On Fri, 14 Aug 1998 12:01:45 -0400, Sean Breheny wrote:

>>
>> I have had a 68HC11 that this was exactly the problem.  Pullups on the unused
 ports fixed it.
>
>Was the CPU watching the state of these inputs in any way? Is it capable
>of causing an interrupt depending on the state of these pins?  I could

It was a long time ago... I remember it was a unused port... Maybe porte the A/D
 port I think... So it's possible a spurrious AD
interupt was causing it but we reviewed the code pretty tightly.

I remember spending a large ammount of time trying to find a wierd intermittant
problem.   It was a controller for a stepper
motor and I think it would glitch and screw up the sequence every now and then.
 What I remember is that we did the pullups
as a last desperation attempt because we had tried every thing we could come up
with.  I think it mentions something about it
in the databook.  Added pullups, problem disappeard.

Since then I have just allways added pullups.


--
Richard A. Smith                         Bitworks, Inc.
rsmith@bitworks.com               501.521.3908
Sr. Design Engineer        http://www.bitworks.com