Chris McSweeny wrote :
 > I presume that if I use a 12f683 where the latches are
 > set to low be default according to the datasheet, I will be
 > safe in simply switching from input to output,...

Not at all. Besides of never trusting "defaults",
the problem is this :

Say you want pin-1 to be OC with an external pullup.

You set pin-1 "low" and then "toggle" tris-1.
All is fine so far, if there wasn't any other pins used !

Now say you also have another pin-2 that is an output.

What will happen is that when tris-1 is "input" the pin-1
will be pulled high by the external pullup. As expected
and by design when using an "open collector bus".

Now if you bcf/bsf to pin-2, you have an R-M-W operation.
The whole GPIO will be read (incl pin-1 that now will be
read as "high"), pin-2 is set according to the bcf/bsf and
finaly the whole GPIO is written back. This will now set
pin-1 "high" !!!

So the solution ot to always set pin-1 "low" right before
setting tris-1 as an output.

This gets a little more complicated if you have multiple
OC pins with external pullup's, but in general it=E4s the same.

Jan-Erik.


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