> -----Original Message-----
> From: David VanHorn [SMTP:dvanhorn@CEDAR.NET]
> Sent: Thursday, April 26, 2001 12:08 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [PIC]: Pin impedance
>
> 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.
>
> I first discharge the cap through a known resistance to get a nominal time
> value.
> Then I discharge through the variable resistance, and I had expected that
> 4.7k would take roughly 5x as long to discharge as a 1k
> would.  Unfortunately, this is not the case.
>
> Discharging through various multiples of 1k, here's the timing values I
> get:
>
> Test run:
> Capsense value (nominal "1") 07Bh
> Set     Time    Delta
> 0       00A             All shorted
> 1       079     6F      Note that the delta is roughly the same as the
> nominal value,
> as you'd expect.
> 2       0E2     69
> 3       12D     48      However, as R increases, the delta shrinks rather
> a lot.
> 4       187     5A
> 5       1B7     30
> 6       1EA     33
> 7       214     2A
> 8       241     2D
> 9       25C     1B
> A       27A     1E
> B       292     18
> C       2AD     1B
> D       2C2     15
> E       2CE     0C      (anomaly?)
> F       2E3     15      All open
>
> I expected to be able to divide the time by the nominal value, and get
> 0-15
> out of it, but obviously that's not working very well.
>
> The charge routine runs through the fixed resistor, and charges FAR longer
> than required, before each discharge,  in order to get up into the very
> flat part of the charge curve, so that the measurements would be
> repeatable.
>
> Is this showing me something unusual about PIC output pin impedances, or
> ???
>
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.

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?

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.

Regards

Mike

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