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PICList Thread
'16C74B'
1999\03\23@111518 by Jean-Paul Hemelaar

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Hi everyone,

I have two problems with the 16C74B /JW:

One of these chips has 0000 written to it's data memory. It is not
possible to erase the contents of the chip. However, it
is possible to write to the fuses and the ID locations of the chip.
After calling the distibuter of the chip I got the answer that it is
possible to protect the chip in a way that it can NOT be erased...
Is this a new function (I worked with 16C74A/JW before and never
had this problem) ? Has anyone else had this problem.

Second question:

When I use the TMR0 interrupt it seems as if this timer is a 16 bit
counter instead of a 8 bit counter. I use a 8MHz oscillator and a
divider of 1:256; result: an interrupt every 32 msec. When I look in
the datasheets it should (still) be a 8 bit counter.

Kind regards,

Jean-Paul Hemelaar
DATEQ b.v.

1999\03\24@012638 by Tjaart van der Walt

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Jean-Paul Hemelaar wrote:
>
> Hi everyone,
>
> I have two problems with the 16C74B /JW:
>
> One of these chips has 0000 written to it's data memory. It is not
> possible to erase the contents of the chip. However, it
> is possible to write to the fuses and the ID locations of the chip.
> After calling the distibuter of the chip I got the answer that it is
> possible to protect the chip in a way that it can NOT be erased...
> Is this a new function (I worked with 16C74A/JW before and never
> had this problem) ? Has anyone else had this problem.

This is unfortunately the fate of a code-protected JW part.
Microchip decided in their infinite wisdom that windowed
parts don't have to be eraseable when code protected.

The reason for this is as clear as mud, but then Microchip
moves in mysterious ways...

Some PIClisters have reported success with leaving the
part in a UV box for a *long* time to erase. You don't
have to worry about damage to the part, because it is the
programming that is destructive, not the erasing. (besides
- it's screwed anyway)

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1999\03\24@055006 by Jim Robertson

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At 16:56 23/03/99 +0000, you wrote:
>Hi everyone,
>
>I have two problems with the 16C74B /JW:
>
>One of these chips has 0000 written to it's data memory. It is not
>possible to erase the contents of the chip. However, it
>is possible to write to the fuses and the ID locations of the chip.
>After calling the distibuter of the chip I got the answer that it is
>possible to protect the chip in a way that it can NOT be erased...
>Is this a new function (I worked with 16C74A/JW before and never
>had this problem) ? Has anyone else had this problem.

This unerasable code protection problem is not new to the 16C74B,
it was always there with the 16C74A and most other PICs, nothing
has changed.


Jim



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1999\03\24@063518 by Marc

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> This is unfortunately the fate of a code-protected JW part.
> Microchip decided in their infinite wisdom that windowed
> parts don't have to be eraseable when code protected.
>
> The reason for this is as clear as mud, but then Microchip
> moves in mysterious ways...

I know of a codeprotected COP micro that has been read out "at
home". The attacker used a cardboard mask with a tiny hole in it.
He used the windowed part to practice: he placed the mask on
the chip at one position, and erased. Then he checked if the
code protection has been erased. If not, he started over with
a new mask position.

Once he found where the code protection bits are placed on
the die, he noticed that they were far away from the code
itself.

He used acid to "un-package" the real attack target (which
had no window) and used his knowledge about the security bit
position and his mask to erase just the bits, not the code
he was interested in.

I think this was done with the "cop cards" (pay tv pirate
decoders).

Protecting the code protection bits from erasure is a
counter measure to this type of attack. Having only one
die for both plastic & windowed packages is a cost saver,
with the disadvantages you noticed. I, too, have a code
protected JW on my desk since a week :-(

1999\03\24@075400 by paulb

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Marc wrote:

> I know of a codeprotected COP micro that has been read out "at
> home". The attacker used a cardboard mask with a tiny hole in it.

> Protecting the code protection bits from erasure is a
> counter measure to this type of attack.

 That's about the size of it.  But I surmise similar work with a SEM or
whatever they call it would just as likely enable location of the CP
bits and at a higher power level, erase or ablate them.

 All dead easy if you have a specialist RE facility and make your money
therefrom.

 I think of it (CP) more as "service protection".  Doesn't protect the
manufacturer much from a larger organisation copying their work, just
from small operators and users trying to back-up and service their
investments, bug-fix the code and enhance it.

 Of course, that is just from my viewpoint, as that is what I RE for.
Sneer if you will.
--
 Cheers,
       Paul B.

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