Well, the nature of my app is that everything is scheduled, so the main
routine is just loop:goto loop.  I should only have to worry about nested or
recursive calls that might exceed the stack depth, right?  8 certainly isn't
much to work with, I agree with that!  Fortunately, my ISR just checks the
time vs. each thing that might need to be done and calls them if there's a
match, one at a time.  I pretty much invented my own scheduling routine.
Couldn't find zippo on the subject @ Microchip.  Suggestions eagerly
accepted.

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Olin Lathrop
Sent: Thursday, July 05, 2001 1:57 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: F877 UART in ISR
>    What's wrong with using the stack with calls from within an ISR?

It's not inherently wrong, but it could be a lurking gotcha.  Your main code
has a certain call tree, which hopefully requires some finite maximum number
of stack locations.  If interrupts are enabled everywhere, then the stack
location used by the interrupt routine must be left available at all times.
If the interrupt routine calls other subroutines, then ALL these additional
call stack locations must be left available whenever interrupts are enabled.
If your interrupt routine calls one subroutine, then it uses 2 call stack
locations.  Your processor (16F877) 8 stack locations, so your main code can
now use no more than 6.  If that's not a problem, then it's OK.  But you
should be aware of the cost of calling subroutines during an interrupt every
time you do it.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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