[time-nuts] Wikipedia Dual-modulus prescaler item

Brooke Clarke brooke at pacific.net
Thu Dec 21 14:53:01 EST 2006


Hi Peter:

I think the beginning of the idea goes back to pen plotters where you 
may need to draw a line where the end points are not on the grid of 
integers of possible pen positions.  But the remainder may be off by 8 
bits.  For more on Bresenham's algorithm see:
http://www.pacificsites.com/~brooke/PClock.shtml#BA
The problem with it is that the jitter can be as large as the register. 

It's possible to get the same effect but with a jitter of only one clock 
period.  To do this you divide the input frequency by N for X cycles 
then divide it by M for Y cycles.  For example you can divide any 
oscillator frequency down to 1 PPS and the jitter in the 1 PPS period 
will be only 1 uC clock period.  You don't need to use dual modulus 
counting if the input frequency can be factored into numbers that are 
smaller than the register size of the micro controller.

Have Fun,

Brooke Clarke

w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com



Peter Vince wrote:

>David Forbes recently mentioned the Wikipedia item on dual-modulus
>prescalers:
>
>     http://en.wikipedia.org/wiki/Dual-modulus_prescaler
>
>I have looked at that article, and either my brain has slipped out of
>gear, or there are some errors in both the diagram and the words:-(  I
>think I can see the sort of thing they are trying to achieve, but was
>hoping someone here could provide a clearer explanation please?
>
>Thanks, Peter
>
>
>
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