Years ago we sensed the AC line for counting timing pulses with a current limiting resistor to a port pin. I seem to remember an ap note possibly by John Day on that subject. I think it was done on a 16C54 but not sure.
Larry G. Nelson Sr

> 
> From: olin_piclist@embedinc.com (Olin Lathrop)
> Date: 2005/10/26 Wed PM 01:58:37 EDT
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Subject: Re: [EE] Inputs over Vdd / under Vss with current-limit
>  resistors... ?  and "input clamp current" spec?
> 
> Jesse Lackey wrote:
> > 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.
> 
> So what's the question on RA4?  Do you think "absolute maximum" is ambiguous
> or somehow doesn't apply to you?
> 
> > 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?
> 
> I take it to mean that if you put more than 20mA thru the protection diodes
> they can go poof.
> 
> > 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...
> 
> A small current is OK in some cases.  Even small currents can upset A/D
> readings.  Also this current must not be on when the chip is powered up, or
> it could go into SCR mode.  100uA applied after the chip is fully powered on
> and if you're not using the A/D is probably OK.
> 
> > 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...
> 
> What's the cost of a failure?  Compare that to the $.05 for the extra
> transistor and resistors.  Unless this is a really high volume product doing
> it right sounds like the clear choice.
> 
> 
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