For fixed values like this, I generally divide in one of two ways...

- Subtract 5 continuously in a loop, until the remainder is less than  
5.  (Then process the remainder as required).

- Convert 1/5 (=0.2) into a value that is <integer>/256 (or even  
<integer>/65536, or other depending on precision required).  Then  
multiply by that integer and shift to divide by 256 etc.  To find that  
integer, I generally would keep multiplying 1/5 by 2, and look for as  
whole a number as possible.  For 1/5, multiplying by 256 is close to  
whole at 51.2.  51/256 = 0.1992, which is 0.4% error from 0.2.  If  
that's acceptable, then the code just needs to multiply by 51, then  
drop the right-most byte.  You can even check the MSB of the  
right-most byte and increment the next byte if required to round the  
result.

FWIW, there are some divide routines in the PIClist source-code library.

BTW, I've done C to F before, and I did the math for 9/5 at once.  I  
used 717/512 = 1.40039 which is 0.03% error.

Cheers,
-Neil.



Quoting alan smith <micro_eng2@yahoo.com>:

> Processor is a 18F23K20
>
> I've never had the requirement to do much math with a PIC, other   
> than simple divide by 2 or 4 (shifts), add and subtract
>
> So looking for a little help in implementing this
>
> temp_F = (temp_C * 9 / 5) + 32;
>
> The temp_C * 9 can be done with the MULLW instruction.  Just not to   
> sure about the divide by 5 part?
>
>
>
>
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