(Reposted with topic tag)

 * *

A contradiction:

>> Now, the program itself frequently _moves_ 32-byte
>> blocks from program memory to RAM, but
>> never writes in program memory.

Versus:

>> I normally use an USB bootloader to load the actual software.

Maybe you'll see it as semantics, but "never writes to program memory"
isn't accurate.  What you mean is that you don't do that intentionally.
But there is a routine present in  memory at all times that will cheerfully
write to program memory if that routine is entered.  I'm not trying to
split hairs.  I remember a long discussion years ago here in which this
sort of thing was hashed out.  You have actually run some good experiments
for us that pretty much points to the block erase function as the culprit.
Meaning, it's not some hardware glitch that causes blocks to disappear.
Instead, it's probably "random code execution".

So, what to do?  Everything Dom just said applies, and I won't repeat
that.  Another level of protection might be to make it more difficult for
the bootloader code to write to program memory (like "secret unlock
codes").  But you can only be so clever, because there will always some
place you can jump to that knows all the tricks to do the erase.  As well
as the core erase routine itself.

Microchip has some parts (don't recall--might be only the 32-bit) that
allow you to access the configuration once and only once at power on, then
protects it.  This could allow the bootloader to access normally protected
space.  But that's not a guarantee if there are startup glitches.

There are situations where there's a cause and then there are subsequent
preventions to guard against the actions of the cause. In my humble
opinion, it is best to prevent the problem in the first place, than to try
deflecting it later.  I think attacking possible startup glitches and power
problems is the way to do it.  By the way, where I worked, we always used
an external brownout/reset controller (simple 3-pin device) and have never
experienced this particular problem.  And I did have a bootloader most
times.
--=20
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