Reply embedded.

Jan-Erik Soderholm wrote:
> 
> Dr Skip wrote:
> 
> And with some resistor between the I/O pin and the base ?
Not that I've seen on other designs, but may be a good idea? OK, pull up, pull 
down, or series. Which and why?

> What values of that (the base) resistor and of the pullup ?
My first thoughts were 10k for pullup. Advice?

> What kind of transistor ? Bipolar och mosfet ?
It has a base - bipolar. For discussion though, the only reason they are there 
are to firmly demonstrate that these have no other outputs or drives on the 
port pins.

> No, not in theory. But you could be hit by the
> read-modify-write problem. There are both timing
> issues (how fast after setting pin 3 and 4 are the
> other pins set/cleared?) and configuration issues
> (such as not shut off analog pin funtions).
I can see the initial timing one, but are you saying that if bits 0 and 1 
(let's say) are set to be analog inputs, that it changes the stability of bits 
3 and 4 in this example?

> 
> But, not as "glitches" but more likely as permament changes
> of pin 3 and 4 (until they're set back to whatever they
> should be by some other code).
Under what conditions, other than the initial setting as you mentioned?

> 
> And again, are you actualy seeing something that
> makes you ask ? Or is it just in general ?
I have a design that has additional hardware that is edge triggered and 
connected to the pic output on a few bits of a port. Triggering when it isn't 
supposed to, is in a momentary blip, while other pins are changing would be 
bad. It's better to know if that's even a possibility before designing, than to 
assume it "shouldn't" and going nuts when all sorts of unexplained triggering 
happens. The very fast blips are the hardest to find too... ;)

-Skip
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