I'm not sure I understand what you disagree with.  I am saying that the
Abs. Max. Ratings and the Input Low and Input High specs are two distinct
issues.  One is a limitation that you must not exceed without introducing
"nar nar" as you say.  The other is a specification of how the chip behaves
under normal conditions, or specifically what the voltage thresholds are
for the input logic.

I injected 2ma DC from a constant DC source, not from field wiring but a
regulated lab power supply.  It never exceeded 2mA and it makes the chip
malfunction.  However, I didn't measure the voltage on the pin.  Its quite
likely that it went below -0.6V and exceeded the voltage limit without
exceeding the current limit.

My point in bringing up this issue was to warn about relying on the
protection diodes when significant current is present, and to present some
details about what happens.

-Jim Hartmann





Dennis Plunkett <dennis@RDD.NECA.NEC.COM.AU>@MITVMA.MIT.EDU> on 10/11/1999
05:23:38 PM

Please respond to pic microcontroller discussion list
      <PICLIST@MITVMA.MIT.EDU>

Sent by:  pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>


To:   PICLIST@MITVMA.MIT.EDU
cc:
Subject:  Re: PICs: Good Design More on (ab)use of PIC protection diodes &
      bad design


At 11:15 11/10/99 -0500, you wrote:
>Thanks,
>
>The 16C621 Absolute Maximum Ratings says "Voltage on any pin WRT Vss :
>-0.6V to VDD+0.6V".
>
>I think there is some confusion as to what the DC Characteristics "Input
>Low Voltage" and "Input High Voltage" mean.  I believe that these
>specifications do not dictate limits to what can be applied to the pins.
>These specifications dictate what Microchip guarantees will be the
>thresholds of the input logic circuit.  An input voltage will produce a
>zero input guaranteed if the input voltage is VSS.  But the threshold
could
>be as high as 0.8V maximum (ttl buffer).
>
>Anyone care to differ?
>
>Jim Hartmann
>


YES I DO!
If you look you will see that Microchip just like all other manufacturers
will have a little note that says
"Stresses abaove and beyond the maximum limits may cause dammage".
The maximum limits are that that can be applied to any pin on the device.

I think that you are getting it wrong
"The 16C621 Absolute Maximum Ratings says "Voltage on any pin WRT Vss :0.6V
to VDD+0.6V""
This is it, go anywhere over these limits and BANG!
In your case you are lookingat why and what current can be sustained. Now
you need to do two things to get this one rapped up. In an earlyer mail,
you indicated that 2mA of negative current casued the PIC to go nar nar.
OK, so was this current constant? Did you monitor the current with a CRO to
see if at any time during that application that the current was in excess
of 20mA? Did you monitor the pin votlage and the supply votage to see if
the supply exteneded beyond 6V (Max limit?) and that the pin voltage was
not more than -0.6V? If you performed this and it was within these limits,
then contact Microchip, as it would seem that you have found an error.


Dennis