Phillip wrote :

> I don't know about making sense but it does fix my problem.
> I have two ISR's in my program.

That are called from some common interrupt entry point ?
Or how are they entered ?

> One is a timer that expires so I can check/scan/de-bounce 
> button presses and the other receives characters on the
> first serial port.

The second is triggered by the USART interrupt, right ?

> Below is the code of my timer/button ISR routine below.
> Look at the first line.

OK :

>   PIR2bits.TMR3IF = 0;  //disable the timer3 interrupt 

The comment is wrong and shows a slight missunderstanding
of the way interrupts works. "clear TMR3 interupt flag" is right.
That does *NOT* "disable" anything.

> If I comment it out my program will no longer work.

Of course.

> If I halt the debugger whilst it is commented out it stops in 
> this routine that is being called over and over...

Of course.

>....because the hardware is transferring control to this routine?

> I do believe that it is doing a retfie (underneath) because I 
> don't enable PIR2bits.TMR3IF on exit and it works fine the next 
time.

Again shows a missunderstanding of how interrupts work.
the TMR3IF flag is set whenever the TMR3 overflows. Your
code should only clear it (in the ISR).

> So I don't believe you when you say that the hardware 
> disables interrupts during an ISR event...

They are.
*Read* the chapter on interrupts in the data sheet.

> ..now I may be mixing my chickens with my ducks
> because my button ISR is a low level ISR and the original 
> serial IRS was a high level...

Are you using high/low interrupt priority ? That makes things
a little more complex. Do you realy need that with only two
interrupt sources ?

> ...but disabling interrupts does fix them both.

But you aren't "disabling interrupts" !


> as you can see I'm much clearer 
> on what fixed it
> than what was going wrong....

No, you aren't.

> 
> void detect_button_push_ISR(void)

This can't be the main ISR. Doesn't that need some keyword
in the function header saying "this is an ISR" ?

Best Regards,
Jan-Erik.



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