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:
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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