Good Design More on (ab)use of PIC protection diodes & bad design
Dennis Plunkett email (remove spam text)
At 11:15 11/10/99 -0500, you wrote:
>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?
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
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.
See also: www.piclist.com/techref/microchip/devices.htm?key=pic
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