Wouter wrote:

>So your potential customer/data thief can put any program in the ROM he
>wants. In that case I don't see how you can prevent that he reads the data
>EEPROM, while at the same time your program can read it. The best you might
>do is that he can not make sense of what he reads, but that is anoter
>matter.

I believe there is another option, and this is what I'm working towards.

I'm assuming that it is not possible to selectively erase, say, the
program memory without also causing the EEPROM contents to revert to
0xFF for every cell. I.e. bulk erase erases both program memory and
EEPROM memory.

Is my assumption correct?

If it is, then if you don't leave any unprogrammed (0x3FFF) cells in
program ROM, then it's nearly impossible (I believe) to "put any
program in the ROM he wants" without first erasing the device
entirely, because to bulk erase program memory you'll end up erasing
EEPROM memory, too.  You would have to be very lucky to find just the
right combination of opcodes in the existing code to allow you to
overwrite them (i.e. turn 1's to 0's) to exactly match the EEPROM
reading program you want / need.

Therefore to try to read the protected EEPROM via a (new) internal
program, you must bulk-erase the part, and with that, the EEPROM
contents are destroyed.

My intention is for the (my) program to refuse to work if it can't
find a particular "key" in EEPROM. Therefore protecting the EEPROM is
what I need to do.

Sound plausible / correct / possible?

Regards,



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   Andrew E. Kalman, Ph.D.   aek@pumpkininc.com

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