The chips are not useless.  They just won't run at the same speed as they did
from the factory unless you put the same value back in for OSCCAL.  You could do
a series of "guesses" with calculations in between until you have the correct
oscillator speed (or at least a speed close enough for your application).  A
quick check of my 12C509 data sheets shows that changing the OSCCAL value from
one extream to the other moves the frequency about 250,000 Hz (at room
temperature). Temperature and supply voltage can change the internal frequency
even more than that of changing OSCCAL.  If you really need a more accurate
clock for the processor, use a crystal or resonator.  If not, don't worry about
OSCCAL too much.  Your programs will still run.

Sean Alcorn - Avion Sydney wrote:

> Hi all,
>
> When I first started mucking around with PICs, I bought 6 x PIC12C509A/JW
> and 6 x PIC12C671/JW - which are not cheap devices. I have since found out
> that erasing these with a UV Eraser actually erases the OSCCAL data for the
> internal clock.
>
> Apparently, I should have read this data when the chips were new, recorded
> the number for each chip, then re-written the calibration data each time I
> write to that particular chip. This I did not do. :-(
>
> Is there any method of retreiving this data or are the chips useless? I
> assume that I can still use them with an extenal clock, but then that leaves
> less I/O.
>
> Regards,
>
> Sean
>
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