[time-nuts] Frequency division by 81

[ed breya]

For a 10 MHz clock, 74HC would be fine. For small numbers like 81, a 
couple of 74HC163s would do it, and be good to go since they're 
synchronous anyway.

For large numbers, my go-to divider is the 74HC4040 12-bit ripple 
counter. It can be rigged for any fixed integer divide ratio from 3 to 
4095 with a simple diode gating arrangement for feedback to the reset 
input. For divide by 81, three diodes (64+16+1) and a resistor comprise 
the feedback circuit. The bigger and more numerically complicated the 
divide ratio, the more diodes are needed. The straight binary 2^n ones 
are of course trivial. The output edge may be re-synchronized with DFFs, 
as Gerhard mentioned. This is advisable especially for precision time 
and frequency work. Also, if you use a dual FF anyway, one can be used 
for re-synchronizing, and the other can be used in the feedback, to 
improve the reset action. The reset is asynchronous, so there is a risk 
of race around from the outputs to the reset, but the prop delays tend 
to eliminate it, and I've never had a problem with it.

The 74HC4020 may also be used, depending on the divide range needed - 
some of its binary stage outputs aren't available, while the 4040 has 
all of them. The 74HC393 also can work this way, for up to 255. I've 
used these counters many times this way, but can't remember the exact 
hookup, without going back to notes on some of my projects, or figuring 
it out again from the datasheets. It's quite simple, but if anyone 
needs, I can revisit and explain it.

Ed