We had to do something similar.  Fortunately we were using a Xicor quade2pot
for setting various parameters in the system and it happens to have a load
of spare registers that we use to store calibration info.

I guess you could pad the thermistors/pots to get a nominal value but I
don't suppose you want the hassle of SOT.

As long as you know the location of the calibration factors then you can
just modify the HEX file, but you will have to adjust the checksum at the
end of the line.

Good luck

Mike Rigby-Jones

> -----Original Message-----
> From: Lawrence Lile [SMTP:lilel@TOASTMASTER.COM]
> Sent: Tuesday, May 04, 1999 2:56 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      In System Calibration
>
> I've been dreading a project that may require in-system calibration.
> Here's
> the scenario - I'm using some cheap, poor tolerance thermistors and
> potentiometers in a controller.  I have a number of these built up, and
> found their behavior changed quite a bit from unit to unit, ostensibly
> because of tolerance variations.  (I'm not really sure)
>
> I could tighten the tolerance by spending money, but the circuit must be
> absolute minimum cost.  So I'm imagining a system that:
>
> 1.  Does a final check on resistances in the circuit  (and maybe a quality
> control check too?)
>
> 2. Calculates two or three constants based ont he resistances in the
> circuit
>
> 3. Creates the proper code (would it have to COMPILE or just modify the
> right bits in a hex file?)
>
> 4. Squirts the program into a PIC either in- circuit or just before
> insertion
>
> It sounds really complex and capitol-intensive.
>
> Has anybody worked with these before?  Is there commercial equipment that
> does this task automatically?