Hello all,

I have a design whereby I want to sense the presence or absence of a 
voltage that is well above +5V; maybe up to 15V, say.  This voltage 
could be negative as well, at least until the user figures out they have 
their DC input backwards...

I also want to use A4 to switch a pfet, and when the pfet is not being 
driven (i.e. A4 set to high, i.e. floating) it will be pulled up to 9V 
or so through 100K.

Now reading the absolute max ratings, of course, you can't do either of 
these things.  Any pin except A4/MCLR is Vdd+0.3V max; A4 is 8.5V max.

There is another spec that says input clamp current (IIK (VI < 0 or VI > 
VDD)) is +-20mA.  (This is the 18F1220/1320 datasheet FWIW.)  I presume 
this doesn't mean the PIC clamps the current to +-20mA... what does it 
mean, exactly?

So, if I current limit to a milliamp or less, maybe even .1mA, am I ok 
here?  I don't see the harm in forward-biasing the input protect diodes 
with a small current, but then again I don't know much about chip design 
norms...

I can add transistor switches etc. if need be, but if the above is 
workable it is definitely preferred.

This is going to be a commercial product, so unfortunately solutions 
that "cheat" and will "normally work at least for awhile" aren't going 
to cut it...

Thanks for any tips...
Jesse
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist