NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/comp/sys/mac/hardware/21616

Text File | 1992-11-15 | 3.4 KB | 69 lines

Newsgroups: comp.sys.mac.hardware
Path: sparky!uunet!decwrl!pa.dec.com!rdg.dec.com!decvax.dec.com!decvax!strange
From: [e-mail] (Steve Strange)
Subject: Re: 25MHz IIsi and my engineering friend
In-Reply-To: [e-mail]'s message of Sun, 15 Nov 1992 12:17:08 GMT
Message-ID: <[e-mail]>
Sender: [e-mail] (Usenet News System)
Nntp-Posting-Host: squeeze.zk3.dec.com
Organization: Digital Equipment Corp.
References: <[e-mail]>
Date: 15 Nov 92 11:48:32
Lines: 55

In article <[e-mail]> [e-mail] (Matthew Friedman) writes:

   1. (The Biggie First) He said that increasing the clockspeed of a machine
   should _NOT_ increase heat in a microprocessor if that chip is designed to
   handle the increase. He said that an increase in heat would _only_ occur if
   there were capacitances in the chip, or "wires" that were too close to each
   other to handle the increased frequency and "energy" was jumping from one to
   another. He said it was too difficult to explain why an increase in clockspeed
   would not cause an increase in heat ina properly designed chip. (My being a
   film major did not make things easier for him!)

This is incorrect.  A CMOS chip *will* generate more heat if run at a
high clock rate.  CMOS transistors only draw significant power when
they are are switching from one state to another (i.e. from logical
"1" to "0" or back again).  If you increase the rate at which these
transitions take place, more power will be drawn.  That power has to
go somewhere -- it all ends up going to heat.  If the chip is run at a
high enough frequency that it gets hot enough to damage itself, it
needs a heat sink.  All the heat sink does is reduce the temperature
at which an equilibrium is reached between the generation of heat in
the chip and the radiation of heat from the chip and heat sink.

   So, if this is true, the increase in heat would seem to be an
   indication that the microprocessor is _NOT_ suitable for higher speeds.

As long as the heat sink keeps the operating temperature low enough,
the higher speed is not a problem.  Unless, of course, the chip
doesn't logically function properly due to propagation delays in the
chip or, much more likely, in the wires leading to and from the chip.

   2. The only damage this procedure could do, he said, would be through the
   increased heat. So attaching a heat sink would make it safe. All the other
   problems he said would be data errors, and could be fixed by swapping back
   to the original clockspeed.

I agree totally with this.

   3. He told me it is a common practice in ibm's to upgrade to a higher
   clockspeed simply by swapping to a new microprocessor (apparently theirs
   include their own timing crystals.) This would suggest
   that the rest of the machine is safe from physical damage, even if an apple
   is not an ibm.

Sounds reasonable to me.


    Steve

--
+---------------------------------------------------------------------------+
|   Steve Strange            |   Opinions expressed are my own and not      |
|   Digital Equipment Corp.  |   necessarily those of my employer, or       |
|   Nashua, NH 03062-2698    |   of anyone else, for that matter.           |
|   [e-mail]      |                                              |
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