Path: utzoo!attcan!uunet!lll-winken!ames!amdcad!weitek!practic!vlsisj!davidc
From: davidc@vlsisj.VLSI.COM (David Chapman)
Newsgroups: sci.electronics
Subject: Re: PC Crystals
Summary: It's not so easy
Keywords: PC-AT (tm IBM), Crystals, Games
Message-ID: <15233@vlsisj.VLSI.COM>
Date: 27 May 89 01:09:44 GMT
References: <306@edstip.EDS.COM>
Reply-To: davidc@vlsisj.UUCP (David Chapman)
Organization: VLSI Technology Inc., San Jose, CA
Lines: 58

In article <306@edstip.EDS.COM> alf@edstip.EDS.COM (John Hamill) writes:
>
>I have a Joysystems 286 AT clone.
>
>When I play some games on it I get killed right away, becouse the
>clock speed is so fast (10 MHz).  Im thinking of getting a 10MHz
>crystal to slow the speed down to 5MHz.
>
First of all, a 5 MHz AT clone is still going to be several times (3x? 5x?)
faster than a PC or XT clone.  You're using a much more powerful processor.
So installing a slower crystal won't help you that much.  Besides, most people
really want a _faster_ computer :-)
>
>          I would like to know if I can install a switch to the two
>crystals with out any problems?  Will a wire running out to the back
>of the computer box be too long and affect how the crystal works?  Do
>I need to have a switch on both leads of the crystal?  Is there a way
>through software to slow down the clock speed with out changing the 
>crystal?  Any help would be helpfull.
>
Crystal oscillators are complex beasties (I won't touch 'em - they're _analog_
and I only know how to count to one :-).  What I _do_ know is that you want
the crystal leads short - it reduces the inductance.  You should be able to
switch in only one lead, but since DPDT switches are cheap I'd change over
both of them.  Make sure you don't damage the original crystal when you
unsolder it!

One other thing:  if you do this, you should probably only change the speed
while the machine is powered down.  With the proper hardware, you could do
the switch while the machine is running, and maybe not even require a reboot,
but what you're doing is at the very least going to require that the machine
be reset after a switch.  Ctrl-Alt-Del won't work either - you'll need a reset
button (why IBM didn't provide a reset button on a machine without protection
from software errors, I'll never know).

It's possible to slow the machine down in software - you write/buy/borrow
(freeware) a TSR (terminate & stay resident) driver that grabs the clock
interrupt and spins for a few cycles.  I know there's one out there in the
public domain (shareware?).  Try some of the disk sellers.  My own favorite
(no, I don't work for them) is Public Brand Software in Indianapolis.  I
think they have "game slow-down" utilities on one of their disks somewhere.
Their phone number was 1-800-IBM-DISK until International Business Machines 
complained; now it's 1-800-426-DISK (still the same, actually).  They'll be 
happy to send you a catalog.

I really think the software method will be preferable.  Since the delay loop
is in software, you could probably slow the machine down enough that you could
start winning :-)

		David Chapman

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		David Chapman

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