21MX proms (per request

[Eric Smith]

On Fri, Sep 11, 2015 at 12:41 PM, GerardCJAT <gerardcjat at free.fr> wrote:
> HOW OFTEN theses old PROM fail  ??

Note that some bipolar PROMs suffer from fuse regrowth, where bits
that have been programmed gradually revert to an unprogrammed state.
This was a big problem with NiCr fuses. Later bipolar PROMs used
different fuse materials; TiW was one of the later choices. I don't
know whether TiW or other newer fuses have regrowth issues, but if
they do, it's slower than NiCr.

It is possible to recover the bits from a bipolar PROM with regrowth
by decap and optical inspection, because the regrowth happens in
relatively narrow tendrils which are visibly different than an
unprogrammed fuse.  I've actually seen that done.

There is speculation that it could be done without decap by trying to
blow each unblown fuse and measuring the energy required to blow the
fuse; a regrown one should require less energy. I'm not aware of
anyone actually having done it that way. I wouldn't trust a
one-of-a-kind PROM to that process.

Obviously it's best to dump the PROM contents before regrowth becomes an issue.