Hi Andreas,

the only way I have blown a pin of a 51 device within the last 10 years
was by applying +12V on the pin.

Please can you tell more about the blown pin: Is it permanently
short circuit against GND or VCC or can it still work as input ?

I see no way to blow it by software.

The only way I see, was applying voltage outside the operating level
(below GND or above VCC). This can also be happen, if VCC = 0V
during power down and 3V applied on the pin.
This can be done by separate powered circuits or by capacitors
especially during power up or power down.

If you not sure, that this can be happen, add 2 shottky diodes against
GND and VCC and a 100 Ohm resistor in series at this pin.


Peter