> To be clear, I want the application to be able to read and write itself
anywhere
> in memory, whether copy protected or not.  If you insist on it, you may
disable
> this feature with a config bit.  I like that feature of being able to
block
> external access to individual blocks.  This is good to allow me to LVP or
HVP
> burn configuration data, etc. into an otherwise unreadable chip.  I also
like
> being able to selectively control access to the EEPROM.
>
> I want to be able to burn the chips using standard LVP or HVP modes after
they
> are plunked onto the board.

I've been following this thread with interest because I've done a couple
dozen PIC projects and never had occasion to care about code protection.
I'd be curious to hear what applications warrant this kind of paranoia.  I'm
not saying it isn't warranted on occasion, but am surprised by the hornet's
nest this topic has stirred up.

Even if someone could read every bit on the chip, that would get them only
to a dissassembly listing at best.  This is a long way from useful source
code that can then be modified in a meaningful way.

If I was really paranoid, however, I could claim that partial over-write
access without requiring a complete erase and unlimited internal read access
is a security hole.  I could program a snoop program into the bank 0 that
can then read out the remaining banks for me.  Using a different chip, I
could program the snoop program into some other part of memory and just
insert a jump or whatever at 0, an interrupt vector, or someplace I think
execution might get to.  It might require some experimenting with multiple
chips, but the whole program could eventually be discovered.  Therefore
partial external write access IS read access.  The only secure external
write access is one that requires complete erasure regardless of how little
is written.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@cognivis.com, http://www.cognivis.com

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