interrupt car take
place, its specific enable bit must be sel to L

t rter'| pt Latency Beware thal the interrupts are not handled as soon as they oocur,
The PBP compiler inhibits all interupts while it executes an instruction. I1 an intenxpt
occurs while an instruction is being executed, the inlerlupt will be addressed atler the
jnstruction is complete. This cdn lead to a particula-rly long delay in the case of a long
PAUSE instruction. PAUSE instuctions should be broken up into smaller pauses in
loops to reduce this latency so the largest latency encountered can be tolerated by the
task at hand. Though intenupts may occur exactly and as frequendy as programmed. they
may not be handled immediately by the proglam. The programmer needs to be aware of
this condition when programming for time-critical applications. (Assembly language pro
granming does not have the problem to this extent. It depends on how the assembler is
designeal, but even so it tal:es some time to gel the irteruprhandling task accomplished )
Ciohal tnterrupt Enebre lGtE) Bit The GIE bit in the PIC allows you to shut off
all intenupts when it is setto 0.Itis located at INTCON.7. On two ofthe timers Timerl
and Timer2 this global intenupt enable bit must be enabled before ary interupt can
occur Time runs all the ti me and is nol affected by the global intemrpt enable bit See
datasheet page 22.
whenever any inteffupt flag is set, the GIEbitis tumed offby the opcrating system
W]rcn the flag is cleared by you (with the software), the GIE bit will be reset a tomat
ically by the operating system. This means no other inlerrupt can occul until you take
care of the intemrpt that has occurred. If you are using more than one interrupt, this can
cause problems, and addressing these probleDts is beyond the scope of this book.
Beginners arc advised to use only one intenupt in their programs till they get proficient
enough wifh the PlCs to create nore complicated programs.
(For those who have an insatiable need to try thesethings out, the solution to thc pre-
viously mentioned problem has to do wilh replacing the irterupts very quickly with