piclist 2001\04\26\114841a >
Thread: Pin impedance
face BY : David VanHorn email (remove spam text)

>Can't answer this one directly, but I spent about a week trying to get this
>method to work accurately for a thermistor measurement (although I was
>charging the cap, and linearity over the supply range was the problem I
>had).  Gave up in disgust in the end.  As Bob says, the Schmitt trigger
>input on RA4 is a little unpredicatble.  Using a comparator to sense the
>discharge point gives a huge leap in accuracy.

I'm using RB5 to sense the cap, the comparator's busy on other stuff.
A4 is used, but only to discharge.

>Looking at your figures, the delta seems to stabilise around 15-1B range as
>the resistance increases.  The lower value resistanaces obviously dump a
>higher peak current through the port, I'm wondering if this upsets the
>switching point of your sense pin somehow?  Are you ensuring the cap is
>fully charged before starting your discharge cycle?

Yes. I scoped the cycle, put in enough time to flatten it, then doubled it.
So, it's charged to a very repeatable point.

>A possible fix might be to introduce a fixed resistance in series with your
>variable one which would move the results down towards the more linear
>region.  Obviously the PIC would have to take into account the offset.

I've switched up to 10k resistors, and I'm going to run another sweep on
the values.

Dave's Engineering Page: http://www.dvanhorn.org
Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9

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