As I also now understand from the other responses.  Slap on my own wrist. 

Thanks,
-Neil.



On Tuesday 04 October 2005 01:06 pm, Howard Winter scribbled:
> That's the way Interrupts work on PICs - when an Event occurs that has an
> Interrupt associated with it, its Interrupt Flag is set, then if its
> Interrupt Enable is set, and if the General Interrupt Enable is set, an
> actual "Interrupt" occurs (similar to a CALL 0x04).  The only thing that is
> stopped ("gated") by the Enables, is the Interrupt itself - the Flags are
> always set by the Event.
>
> There is a good reason for this - if you have disabled one of the
> Interrupts temporarily and during that time an Event occurs, when you
> Enable it you would have "lost" the fact that the Event occurred if the
> Flag wasn't set.  In the case of a real time clock it would lose time, but
> other more serious consequences could happen in other applications.
>
> So to sum up, the correct procedure (unless you *never* clear an Enable in
> the program) when you want to know if an Event needs handling is:
>
>   IF an Event's Enable AND Flag are set
>        an Event occurred and should be handled
>   ELSE
>        ignore this Event
>
> The language doesn't help - some people tend to use "Interrupt", "Flag" and
> so on ambiguously.  I prefer these definitions:
>
> Interrupt: The act of passing control to the Interrupt Service Routine
> Event: Something which happens, and which may cause an Interrupt
> Flag (or Interrupt Flag): A bit which indicates that an Event has occurred
> Enable (or Interrupt Enable): A bit which indicates that an Event will
> cause an Interrupt
>
> Of course there are two types of Enable - the General one which prevents
> all Interrupts from occurring, and the specific ones which relate to
> individual Events, and both have to be set to allow an Interrupt to occur.
>
> Cheers,
>
>
> Howard Winter
> St.Albans, England


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