Since your code seems simple I guess you may have a
hardware problem! If I understand your circuit properly
you are using one pin to ground the C through R, then
the other pin to sense the voltage. And the third pin
hopefully floats so it doesn't affect anything.

Since it is not detecting low, my guess is the voltage
on the capsense pin is not going below the bottom
threshold. This could be caused by three things;

* there is a current leak holding the cap voltage high
* there is not enough current drain to pull the volts down
* voltage mismatch with pin types

Put a voltmeter on the cap. See if it below the bottom
threshold. Disconnect the float pin to see if that
is leaking to hold the cap volts up. Check the volts
on the low output pin, it may be too high.

My *guess* is you have quite a high value resistor
from the cap to the low pin, the PIC low voltage
can be as high as 0.6v. The low input threshold is
about 0.8v for the TTL inputs and 0.2v for the schmitt
inputs.

If you capsense is schmitt input that might explain
it?? What value is your discharge resistor??
-Roman









David VanHorn wrote:
>
> I'm having trouble with an I/O function.
>
> I have a capacitor, connected through three resistors, to three I/O pins.
> The Capsense pin connects to the cap through a 1k resistor.
> The other two pins are varying resistance.
> If I set them as inputs, then the resistances don't matter.
> If I set either one as output, then it charges or discharges the cap at a
> rate determined by it's resistor.
>
> The basic idea is to read the cap RC time constant, as a form of A/D converter.
>
> Everything's fine, till I get to the point where I want to watch the cap
> discharge, until it's low, before taking one of the other pins to output high.
>
> Read_Function:
>         btfsc   PORTB,Capsense          ;loop here till low on cap sense pin
>         goto    Read_Function           ;
>
> The Capsense pin is set as an input, as is one of the other two pins
> The third is an output, and I can see that it is low, and that the voltage
> on the capsense pin is also low, but the code loops here forever,
> indicating that it's seeing a high input.
>
> I've walked through all the banking, and I believe that I'm in the right
> banks when I'm talking to PORTB and TRISB.
>
> I've disabled all peripherals that would otherwise use portb pins.
>
> So, how could I get stuck here, given a low on the Capsense pin of portb?
>
> --
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>
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