piclist 2001\04\25\192121a >
Thread: Pin impedance
face BY : David VanHorn email (remove spam text)

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
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 ???

Dave's Engineering Page: http://www.dvanhorn.org
Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9

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