On 22/05/2011 14:41, Yigit Turgut wrote:
> Hi everyone,
>
> I  brought my old stash alive and found a couple of 18f2550's and
> decided to use them instead of throwing them away. I built a very
> simple 2 channel 8-bit ADC application and it works  as expected. I
> can read the input voltage up to %98 precision which is enough for me
> considering the PIC does it all, it does it's job well. Then I decided
> to use the circuit as an ohm meter to read input impedance. Input
> range is between 0-5V, I attached a 10K pot and connected to a 1.5V
> battery to see the voltage change as a play with the pot but there
> seems to be no change which doesn't make sense. AN0 is input, I am
> sure experienced users on list can find a solution very quickly.
>
> Have a nice sunday!

How is the pot actually connected? You don't show it on your schematic.
What voltage/value are you seeing? (as an aside, I thought the 2550 has=20
a 10-bit ADC?)
If I understand you right, it works okay with some voltage "directly"=20
applied to the input, but not when you apply through the pot?
I can't see any reason the pot wouldn't work too with the ends at=20
Vdd/Vss and wiper to AN0, providing the voltage driving the pot is of a=20
low enough impedance not to raise the total over whatever it is MC=20
recommends (10K generally IIRC) - have you confirmed with a=20
multimeter/scope the voltage is actually present and changing at the=20
pin? (i.e. the pot isn't dodgy or you have an open circuit somewhere)
Also, your circuit doesn't show any power bypass caps in place.




--=20
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