Very good thoughts. I appreciate it.

Quintin Beukes



On Wed, Sep 23, 2009 at 10:12 PM, Barry Gershenfeld <gbarry42@gmail.com> wr=
ote:
>> Further, we eventually didn't ground the PINs because Francois also
>> said that it's "bad". Though we found shorting them against each other
>> only also prevented the PIC from rebooting due to static.
>>
>
>
> Reading this thread brought several things to mind. =A0They are not
> necessarily aimed at your problem, but can be generally applied.
>
>
> If connecting the pins together without grounding them remedies the probl=
em,
> that suggests that at least one of them is an input, and that at least one
> of them is an output. =A0In effect, it ties the inputs to whatever logic =
level
> the output pin is. =A0 It also demonstrates that an "approach of static" =
(a
> high impedance phenomenon) would not affect an output pin (or any pin
> connected to it).
>
> If one of the pins is found to be an input, then that's kind of a lesson
> worth considering when approaching the notion of connecting unused pins to
> ground. =A0They say, "Be sure you don't try to drive them high". =A0And w=
e say,
> "I'm careful not to." =A0And then something like this happens, causing us=
 to
> reconsider just how sure about that we are :)
>
> TRIS is not the end-all in pin programming either. =A0I had a situation o=
nce
> where port D was behaving extremely strangely. =A0It turned out the paral=
lel
> port feature had been enabled, which meant some other pin (on port E) was
> controlling its ability to be an input. =A0I looked at the chip in questi=
on
> here, and I see that some of port B can be an A/D channel. =A0There's a
> register to make it not do that, and it has to be set when you start.
> Things like that.
>
> And then the overall lesson. =A0=93When you eliminate the impossible, wha=
tever
> remains--however improbable--must be the truth.=94 =A0Paraphrasing that, =
it says
> that once you know nothing in your circuit could be wrong, you are left w=
ith
> the task of proving it. =A0That's why putting a TRIS instruction in your =
code
> almost certainly sets the port up, but only "almost". =A0You would still =
need
> to show that it actually compiled into your code, that it is actually loa=
ded
> (and gets executed!) on the chip, that it had the intended effect, and th=
at
> nothing else fiddled with the port later. =A0(And maybe another thing I
> haven't thought of!)
>
> Barry
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