Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!pyramid!weitek!weaver
From: weaver@weitek.WEITEK.COM
Newsgroups: comp.arch
Subject: Re: What CMOS cannot do (Re: Surges)
Message-ID: <19882@weitek.WEITEK.COM>
Date: 20 Nov 89 18:35:10 GMT
References: <7000@pt.cs.cmu.edu> <31547@hal.mips.COM> <48393@bbn.COM> <7032@pt.cs.cmu.edu>
Sender: weaver@weitek.WEITEK.COM
Reply-To: weaver@weitek.UUCP (mike weaver)
Organization: WEITEK Corp. Sunnyvale Ca.
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In article <7032@pt.cs.cmu.edu> lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) writes:
>In article <48393@bbn.COM> slackey@BBN.COM (Stan Lackey) writes:
>....
>And when do we see a BiCMOS micro, with a CMOS core and ECL IO? Or
>will the core go BiCMOS too, to avoid all the di/dt issues that the
>ECL people are so quick to point out?
>-- 
>Don		D.C.Lindsay 	Carnegie Mellon Computer Science


BiCMOS implementations look good for the next generation of micros.
These parts may be a bit slower than all ECL, but lower power, and 
a bit denser. I expect that they will look attractive for mid-line
workstations, with ECL, GaAs, and liquid cooled chips possibilities
for the high-end.

BiCMOS is a process that allows bipolar transistors and MOS transistors 
on the same die. The bipolar transistors are not particularly faster than
the CMOS, since they are made of the same silicon.

I see four advantages to ECL over CMOS in this case:

1. low voltage swing (for faster transitions).
2. constant current  (noise through power lines minimized).
3. smooth rise / fall (less noise generation).
4. high output current.

And two disadvantages:

5. bigger. 
6. higher power.

For a chip with ECL IO and CMOS core, most of the power will be going 
to the output pin drivers. So advantage #2 is unimportant for the core.

Internal wires on a chip are very resistive-capacitive, and require little
current. So advantages #3 & #4 are not important for the core.

That leaves advantage #1, which hardly seems worth the increase in 
power and size.

Unless, of course, you really want to pay for speed. But then you would
be better off with the best bipolar transistors that money can buy, and
forget about BiCMOS.

Michael Weaver.