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'Interrupts during table read and AN556'
1995\10\11@040818 by Scott Stephens

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I just read the note on Eric Smith's PIC Page http://www.spies.com/~eric/pic/
mentioning a problem with Interrupts occuring during a table read operation.
AN556, page 5-3, bottom, under bold heading Iterrupts states that "...if the
interrupt occurs just before the "movwf pcl" instruction...On return from
interrupt ,the program will go to the intended offset of the table +1...a
very undesirable result, so interrupts must be disabled during a table read.."

Eric states on his page that at a seminar the Microchip rep's said this
wasn't a problem.

Anyone aware of this, will MPSIM handle is the way a 16C84 will?

1995\10\12@011655 by Andrew Warren

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Scott Stephens <spam_OUTstephnssTakeThisOuTspamKIWI.PYROTECHNICS.COM> wrote:

>AN556, page 5-3, bottom, under bold heading Iterrupts states that
>"...if the interrupt occurs just before the "movwf pcl"
>instruction...On return from interrupt ,the program will go to the
>intended offset of the table +1...a very undesirable result, so
>interrupts must be disabled during a table read.."
> ....
>Anyone aware of this, will MPSIM handle is the way a 16C84 will?

   Scott:

   This is NOT true.  Interrupts do NOT need to be disabled around
   table reads.  As discussed at some length on the Microchip BBS, if
   this WERE true, interrupts would also have to be disabled around
   GOTOs and CALLs.

   MPSIM handles interrupts and PCL writes exactly as a 16C84 would.

   -Andy

--
Andrew Warren - .....fastfwdKILLspamspam@spam@ix.netcom.com
Fast Forward Engineering, Vista, California

1995\10\12@104926 by Jeff D. Pipkins

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From:    Scott Stephens <stephnssspamKILLspamKIWI.PYROTECHNICS.COM>
|
|I just read the note on Eric Smith's PIC Page
|http://www.spies.com/~eric/pic/
|mentioning a problem with Interrupts occuring during a table read
|operation.  AN556, page 5-3, bottom, under bold heading Iterrupts
|states that "...if the interrupt occurs just before the "movwf pcl"
|instruction...On return from interrupt ,the program will go to the
|intended offset of the table +1...a very undesirable result, so
|interrupts must be disabled during a table read.."
|
|Eric states on his page that at a seminar the Microchip rep's said
|this wasn't a problem.
|
|Anyone aware of this, will MPSIM handle is the way a 16C84 will?

Scott,

I was one of several who found conflicting results and suspected
that the problem didn't actually exist.  A couple of people at
Microchip, including the author of AN556, have since told me
that this is indeed the case -- the problem does not exist, nor
has it ever existed.

My WWW connection is not working right now, but if someone were
to send me Eric Smith's email address, I would be happy to
forward to him my email archive on this problem, including the
note from D'Souza.  For that, matter, if anyone else wants a
copy, you're welcome to it, just send me an email address.

Cheers!
--Jeff

Jeff Pipkins <.....PipkinsKILLspamspam.....bangate.compaq.com>  -------------------
Opinions above are mine personally; use only at your own risk.

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