Rolf wrote:
> Uhmmm... because I was worried that at 3am some morning while my brain
> was fried I would erase the chip instead of program it...

Good programming software makes it difficult to just erase the chip without
first reading the calibration value then writing it back afterwards.  You
can't just erase a chip without saving the calibration value using my
PIC_PROG program, for example.  It does perform a bulk erase, but always
reads the calibration value first, writes it to standard output for
reference, does the erase, then write the calibration value back.

> so I first read the chip, wrote the
> callibration number (with a sharpie) on to the chip, and in to my
> code...

On the chip is a good idea, since things can still go wrong.  If power went
down or communication interrupted during the right few 100mS, my programmers
could lose the calibration value also.  In the code is not a good idea
because that code will only work right on one specific chip.  You'll
probably forget all about that when you pick another chip out of the drawer.

> There is not much documentation (that I can find) about safe-guarding
> the callibration value in the program memory. Can you point me to
> something that indicates how to properly deal with the instruction
> (RETLW 0x??) at 0x3FF?

In the linker file specify the program memory range from 0 to 3FFEh instead
of 0-3FFFh.  If you want to you can create a separate region at 3FFFh and
declare it protected.

> Do all programmers use a similar approach to warning the user when
> programming the chip that the callibration data is different?

Actually I think it's not possible to overwrite the calibration value with
my programming software using the default configuration file.


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