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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.micro
Subject: Re: 74S124 as xtal osc - (nf)
Message-ID: <4062@utzoo.UUCP>
Date: Wed, 11-Jul-84 16:07:09 EDT
Article-I.D.: utzoo.4062
Posted: Wed Jul 11 16:07:09 1984
Date-Received: Wed, 11-Jul-84 16:07:09 EDT
References: <3754@fortune.UUCP>, <118@bragvax.UUCP>
Organization: U of Toronto Zoology
Lines: 28

David DiGiacomo observes:

    Maybe I'm old fashioned, but I always use 1/3 of a 7404 + 2 resistors +
    1 capacitor (OK, if you want it buffered it's 1/2 of a 7404).  Total
    cost = $0.50, low power, reliable startup, etc.  If you really want to
    spend more on it you could use a 74HC04.

The trouble with this is that TTL gate oscillators are *known* to be
prone to startup problems; my understanding is that you have to get a
good deal fancier than just 2R+C to get something that will start reliably
under all conditions of chip variation, temperature, voltage, transients,
etc.  I agree that they usually work all right, and I've used them for
some things, but I would hesitate to design them into anything that was
intended for "production" use.

Dunno about 74HC.  CMOS gates work much better as linear circuits than
TTL gates, and I seem to recall that gate oscillators are considered
respectable in CMOS circles, but HC is a slightly different animal and
I would have to look at specs.  (I've given up keeping track of the
sixteen different kinds of fast CMOS, so I don't remember for sure just
what the logic levels etc. of 74HC are.)

Can some of the more knowledgeable hardware types comment on either of
these issues?  I'm just a poor software man; hardware is my hobby, and
I lack formal background and "real" experience in it.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry