Hi,
you should keep in mind that the factory-determined OSCCAL tuning
value can be different for every PIC. Such way, one should READ and SAVE
the said value before manipulating the content of the EPROM (I wrote on
the bottom of the PIC). Then the program should contain exactly that MOVLW
statement, and then as a very 1st one MOVWF OSCCAL.  It is possible when
the programming process does not influence the very last location of the
EPROM then it remains unchanged. But, if you erase it via UV then it is
not true any more.
I hope this helps.  Imre


On Mon, 22 Feb 1999, Chris Morison wrote:

> Dear All,
>
> I'm in a state of confusion over the Internal Oscillator on the
> PIC16C672.  I've previously worked with PIC12's and I am now
> branching into PIC16's.  I've make the PIC16 work with an external
> 4Mhz Xtal oscillator, but when I remove it and re-program the part
> with the options set to internal oscillator, the very same program no
> longer works.  Any ideas?
>
> I also don't understand the logic in placing the OSCAL tuning
> number into the end of the program memory as a RETLW
> instruction.  If I have to erase the part before re-programming, then
> surely I have to include this in the new program, which means
> knowing what the number is.  If this is the case, why don't
> Microchip just quote the number in the datasheet, and let me
> program it in however I like?  What does this default tuning number
> tune the clock too anyway?  Can anyone tell me what it is,
> because I erased the part before I was interested in the internal
> oscillator and read that bit of the datasheet?
>
> Unless I've missed or misunderstood something, it seems totally
> illogical to supply this information pre-programmed onto the part.
>
> Can anyone alleviate this pain in my head?
>
> Cheers.
>
> Chris Morison
> --
> Research Student
> Manufacturing & Mechanical Engineering
> University of Birmingham
> Birmingham, B15 2TT
> Tel: +44 (0)121 414 4185
> Fax: +44 (0)121 414 3958
> e-mail: c.n.morison@bham.ac.uk
> web:    http://www.arcsite.de/hp/chrismorison/
>
>