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Thread: Pin impedance
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face BY : Roman Black email (remove spam text)



David VanHorn wrote:
>
> I've hit an Funky problem.
>
> For many reasons, I'm discharging a 0.1uF cap through a selectable
> resistance, using a pic output to discharge it,  and a pic input to sense
> when it's done.

First, are you charging and discharging through
separate pins? They may have different properties.

The PIC output low (current sink) saturation
voltage varies with current. By varying resistances
there is a variance in current and also discharge
curves. The output sat curves are in the datasheet.

Maybe try using a second series resistor
so your variable range is only 20% of the total,
not 95% of the total. This should flatten it out
a bit, also maybe try using higher value resistors
to get the currents lower.
-Roman

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