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
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