Path: utzoo!mnetor!uunet!husc6!bloom-beacon!tut.cis.ohio-state.edu!im4u!ut-sally!utah-cs!utah-gr!uplherc!esunix!jsnow
From: jsnow@esunix.UUCP (John Snow)
Newsgroups: sci.electronics
Subject: Re: 6502 crystal clock
Message-ID: <803@esunix.UUCP>
Date: 18 Apr 88 18:55:25 GMT
References: <4641@bloom-beacon.MIT.EDU>
Organization: Evans & Sutherland, Salt Lake City, Utah
Lines: 34

From article <4641@bloom-beacon.MIT.EDU>, by gordo@athena.mit.edu (Garet G Nenninger):
> 
> Anyway, right now I'm looking at using a crystal clock oscillator
> instead of a crystal and some gates.  However, I've seen it hinted
> that you can hook a crystal directly to the 6502 and get it to
> oscillate.  Does anyone have anything on this; I have never seen
> anything concrete?

I'm looking at an old Synertek data book that shows an example of doing
this very thing.  It shows how to do it with a crystal, 1 inverter from
a 74LS04 package, a 10pf cap, a 330K resistor, and two diodes.  Let's see
if I can describe how it is connected.

The phase2 output of the 6502 feeds the input of the inverter.  The
cap goes between phase2 and phase0.  Connected to phase0 are the two diodes.
One diode has its cathode connected to phase0 and its anode to ground.  The
other diode has its anode connected to phase0 and its cathode to +5V.  Also
connected to phase0 are one side of the XTAL and one side of the resistor.
The resistor and XTAL are in parallel and the other side of each goes to
the output of the inverter.  They also show the output of the inverter
feeding the input of another inverter.  The second inverters output is
designated as the phase2 system clock to the rest of the system.

I seem to remember some old Rockwell 6502 documentation that showed a
simpler circuit involving a crystal.  It also showed how to use just a
resistor and capactor to make the 6502 oscillate if precision timing was
not important.  However, I haven't been able to dig up this old documentation
yet.

-- 
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John F. Snow                         UUCP: {ihnp4,decvax}!decwrl!esunix!jsnow 
Evans & Sutherland Computer Corp.    Compuserve: 71550,1152    BIX: johnfsnow
Salt Lake City, Utah