On Thu, 28 Aug 1997 09:56:55 +0000 Lawrence Lile <lilel@TOASTMASTER.COM>
writes:
>PIC-nikkers:
>
>What happens to the OSCCAL number when you UV erase a windowed
>PIC12C508?  Is the OSCCAL even programmed into windowed parts in the
>first place?

It is erased.  The cal value is merely a MOVLW instruction in the highest
program address.  (The MPLAB/PICSTART software doesn't allow storing an
instruction there, but physically the EPROM is continuous.) After a
reset, the 12-bit core sets the PC to all ones, which causes the MOVLW
<cal value> to be exectuted first.  The PC then rolls over to all zeros,
where the first user instruction is stored.  Without the cal value
instruction, the W register would be undefined after reset, so assuming
it has a cal value will cause unpredicatble results.

As part of the manufacturing process, the factory determines the correct
value and programs it into the EPROM, leaving the rest of the EPROM blank
just as if it were a plastic chip.  Everything about a window chip is
exactly the same as an OTP one except the package, and additional testing
so it is qualified for all oscillator settings.

Before erasing the chip the first time, read the factory's value out and
remember it for future reprogrammings.