Dan,

The question would be - what is a "safe" current in each variety of PIC's
internal input circuit?  Where did you get your figure of 25mA from?

BTW, I currently have an R-2D-R==>pin circuit at each input of my PIC in an
automotive (headlight reminder) project (hobby only).  I've also been concerned
at presenting a possible Vcc+0.6V at all the inputs, which it will currently do
if the input circuits inside the PIC draw enough current.  All the inputs are 0
==> +(11 to 14)V, so the first R has to limit the current due to a 14V to 5.6V
drop initially, and the second R limits the current into the PIC.  I've
currently chosen 10k for each.  I tried putting a third diode in-line with the
second R (before input pin), but later realised and confirmed that this leads to
crazy behaviour when the input is supposed to be near ground (had to ground
input pin with my finger to make it behave).

Anyway, the PIC hasn't blown up yet, but I'm open to suggestions - what was the
problem with a 5v1 zener again?  The inputs also have a 0.1uF cap between the
junction of the external clamping diodes (before the second R) and ground.

Vcc (+5V)
----------------------------+--------------------------------
                            |
                         D1 ^
                     10k    |        10k
 (From Car) o------[ R1 ]---+--+---[ R2 ]----> PIC input
  (0-14V)                   |  |
                            |  = C1
                         D2 ^  | 0.1uF
GND                         |  |
----------------------------+--+-----------------------------

Cheers.

Dan Michaels wrote:

> Bob.A wrote:
>
> >
> >The point I was making here is that using a (conventional silicon) diode,
> >and the R-2D, R-2D limiter you are still stressing the chip pins at greater
> >then 0.3V, which is their specified absolute maximum rating.
> >
> >Now, I know that in reality you have limited the current to no worse than
> >0.6V/R in the second stage, but this is still, apparently, out-of-spec.
>
> [hint - with no topic/etc, I couldn't even find the thread this
> is listed under].
>
> I don't understand what the argument is here. Seems to me a
> 2D-R --> pin protection ckt is perfectly correct, as long as R
> is chosen to limit the max current into the clamping diodes on
> the pin to a safe value. Chose R > 20 ohms [0.6v/0.025A], and
> go. You will "never" actually see Vcc+0.6v on the pin, due to
> the IR drop.
>
> I have a more interesting question, however.
>
> Say you use the 2D but no R. And say you overvoltage it, and
> run too much current thru the internal diodes, and blow them.
> Well, you still have the external diodes there, and seems they
> will forthwith protect the chip from the stresses the built-in
> diodes where in there for in the first place - ESD. And they
> will certainly be much more robust than the internal diodes.
>
> No ????????/  [any takers?]
>
> - danM
> ===============
>
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