David VanHorn email (remove spam text)
>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.
I'm using RB5 to sense the cap, the comparator's busy on other stuff.
A4 is used, but only to discharge.
>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?
Yes. I scoped the cycle, put in enough time to flatten it, then doubled it.
So, it's charged to a very repeatable point.
>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.
I've switched up to 10k resistors, and I'm going to run another sweep on
Dave's Engineering Page: http://www.dvanhorn.org
Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9
http://www.piclist.com hint: To leave the PICList
In reply to: <0F60BEDDC090D41185D300508BCFA39EE81539@zpgty002.europe.nor tel.com>
See also: www.piclist.com/techref/microchip/devices.htm?key=pic
You must be a member of the
piclist mailing list
(not only a www.piclist.com member) to post to the