[time-nuts] Frequency Dividers

[Hal Murray]

> I have been seeing a lot of traffic concerning making 10MHz frequency
> dividers using PIC's. While they provide an elegant solution to
> providing an accurate 1PPS from a precision source, I have to ask if
> there is a reason for going this route? I am just using three HCT40103
> down counters hooked to a DS4000 to get what I think is a very stable
> 1PPS. Am I missing something? I realize 40103's are as old as dirt (I
> guess I am showing my 4000 series CMOS days), but the HCT series have
> plenty of bandwidth. 

The answer probably depends upon the context.

For low volume, design time and/or time to market is probably more important 
than small differences in cost.  That means you normally use the tools you 
have and/or are familiar with.

For high volume, parts cost is important.   Old, mature, parts are often 
inexpensive.  TI says $0.59 for the CD74HCT40103, qty 1K.  Strange, Digikey 
says $0.36.  That seems high to me for a jelly-bean type part so I'd guess 
that one is old enough to be past mature.  (TI says they got it from Harris.)

Digikey says $0.96 for the PIC12F629, Q100.  So even ignoring the board 
space, it looks cheaper.  (That's ignoring the NRE for the software and the 
cost of programming each chip.)

The main reason I'd choose one over the other for a hobby project is the mood 
I'm in and/or unspecified design options.  I happen to get a kick out of 
writing small, clean chunks of code, so I would probably go that way all 
other things being equal.

The other main advantage of software is that design changes are often easier. 
 (That's assuming the programmer is still around.)



> I understand that the micros work fine, it just seemed like you would
> need a second one to discipline the OCXO because of the timing
> constraints on the divider. 

It's often possible to do two things with one CPU.  It's much more 
interesting if they are both timing critical, especially if you are counting 
cycles to get very tight control.

A second CPU may be cheaper/simpler, but sometimes it's fun writing that sort 
of code.  For something as simple as a clock divider, I'd probably try to 
push all that work into the timer hardware.



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