Path: utzoo!mnetor!uunet!oddjob!hao!ames!eos!aurora!labrea!decwrl!decvax!mandrill!neoucom!wtm
From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: comp.sys.ibm.pc
Subject: Re: caution when cranking up the CPU clock
Message-ID: <1034@neoucom.UUCP>
Date: 2 Mar 88 15:55:12 GMT
References: <49@vsi.UUCP> <5950@netsys.UUCP>
Distribution: comp
Organization: Northeastern Ohio Universities College of Medicine
Lines: 36
Keywords: 80x86, Intel, many MHz
Summary: Speeding up chips makes them get hotter


In a CMOS chip, power dissipation is proportional to clock speed.
This is due to the fact that the principal cuase of power
dissipation is that the complementary transistors in a gate's
output stage are both in their ON states for a brief instant as the
gate switches HIGH-->LOW or LOW-->HIGH.  Essentially the +5 is
connected directly to ground through the transistors.  

Incearsing the clock rate increases the number of state transitions
per second and thus the average power consumed goes up.  The
attendant rise in temperature may casue the chip to fail.  A low
speed chip may run just fine at an elevated clock rate, but will
fail prematurely due to thermal stress.  808x CPU
chips are NMOS, thus they should have a power consumption
independent of clock rate.  Many clones do use NEC V series 808x
work-alike chips which are CMOS.

Some of the cheapo turbo clone boards I've seen for sale at local
computer flea markets attempt to get away with using cheap parts by
gluing their own heat fins onto the chips.

Chip capabilities aren't the only problem associated with
increasing a system's clock rate.  At higher clock speeds, race
conditions (the time it takes a logical condition to propagate
though a logic network) can cause erronious addresses, etc to
appear on the the bus.  Race conditions may not occur consistently,
and thus are difficult to trace down as the cuase of system
crashes.

On a system that I depended on reliability, I would probably decide
to use a turbo coprocessor card, where the entire logic on the
card (and chips) have been designed to operate properly at high
clock speeds.


--Bill