At 09:37 AM 5/8/00 +1000, you wrote:
>Hi PICster's,
>I have had a harrowing experience yesterday with a couple of old PIC16C55s.

Ok, it sounds as if you have experienced what is called "latch-up".

All CMOS chips will do this, PIC's are relatively immune, but what happens
is this..

Somehow the protection diodes on at least one input get forward biased, that
means that the voltage must be a bit above Vdd or a bit below Vss (by a
diode drop).  Fine, they are designed to work this way, if , *however* the
current has for even a microsecond exceeded the maximum (in the low 10's
of mA) then latch-up occurs.

Essentially there is a parasitic SCR present in all these chips between Vss
and
Vdd, just waiting for a gate signal. When it turns on, you get about 1V from
Vss to Vdd, even at 100's of mA. Like all SCR's it will stay on until the
current from Vdd to Vss drops below the holding current of a few 10's of mA
(like when you notice it is getting hot and cut the power).

If the power supply is current limited, the chip may even survive this nasty
experience, though high temperatures may damage or degrade it.

It is often caused by applying a signal to an input without any series
impedance before power is applied to the chip, or by static discharge into
an unprotected input.

Hope this helps,

Best regards,
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