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From: dale@lamont.ldgo.columbia.edu (dale chayes)
Newsgroups: sci.electronics
Subject: Re: Sidereal Time Clock
Summary: Microcontroller anyone....?
Message-ID: <3023@lamont.ldgo.columbia.edu>
Date: 13 Dec 90 23:51:50 GMT
References: <billq.661025394@s.ms.uky.edu> <1833@umriscc.isc.umr.edu>
Distribution: sci.electronics
Organization: Lamont-Doherty Geological Observatory N.Y.
Lines: 46

In article <1833@umriscc.isc.umr.edu>, robf@mcs213j.cs.umr.edu (Rob Fugina) writes:
> My roommate just asked me to design a clock for him.  It's for astronomical
> purposes.  Relatively simple, really, but I need a little advice and some
> chip numbers.
> 
> The clock has to count from 00:00:00 to 23:56:03, then reset back to zero.
> I figured it would be easy enough to design an oscillator with a 555 to

A lot  of discussion about counting and setting an stuff omitted .....

> 
> The whole thing would be made up of a 555, 6 counters, 6 BCD-to-7 seg
> decoders, and 6 LED digits.
> 
> Oh, and I nearly forgot...the most complicated part will be using
> logic gates to reset 0 at the right time...I'm looking at two 24-input
> NAND gates or something similar.  Advice, please!


For my time and effort, I would chop all the discrete parts and do
it in a single chip microcontroller. There are lots that would work
just fine. Use one you are familiar with (or pick one and learn.) 

Use the crystal as the time base and count either with one of the built
in timer/counters, or  write a software timing loop. (I'd go for the
counter/timer to make live simple. Let it overflow once per second
or 10 times per second.  Keep track of time in software (then the
end-of-sideral-day rollover is trivial. (If you do it in hardware,
you into another bunch of '688s or other comparator.)

You should be able to mux the 7-seg displays right off a pair of
eight bit ports to get plenty of digits directly.

If I was going to do it, I'd use an 8751 because I'm familiar with
it but I might choose one of the 68HCxxxxx processors if I was starting
from scratch.

In any case, I would provide a battery backup if you friend is going
to depend on the clock. 

Dale
============
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Dale Chayes Lamont-Doherty Geological Observatory of Columbia University
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