> > I have been asked to divide a digital signal by 3.

> 3 is an akward number.  My first reaction is

All good thoughts.
I've tried to think of them all already - alas the situation is fairly well
constrained.

> to step back and take a system
> wide view.  Why 3?

The customer says so :-)
It's the input to a console for a piece of "rotating machinery" for a
consumer application. The speed is much faster than they are normally used
to because the equipment it's being used with is different than usual and
the sensor is on a motor pulley running at up to 4000 rpm * - usually it's
on a slower moving pulley but in this equipment there is no slower moving
wheel. The divide by 3 happens to match a standard setting in the equipment.
The processor is mask programmed and produced in zillion quantities. (*
spits magnets nicely when you use 5 minute epoxy to fasten them :-) ).

> How is the signal being used?  Could the rest of the
> system be easily modified to not require the divide or deal with a divide
by
> a power of 2?

I asked that. They are still considering if any power of 2 division will
strike some other standard ratio but the answer will probably be , "no".:

> Is there a programmable part elsewhere in the system that can
> compensate?

No. As above. Mask programmed.

> Are there any other spare gates from the rest of the circuit?  If there
are
> spare inverters, these could be specified to be schmitt trigger and front
> end the flip flops.

No spare gates.

> If not, 2 CMOS inverters and two resistors make a
> schmitt trigger.  If there are no suitable gates, you can do it with two
> transistors and a few resistors.

I built a 2 transistor Schmitt trigger to trial the result when using JK
flip flops for the divide. Haven't had to do that for a few decades :-)
Looked reasonably nice on the scope - didn't pursue it too far. The JK's
didn't like it though - better but not clean enough for 100% reliable
operation. Probably still not meeting the JK rise/fall time specs. I could
have tried to improve the Schmitt and may well have succeeded but the
component, board area and loading costs would have been in excess of the 2
diodes plus resistor required by the CD4040 solution. (2 transistors, 3+
resistors, probably a speed up cap by the time I was finished.)

The 4040 solution works with hall and reed sensors up to 4600 rpm and as
slow as I wished. Would be fun to get down to a true 1 chip solution but I
doubt it's going to happen economically. A dual JK with a Schmitt clock
would do it - but would proibably be non-standard enough to cost more
overall. Some of the programmable/presettable dividers  have beckoned but
the lack of Schmitt input makes the 4040 solution about as good.

All good fun though.



        Russell McMahon

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