Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!decwrl!mejac!orchard.la.locus.com!fafnir.la.locus.com!dana From: dana@locus.com (Dana H. Myers) Newsgroups: sci.electronics Subject: Re: 80386. Message-ID: <1991Jan10.061457.2817538@locus.com> Date: 10 Jan 91 06:14:57 GMT References: <1990Dec18.234020.2491@uoft02.utoledo.edu> <1990Dec28.210731.10685@zoo.toronto.edu> Organization: Locus Computing Corporation, Inglewood, CA Lines: 53 In article esmith@goofy.apple.com (Eric Smith) writes: >In article otto@tukki.jyu.fi (Otto J. Makela) writes about using a 16 MHz 80386DX in a 25 MHz circuit: > >> Actually, if it boots, it'll probably be (reasonably) reliable. The 80386 >> is such a complicated beastie that if it's even slightly flaky, it'll >> probably crash immediately. > >Maybe, maybe not. Be more specific. One thing I recall with the 286s was that lower speed parts would run DOS ok at higher clock rates, but then they'd hang or crash when you try to run Unix (or any other protected mode program). It seems the MMU couldn't keep up at the higher speed. >> It'll probably run hotter 'en 'ell, though. > >Not unless the 25 MHz chips run hotter 'en 'ell. >The 16 and 25 MHz rated parts have the same circuitry implemented in the >same (mostly) CMOS process, so the power dissipation of the two will be >nearly identical at the same clock speed. The 25Mhz chips DO run hotter than the the 16Mhz parts. CMOS power consumption occurs during transitions. There are two reasons why;(1) current must flow through the gate capacitances (standard 1/(2*pi*f*C) stuff) and (2) there is a brief instant when both the P and N channel FETs are conducting. Resultantly, the power consumption of CMOS parts is roughly proportional to frequency of operation. A 25Mhz will consume about 25/16 (or 150%) of the power a 16Mhz part consumes. >The 16 MHz part is simply not guaranteed to run at 25 MHz. It may never >have been tested for 25 MHz operation, or it may have failed 25 MHz test >but passed 16 MHz test. Or it may have failed some aspect of the 25Mhz test that the user won't encounter :-). >In general, I personally consider it to be very poor engineering practice >to push parts past the manufacturers specs just to save a few $$$. I >trust a lot of data to my computer, and that's worth more to me than a >one time savings of less than $100. I wouldn't knowingly buy any product >in which such things are done. Good advice, even if I wouldn't hire you to design CMOS computers for me :-) :-). Dana -- /* * Dana H. Myers KK6JQ | Views expressed here are * * (213) 337-5136 | mine and do not necessarily * * dana@locus.com | reflect those of my employer *