> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of alan smith
> Sent: 13 May 2010 13:39
> To: Microcontroller discussion list - Public.
> Subject: [PIC] converting C to F
> 
> 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?

What range of input values do you need to cover?  By adding an offset
you can use a close approximation for your fractional calculation and
spread the error out.  e.g. 

temp_F = ( ( (temp_C * 205) + 112 ) >> 8 ) + 32 + temp_C

This gives results accurate to within +-0.4 Fahrenheit over the range
-50 to +200 degrees Celsius

Mike

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