[time-nuts] 50/60 Hz clocks

[Hal Murray]

magnus at rubidium.dyndns.org said:
> On the other hand, it would not be difficult to make a DDS which hit  60/
> 10000000 exactly. Reducing it by 20 on each side you get 3/500000 so  a 19
> bit accumulator (mod 500000) incrementing with 3 on every 100 ns  period
> would do it.

Neat.  Thanks.

I'd noticed that adding in decimal rather than binary would make exact target 
frequencies in some cases, but I hadn't generalized to adding modulo N.  
Using N of 10,000,000 with a 10 MHz clock gets you all exact integer 
frequencies in the audio range.


> A LUT for sine would be possible. Playing a few  tricks with the LUT table
> (realizing that the LUT would be walked  through three times with three
> different start-alignments) converts it  into a LUT of the same size and a
> increment by one or decrement by one  counter modulus 500000. A decrement by
> one counter allows wrap-around  loading with 499999 easy. CPLD or CMOS/TTL
> implementations would be  trivial for the counter. The LUT will be large...

More neat.  Thanks again.  It's just a simple state machine cycling through 
some collection of states.  If we are willing to rearrange the LUT/ROM, we 
can simplify the next state calculation from a modulo adder to a re-loadable 
counter.



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