[sdiy] digital clock schematic

[Paul Maddox]

Tuj,

> I'm looking for a schematic for a square-wave osc (TTL or similar) that
can be locked to various frequencies.  I want something were I can input the
desired frequency as a binary number and have the osc produce it to within 1
Hz.  Range needs to ideally be 1KHz to about 2MHz.
>

You can do better than that ;-)

> I was thinking the way to do this would be to use a crystal osc at say
10MHz, then compute 10MHz/desired frequency to get the ratio, and use a
counter to count the oscillations, resetting when it matches the ratio
number.
>
> Am I on the right track?

what you've described is a DDS/Phase accumulator oscillator. and yes, you're
bang on track.
I used this very method for the oscillators in my Monowave.

Lots of examples of these on the net, though most commercial DDS generators
put out a sine wave.
This one, http://www.myplace.nu/avr/minidds/, will give you various
waveforms, under serial control.
Whats neat is that its basically, one chip and a DAC, and this will give you
a variety of waveforms, with a resoloution from 0.07hz upto around 300Khz.
But if you only want a square wave output, just use the MSB of the output,
ditch the DAC, and you can go to around 5Mhz!

Paul

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