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PICList Thread
'OTP overblow.'
1996\05\24@045112 by Andy David

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Hello,

is it true that the pic 16c73 OTP has a small area of ROM (64 words?)
that can be overblown after the chip has been programmed? When we
were looking around to choose which microcontroller to use in our
application (this was months ago), we were told by the consultants
advising us that this overblow area exists and is usually used for
serial numbers or maybe some constants which aren't defined when
the pic is programmed. Of course, something as useful as this has
been fully exploited by our design spec,  but I've not managed to
find any confirmation of this overblow rom in any of my microchip
books...

regards,


- Andy David.

1996\05\24@060926 by fastfwd

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Andy David <spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU> wrote:

> is it true that the pic 16c73 OTP has a small area of ROM (64
> words?) that can be overblown after the chip has been programmed?
> When we were looking around to choose which microcontroller to use
> in our application (this was months ago), we were told by the
> consultants advising us that this overblow area exists and is
> usually used for serial numbers or maybe some constants which aren't
> defined when the pic is programmed. Of course, something as useful
> as this has been fully exploited by our design spec,  but I've not
> managed to find any confirmation of this overblow rom in any of my
> microchip books...

Andy:

You need better consultants.  The PIC16C5x and 16C71 parts have
code-protection that works as you describe; even after the
code-protection bit is turned on, the first 64 locations can still be
programmed.

The 16C73's code-protection works a little differently; there are two
code-protect bits, and you can selectively protect the entire code
space, the upper 3/4 of the code space, the upper half of the code
space, or none of the code space.  Unprotected areas can be read and
programmed.

-Andy

Andrew Warren - .....fastfwdKILLspamspam@spam@ix.netcom.com
Fast Forward Engineering, Vista, California
http://www.geocities.com/SiliconValley/2499

1996\05\24@074602 by Andy David

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Andy Warren,


>The 16C73's code-protection works a little differently; there are two
>code-protect bits, and you can selectively protect the entire code
>space, the upper 3/4 of the code space, the upper half of the code
>space, or none of the code space.  Unprotected areas can be read and
>programmed.

thanks for straightening that out, I've found the relevent paragraph
now... so even OTP devices can have their unprotected ROM
reprogrammed? I assume it's just the one extra time.


- Andy.

1996\05\24@125624 by Martin J. Maney

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On Fri, 24 May 1996, Andy David wrote:

> thanks for straightening that out, I've found the relevent paragraph
> now... so even OTP devices can have their unprotected ROM
> reprogrammed? I assume it's just the one extra time.

It's not a general reprogramming: you can only change 1 bits into 0
bits.  This is the same as you start out with for the whole OTP chip.
Until the other CP bit is set (and since they get set by being programmed
to 0 they can be set in two steps) you can program areas left in the
erased state.  If you're crafty you can change already programmed code,
but only by changing 1s to 0s.  This could still be very useful if those
1s and 0s are in the data part of a literal instruction...  :-)

(yeah, I thought about doing something like this - the idea was that I
could put two different builds on one EPROM part before I had to stop and
erase it, since the code (so far!) fits well within either of the 2K
banks.  But the practical difficulties of making sure the code assembled
for the correct placement and arranging to have the flag bit get
reprogrammed, etc, made this too much trouble for so little benefit.)

1996\05\24@144138 by fastfwd

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Andy David <PICLISTspamKILLspamMITVMA.MIT.EDU> wrote:

> thanks for straightening that out, I've found the relevent paragraph
> now... so even OTP devices can have their unprotected ROM
> reprogrammed? I assume it's just the one extra time.

   Yes, just once per bit.

   -Andy

Andrew Warren - .....fastfwdKILLspamspam.....ix.netcom.com
Fast Forward Engineering, Vista, California
http://www.geocities.com/SiliconValley/2499

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