Had a similar problem with an 8V field bus supplying a PIC via a linear 5V
regulator.
When the 8V field bus had a partial short, it dropped the line voltage to 3
or 4V.(Current limited over a long line)
When the field partial short was remove, the linear regulator which was
driving hard
to maintain voltage, let a spike of high voltage through before it could
regulate properly.
This was corrupting the EEPROM on the PIC but did not affect the flash. A 5=
V
transorb across the PIC fixed the problem.

Paul


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