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From: ralf@b.gp.cs.cmu.edu (Ralf Brown)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 20 Mhz 386 SX (?)
Keywords: Citrus and how do you do it?
Message-ID: <7982@pt.cs.cmu.edu>
Date: 14 Feb 90 18:57:28 GMT
References: <1109@watserv1.waterloo.edu>
Distribution: comp
Organization: Carnegie-Mellon University, CS/RI
Lines: 39

In article <1109@watserv1.waterloo.edu> ssingh@watserv1.waterloo.edu ($anjay "lock-on" $ingh - Indy Studies) writes:
}3.) BTW, WHY does the componentry heat up?

The following is true for CMOS, and partially true for TTL devices:

	A CMOS switching element has two transistors, one between the
	supply voltage and output, and one between the output and ground,
	like so:

                 power supply
                       |
                       |
                _____|/
               |     |\
               |       |
     input ----+       +------- output
               |       |
               |_____|/
                     |\
                       |
                       |
                    ground

The two transistors of the switching element have opposite polarities, so that
one is off when the other is on.  Due to this architecture, the current drain
at rest is very low (only enough to drive the gates of the switching elements
connected to the output).  However, when the input switches from one state to
the other, that switch is not instantaneous, so for a brief period of time,
both transistors are partially on.  We now have what amounts to a momentary
short between power and ground.  The duration of this short depends on how
cleanly the input switches states, which is relatively independent of clock
speed.  However, the clock speed does influence how often the short occurs,
so a higher clock speed results in more momentary shorts (of the same
duration), causing a higher current draw, which heats up the chip.
-- 
{backbone}!cs.cmu.edu!ralf   ARPA: RALF@CS.CMU.EDU   FIDO: Ralf Brown 1:129/46
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DISCLAIMER? | _How_to_Prove_It_ by Dana Angluin  24. by appeal to intuition:
What's that?| Cloud-shaped drawings frequently help here.