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
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