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

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From: D. C. Sessions (dc.sessions@vlsi.com)
Date: Wed Dec 22 1999 - 13:45:23 PST


Chris.H.Simon@gd-is.com wrote:
>
> DC,
>
> I like your suggestion to start with a clean sheet of paper and ask
> ourselves what should be done to optimize the I/O design.
>
> In my experience working with computer system architects, they always want
> more and more bandwidth, which in the past has meant more and more pins at
> the boundary of an IC and more and more pins through connectors. And also
> increased data rates. To combat the higher data rates (and associated
> issues of noise, SSO, timing, ...) I have suggested to them to use
> differential signalling. The complaint that I get back is that this takes
> more pins. "I don't want to use two pins per signal when I can get twice
> as many signals using single ended." they whine. Of course this isn't
> really true since an increasing number of ground and power pins are
> required, but system architects don't consider these since they don't show
> up on any block diagram.

That's what grouches like me are for :-)
We make nuisances of ourselves pointing out that SSO effects are
the #1 limiter on available performance. That using DC-balanced
signaling cuts SSO effects more than the equivalent number of pins
dedicated to power connections. That the balanced signals cause
less crosstalk and less EMI. That they can cut the cost of their
termination supplies dramatically and reduce the number of PWB layers
required.

Of course, if those considerations aren't important to them, then
we'll be perfectly happy to charge extra for more advanced process
technology, packaging, and core area to make up the difference.

> If it is really true that "Padrings are some of the most expensive real
> estate around, so pin count should be minimized." then why don't we start
> using each precious location on the padring to get more than one signal?

We do. For busses, we crank up the clock rate and lower the number of
lines needed. If we have more bandwidth per pin than a single channel
can use, we use packet burst protocols.

> I'm suggesting keeping differential signalling to alleviate some of the SI
> issues, but putting more than one logical signal on each differential pair.

Basically broadband. The desirability of broadband depends a lot on both
the nature of your traffic and the limitations of your interconnect. For
short interconnects it's not really attractive for quite a while yet (we
did a science project of this sort recently. No, I can't discuss it.)

> With two logical signals per pair I'm back to the one signal to one wire
> ratio that system architects love. See U.S. patent #5,872,813 "Dual
> Differential and Binary Data Receiver Arrangement" as an example. Although
> that patent refers to bipolar ECL-like circuits, I believe that some
> similar concepts could be implemented in CMOS circuits.
>
> The added complexity of the driver and receiver (and a little more power
> due to increased voltage swing) may be worth it if we gain one or more
> logical signals for each precious pin on the IC.

Don't underestimate the grief that that "little more ... voltage swing"
causes. It really messes with your S/N ratio, but worst of all is the
fact that it runs up against the voltage-scaling objective. If you use
voltages larger than the native supply for a technology, you have to
degrade performance to what might as well be the technology appropriate
for that voltage. IOW, if you inisist on 3.3v signaling you're going to
be stuck with 350 nm CMOS performance.

> ***************************************************************
>
> 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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-- 
D. C. Sessions
dc.sessions@vlsi.com

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