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From: timd@hotwheel.dell.com (Tim Deagan)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Why I lower my CPU speed to 17.5MHz
Message-ID: <14901@uudell.dell.com>
Date: 6 Feb 91 15:26:18 GMT
References: <5540@bwdls58.UUCP> <1991Feb5.172330.27155@d.cs.okstate.edu>
Sender: news@uudell.dell.com
Reply-To: timd@hotwheel.dell.com (Tim Deagan)
Organization: Dell Computer Corp.
Lines: 48

In article <5540@bwdls58.UUCP>, mlord@bwdls58.bnr.ca (Mark Lord) writes:
> In article <1991Feb5.172330.27155@d.cs.okstate.edu>
ong@d.cs.okstate.edu (ONG ENG TENG) writes:
> <
> <After upping my 386SX from 16MHz to 20MHz (without heatsink or fan), it
> <worked fine, and still will if I choose to run it at that speed.  But 
> <after many hours of continuous operation, my fixed-HD (I have a
> <fixed-HD and a SyQuest removable where I run off most of
> <my softwares, the fixed-HD contains most "standard" programs like
> <system programs, word processors, graphics editor, terminal emulator,
> <etc, only) my fixed-HD started giving disk-read error.  To get rid of
> <it, I simply do warm-boot (NO turning off the power). 
>  ... guesses about cause deleted ...
> 
> 
> This problem is most likely due to the higher expansion bus speed
> due to the higher frequency clock oscillator (crystal).  A 16-Mhz SX
> typically runs the bus at 1/2 cpu speed, giving a convenient 8Mhz bus
> to match the AT spec.  
> 
> When one speeds up the CPU by changing the crystal, as ONG has done, 
> the expansion bus will also be run faster in most cases.  Since it is
> run at 1/2 CPU speed, this means 10Mhz instead of the standard 8Mhz.
> 
> Not all expansion cards can cope with faster bus rates.

Two points to consider:
1) The difference between a 16MHz CPU and a 20 MHz CPU is generally that
the 16MHz chip failed when they tried it at 20 MHz.  The process is called
sampling.  When INTEL does it they brand the results on the outside of the 
chip, when others do it you call it roulette. :-)  The CPU is not the only 
chip speed sensitive, the entire chip set is speed rated (and generally not
only sampled, but engineered for timing specs relevant to the rated speed).

2) Due to the plethora of fried cards due to the increased bus speeds of 
some 10 & 12 MHz 286's a few years back many card manufacturers have a clause
in their warranty which declares the warranty void if the card is run in a 
system with a bus speed higher than 8.33 MHz.  Check your warranty if you care.

Also, the multi-speed function is usually just a divisor function off the main
clock, rarely is it a different oscillator.

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------------------"Wake up!  Wake up!" - Spike Lee ---------------------
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                     timd@hotwheel.dell.com
                     timd@twaddle.dell.com
			Rev. Tim Deagan