> 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
> a 10-bit ADC?)
Pot and 1.5v battery are connected to AN0 all in series.Ground of
battery is connected to ground of the circuit.

That's correct it has 10bit ADC, I am using most significant 8bits of
it. I am just experimenting.

> If I understand you right, it works okay with some voltage "directly"
> applied to the input, but not when you apply through the pot?
Yes that's correct.

> I can't see any reason the pot wouldn't work too with the ends at
> Vdd/Vss and wiper to AN0, providing the voltage driving the pot is of a
> low enough impedance not to raise the total over whatever it is MC
> recommends (10K generally IIRC) - have you confirmed with a
> multimeter/scope the voltage is actually present and changing at the
> pin? (i.e. the pot isn't dodgy or you have an open circuit somewhere)

I confirmed that pot works by externally testing it with scope and
multimeter using the same source I apply (1.5V battery).

> Also, your circuit doesn't show any power bypass caps in place.

There is a 220nF capacitor connected to VUSB (14) pin of PIC, sorry
didn't mention that.
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