Yes Jim you're right :-)

and the 24 bit version should actually be:

0x1000000 * 1 / 2.367 = 0x6C275E

Regards
Sergio



On Thu, 12 Jun 2008, PAUL James wrote:

> Sergio,
> 
> Don't you mean 0x10000 * 1 / 2.367 = 0x6C27?
> 
> jim 
> 
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of sergio masci
> Sent: Thursday, June 12, 2008 7:20 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] frequency dividing 
> 
> 
> 
> On Thu, 12 Jun 2008, Snail Instruments wrote:
> 
> > At 09:26 PM 6/12/08, you wrote:
> > >The speedo wants 2,458 pulses per kilometre. This works out at 
> > >dividing the wheel sensor frequency by about 2.367 with a maximum 
> > >frequency from the wheels of 320Hz at 150MPH (no I don't plan to go 
> > >that fast! Might as well design to the max speed though).
> > 
> > Express the 1/2.458 as a fractional binary number:
> > 
> > = 0.0110 1100 0010 0111 etc... or = 0.6C27 in hexadecimal
> > 
> > Now make a 16 bit variable, add 0x6C27 to it for every incoming pulse.
> 
> > Every time there is a carry set after addition, send an output pulse. 
> > The division ratio will be very close to 2.367 without large tables.
> 
> This a good way of doing it and in case anyone is wonder how to derive
> the magic number:
> 
> 0x10000 * 1 / 2.458 = 0x6C27
> 
> If you wanted greater accuracy, say 24 bits instead of 16
> 
> 0x1000000 * 1 / 2.458 = 0x6C275E
> 
> Regards
> Sergio
> --
> http://www.piclist.com PIC/SX FAQ & list archive View/change your
> membership options at http://mailman.mit.edu/mailman/listinfo/piclist
> 
> -- 
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
> 
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist