David Venz wrote:
>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?
>

David,

I just checked the datasheet for the 16C7x, and it actually says +/-20 mA.
Max input clamp current, under Electrical Characteristics.

If you use a 330 ohm series R in each line, like I do as a general rule,
then you have ~7v overvolt protection. My personal philosophy is to
NEVER hang an unprotected pin onto a header for external connections,
etc. 330 ohm is a nice choice, IMO.

However, during qualification testing - specifically intended to help
determine what stress levels the internal diodes could handle - I have
purposely applied up to 80 mA via series R, and not had the internal
diodes pop. So, I assume there is a fair margin of safety on the spec.
==============

.......
>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.
>

5v1 should still have a series R upstream, plus I imagine some people
believe the ~0.7v for below gnd swings violates the spec. Also, if the
input signal is high-speed, then the zener capcitance is going to
severely slow the response - prolly not a problem for your situation.
===============


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

If you are having problems with sensing near gnd, you might try reducing
R1 and R2 to the 1K region, or less. You will still have tons of safety
margin with your ckt. Do the calcs, using 20mA max into the pin. For
this ckt, R2 can be very small, and still meet it.

- dan michaels
www.oricomtech.com
=========================

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