[sdiy] frequency counter

[Magnus Danielson]

From: mark verbos <mverbos at earthlink.net>
Subject: Re: [sdiy] frequency counter
Date: Sun, 27 Jun 2004 01:01:39 -0400
Message-ID: <40DE54B3.2080603 at earthlink.net>

> How do you ddivide by 25,000? Aren't all counters powers of 2?

No, not all counters is a power of two. If you take a number of 4018 CMOS
dividers you can program these. But even more importantly, you can hook them
up to longer chains and program them up. Actually, both the CMOS and TTL sets
of devices is full of various counters of either fixed or programable divisions
which would allow you to divide this up. If you use chips with /5 and /2 then
it is interesting to note that 25000 = 5*5*10*10*10 so you would need 5 chips
where two is /5 and the other three is /10.

In short, these counters is built from a power of 2 but then one way or another
(there are many different methods) have a shortend cycle. The length of this
shortend cycle is the division ratio. For some counters is the length locked
and for some it is either directly programable or indirectly by external
decoding and then generation of a reset-signal.

Division by 10, 6 and 24 should be familiar to you since it may be what gets
you up in the morning and keeping time to the office or wherever you work
(assuming you work). Most clocks isn't very advanced in that sense, they do
just this directly, since that is the cheapest, most low-cost way of doing
things.

I would recommend you to look in Don Lancasters TTL and CMOS Cookbooks and you
should also find it in the Art of Electronics (I could check but I am too lazy,
OK). Anyway, there's no big magic about it and if you ask more questions about
it you will get the answer here anyway.

Division by integer + one half (from 1.5 and upwards) is possible, but a bit
messy. Also, for non-even integers you can create 50 % PWM but it is also a bit
messy. For even integer division ratios it is trivial to create 50 % PWM since
you then divide the frequency by half the division ration (if you want to do
10 you first divide by 5) and then divide by 2 and the output will be 50 % PWM.

Cheers,
Magnus