> This is unfortunately the fate of a code-protected JW part.
> Microchip decided in their infinite wisdom that windowed
> parts don't have to be eraseable when code protected.
>
> The reason for this is as clear as mud, but then Microchip
> moves in mysterious ways...

I know of a codeprotected COP micro that has been read out "at
home". The attacker used a cardboard mask with a tiny hole in it.
He used the windowed part to practice: he placed the mask on
the chip at one position, and erased. Then he checked if the
code protection has been erased. If not, he started over with
a new mask position.

Once he found where the code protection bits are placed on
the die, he noticed that they were far away from the code
itself.

He used acid to "un-package" the real attack target (which
had no window) and used his knowledge about the security bit
position and his mask to erase just the bits, not the code
he was interested in.

I think this was done with the "cop cards" (pay tv pirate
decoders).

Protecting the code protection bits from erasure is a
counter measure to this type of attack. Having only one
die for both plastic & windowed packages is a cost saver,
with the disadvantages you noticed. I, too, have a code
protected JW on my desk since a week :-(