Underclocking CPUs?

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dvisic
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Post by dvisic » Mon Aug 12, 2002 2:32 pm

I checked out the How To on making a Silent Thin Client (at <!-- BBCode Start --><A HREF="http://www.ltsp.org/contrib/fanless_howto.html" TARGET="_blank">Hiretechs.com</A><!-- BBCode End -->). One thing that caught my eye (and I have seen comments like this before) is:
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<BR>"In order to bypass the need for a CPU fan, clock your processor down one or two steps and have a nice heat sink."
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<BR>Where can I find more info on this? Is there a certain mhz speed beyond which this simply won't work? On the site, he underclocked a Celeron 366mhz down to 300mhz, and was able to run it with just a heat sink. Would this work on, say, an Athlon XP 1800+?
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<BR>--> dvis.
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Gxcad
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Post by Gxcad » Mon Aug 12, 2002 2:32 pm

Yes, this will work but you will need to unclock and unvolt quite a bit. I can barely run a duron 750 @ 800 @1.47vcore passively. You'd probably have to go down to similar levels. I use Alpha 8045 and AS2 with decent but not excessive case cooling. Also, it is quite difficult (but very possible!) to unlock the multiplier on athlon xp's.
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<br>-Ken
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dvisic
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Post by dvisic » Mon Aug 12, 2002 2:32 pm

<!-- BBCode Quote Start --><TABLE BORDER=0 ALIGN=CENTER WIDTH=85%><TR><TD><font class="pn-sub">Quote:</font><HR></TD></TR><TR><TD><FONT class="pn-sub"><BLOCKQUOTE>I can barely run a duron 750 @ 800 @1.47vcore passively.</BLOCKQUOTE></FONT></TD></TR><TR><TD><HR></TD></TR></TABLE><!-- BBCode Quote End -->Okay, I'll admit my ignorance here...I've never actually done any over- or underclocking. Your statement confuses me. Wouldn't running a duron 750 @ 800 be <!-- BBCode Start --><I>over</I><!-- BBCode End -->clocking?
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MikeC
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Post by MikeC » Mon Aug 12, 2002 2:32 pm

To over- or under-clock, normally you have to change the bus speed of the CPU. A P3-1000, for example, has a bus speed of 133 MHz & a multiplier of 7.5 (Mutiply those 2 numbers and you get 1000). As the multiplier is locked, you can change the CPU speed only by changing the bus speed. This can be done on most motherboards via the BIOS (though some still use jumpers). So you change the bus speed to 100 MHz, and the CPU now runs at 750MHz. With modern mb, you don't have to worry about PCI & AGP clock speeds as they are pretty much independently adjustable w/ most chipsets.
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<br>This change makes the CPU less powerful, sure, but probably still plenty fast. If we're talking about one of the Coppermine cores that run at 1.75V, at default settings, the power is just below 30W. The 250MHz underclock brings the power down to 22W.
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<br>BUT we're not done yet!
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<br>Because the processor is now just coasting, you can probably set its voltage (core voltage or Vcore) down to maybe 1.45V. You play with BIOS settings, bringing it down till you notice instability or the PC doesn't boot. Then you back up a notch. So what does 1.45V do for you? The power now drops to 15W.
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<br>This is probably low enough to run the thing fanless with a large surface-area copper heatsink & good case airflow. Or plop a quiet 80mm fan @ 5V over a decent heatsink & enjoy the silence. <IMG SRC="modules/phpBB_14/images/smiles/icon_smile.gif">
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<br>BTW -- I believe AMD MP CPUs are completely unlocked -- you can set both multiplier and bus to whatever you want. But don't take my word -- confirm elsewhere.
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<br>Places like amdmb.com, overclockers.com, tom's hardware & anandtech are good places to learn about overclocking & unlocking CPUs
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dvisic
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Post by dvisic » Mon Aug 12, 2002 2:32 pm

Hmm...thanks for the informative response! This is definitely an intriguing idea...I'll have to do some more research, then see how much of a performance hit I'd have to take to achieve silence. I can decide whether or not its worth it to me then. Certainly worth looking into, though. Thanks!
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