Re: Fuseable Links - no guarantees??

[wxfield@shore.net (Warren)]

Jim;

        I was under the impression that a fuseable link was literally a
piece of conductive material that you deliberatley 'blow-away' - In most
cases, couldn't you simply 'tap into' the data side of the fuse, and
download the info??


>At 11:44 PM 6/14/96 -0400, Warren wrote:
>>I have never paid much attention to the protection of firmware or the
>>technical issues revolving around such schemes...was wondering:
>>
>>I recently saw an add for a UK based group that says they can take a PIC
>>OTP micro and read the prom (for a fee, of course) - How the heck is this
>>done?? I have my suspicion that they (somehow) magically peel off the
>>ceramic coating (without destroying the chewy center), get a circuit mask
>>and 'micro probe' the I/O of the IC...they then download the secret recipe
>>to the afore mentioned 'chewy center'.
>>
>>Is this close to accurate?? How is it 'done' ???
>
>
>While I have never come even close to needing to attempt this kind of thing,
>long ago it occurred to me that if the "no read" bit was stored in a
>programmable bit, and if the location of that bit was known or could be
>identified, you could expose that particular bit through a tiny mask hole
>and cause the part to be readable again.  Locating that bit (assuming
>there's just one) would be relatively simple:  Take a test part, program it,
>read-lock it, and then expose it to a VERY slowly sliding mask with UV
>behind.  Do this for both axes, to find the bit's location on the chip.
>
>Jim Bell
>jimbell@pacifier.com