From: Pat Zabinski (email@example.com)
Date: Wed Dec 22 1999 - 04:51:07 PST
You're right; I don't think we can solve all these criteria
with a single catch-all standard. So, why not add two more:
* It must allow different forms of termination. This is
sort of hidden within your statement regarding
"multidrop or point-to-point; uni- or bidirectional;", but
let's make it clear. i.e., source-series, far end shunt, etc.
* It must allow for easy (cheap) control of edge rates.
One of the best I/O I like to use for super-high freqs is
CML, but edge rate control is tough. With edge rate control,
you can get "sloppy" in packaging/discontinuities/EMC/etc., which
saves-the-day quite often.
Putting down criteria is the easy part; I'm interested to
see what comes back for "solutions".
> 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
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