> I believe that E(E)PROM cells can also be erased by soft Xrays. The problem
> with Xrays is that they are 10-50 times more energetic than UV (UV quanta
> by definition have energies in the range between 3 and 200 eV, and Xrays
> are above that, with most typical Xray generators using 5,000-50,000 eV).
> Thus, Xrays can induce radiation damage in the semiconductor, so you have to
> be very careful with the dose. On the other hand, a little metalization won't
> prevent the erasure by Xrays.
>
> I suppose that a friendly dentist or crystallographer would let someone use
> their Xray machine; would someone be willing to experiment a little with
> the times necessary?

This would leave us back at square 1 it seems, in terms of security (though
no worse than many other OTP micros).  On the other hand, if X-rays work,
OTP's could be used as EPROMs :-)

Anyway, here's the idea I was thinking of for code-protect: rather than using
a "fuse" (EPROM location), why not have a RAM flag [i.e. latch] for that
purpose?  Require that in order for programming or reading to be performed
on the device, the PC must first feed the device a copy of the program within
it.  Have this behavior apply regardless of whether a "code-protect" flag is
set or not (there wouldn't have to be a code-protect flag).

While some care would need to be taken to prevent an unscrupulous person from
glitching the RAM flag, these semantics would allow a device with known cont-
ents to be verified (or programmed more) and a blank device (whose contents
are known implicitly) to be programmed and verified.

What do people think of this idea?