> =

> In the main code the interrupt enable bits are significant in that  =

> they will prevent a peripheral from generating an interrupt. So the  =

> interrupt flag bits can safely be ignored.
> =

> Once an interrupt (any interrupt) forces execution of the interrupt  =

> service routine the interrupt enable bits become useful flags to tell  =

> which peripheral's service request should be honoured. But now the  =

> interrupt flag bits (all that are set) become significant and cannot  =

> be ignored. Whether the peripheral's interrupt enable bit is set or  =

> not it's interrupt flag bit must be cleared or the processor will be  =

> hung repeatedly reentering the interrupt service routine.
> =


Since a new interrupt won't be generated if its IE bit is clear there will =
be =

no new interrupt generated so the interrupt will not be reentered if only t=
he =

IF bit without its IE bit is set. Figure 9-10 in the datasheet explains thi=
s =

(and the wake up from sleep) better than with 1000 words. The IF bit should=
 not =

be cleared in the ISR if the IE bit is not set since the non ISR code could =

look for this bit. I have actually done this where timer0 caused interrupts=
 to =

generate notes of a certain frequency in a buzzer while the main program =

checked for other conditions, including timer1 with its IF bit, to tell whe=
n to =

stop playing one note and go on to the next.

Here is one way to efficiently check both IE and IF bits for timer0:

isr_tmr0_start
  btfsc  INTCON,TIOE
  btfss  INTCON,TOIF
  goto   isr_tmr0_end
isr_tmr0_on
  bcf    INTCON,TOIF
  ;rest of tmr0 isr here
isr_tmr0_end
  ;check other interrupt sources here
  ...

Note also that you have three conditions to check for with the periferal =

interrupts: In addition to the IE and IF bits there is also the PEIE bit.

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

-- =

http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist