Re: [SI-LIST] : Clean sheet of paper

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From: D. C. Sessions (dc.sessions@vlsi.com)
Date: Wed Dec 22 1999 - 10:31:13 PST


Howard Johnson wrote:
>
> Regarding new I/O technologies:
>
> My personal wish-list for Christmas includes new developments in the field
> of giant-magnetio-resistance (GMR) materials. I don't know if you've
> followed this lately, but some of the recent advances have been
> breathtaking. GMR materials may eventually be used to support some pretty
> neat signaling methods.
>
> Here's the deal: a GMR material changes its resistance in reaction to a
> small (very small) applied magnetic field. It's a bulk material effect -- it
> happens real fast. The guys I've talked to say they can't even measure how
> fast it is. The required magnetic fields as SO small that it can detect the
> current flowing in a signal PCB trace just by being NEAR it (no electrical
> contact required). What you do is put a dot of this material near the point
> of interest, hook up some terminals to the dot so you can measure its
> resistance, and Voila! you have a very sensitive, DC-active, current probe.
> At a distance of perhaps a few mils, as you might imagine in a backplane
> connector application, a typical GMR dot would contribute a load impedance
> on the line of only perhaps a few tenths of a pF. Think about it. A
> backplane receiver that had only a few tenths of a pF would significantly
> improve our ability to make multidrop backplanes.
>
> Rumor is that GMR dots are being designed into magnetic read heads for
> next-generation magnetic storage disks. B.T.W., does anybody have any
> specific information on these projects?

Oh Heavens! That's ancient history. Run a search on EET for GMR
and most of what you get is disk read apps. For that matter, you can
probably go to outfits like ReadWrite (sp?) and get sales material.

> OK, so that's the "good news" part, but what are the drawbacks? Well, at
> present, operating at room temperature, you only get about a 10% change in
> resistance from the best materials. That means your receiver has to work
> with a really tiny signal. BUT HERE'S THE EXCITING PART: researchers are
> reporting development of materials with GMR changes on the order of 1000:1
> at cryrogenic temperatures. If they can make these materials work at room
> temperature, the GMR material will actually represent a "new" device with
> susbstantial power gain, that accepts current in and controls current out,
> in a fundamentally new way. It's like getting a whole new kind of FET, only
> it's sensitive to magnetic fields, not electric ones. Who knows where this
> subject will lead?

GMR processes are compatible with mainstream CMOS processes. Nonvolatile
memory using GMR has been in the labs at Honeywell for quite a while.

> Anyway, I can imagine using the GMR material to make terrific differential
> isolators (suppose a current inside your chip controls the resistance of a
> GMR dot -- the GMR dot is connected to an external bias circuit that
> converts its change in resistance into a measureable signal). The advantage
> of this structure is that, if the GMR dot is fabricated inside the IC (oh
> yes, did I tell you that this can be done??), and if the bias circuit is
> done correctly, it will eliminate ground bounce. Cool.
> GMR dots embedded inside connectors might someday operate kind of like
> super-fast, and super-cheap, optical isolators -- they could eliminate
> circulating common-mode currents.

Actually the Really Neat thing about them is that they don't require
voltage swing _per_se_ to operate, and are insensitive to common-mode
potential. The ground bounce might be there (the drivers still switch)
but nobody cares any more except for the jitter impact.

The cryo/roomtemp stuff isn't really all that important given monolythic
matching. As noted, we do much more challenging stuff with memories now.

> Of course, all this theorizing depends on getting the material operating
> temperature up into a useful range--something over which we as engineers
> have very little control, but it's always nice to dream about what you
> want for Christmas.

Kewl ideas. Alas, the difference between us is that you can afford to take
a slightly longer view than I can. I'll be happy if I can get something
into standards in the next couple of years. (Right now my big headache is
cleaning up DDR II and cleaning up after AGP4x and some telecom protocols.)

> At 04:04 PM 12/21/99 -0700, you wrote:
> >With the year wrapping up and my inbox filling with
> >"Out of Office Autoresponse" messages, I thought I'd
> >kick off something more interesting than the joys of LVDS.
> >
> >In particular, what would we use for signaling if we could
> >start with a totally clean sheet of paper? Rather than
> >immediately jump to a solution, I'm looking for some criteria:
> >
> >* It has to be scalable. Given silicon technology trends, it
> > should migrate gracefully to lower-voltages and less
> > voltage-stress-tolerant semiconductors.
> >
> >* It has to be SI clean. Output impedance should be matched
> > (stringency variable) to the line across the switching range.
> > Inputs switchpoints should be symmetrical and well-defined
> > (ie differential receivers). Power plane proliferation
> > leads to bad SI and wasted money, so separate termination
> > supplies are a Bad Thing.
> >
> >* It has to be versatile. Single-ended, balanced single-ended, or
> > differential; multidrop or point-to-point; uni- or bidirectional;
> > all should be minor variations on the same system.
> >
> >* It should be economical. Wasted power is a Bad Thing, so low
> > swing is a must. Padrings are some of the most expensive real
> > estate around, so pincount should be minimized. Line termination
> > can dominate a PWB so KISS is the rule. Power supplies (esp.
> > ones that can both sink and source current) are expensive and
> > nasty to deal with, so do without (both for termination and
> > funny analog functions in the I/O circuits.)
> >
> >What can we add to the list? Remove? Priorities? (This is
> >engineering, we make tradeoffs.) Where does this take us?
> >
> >--
> >D. C. Sessions
> >dc.sessions@vlsi.com
> >
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> >
> >
> _________________________________________________
> Dr. Howard Johnson, Signal Consulting, Inc.
> tel 425.556.0800 // fax 425.881.6149 // email howiej@sigcon.com
>
> http://signalintegrity.com -- High-Speed Digital Design books, tools, and
> workshops
>
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-- 
D. C. Sessions
dc.sessions@vlsi.com

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