Re: [SI-LIST] : RE: [SI-LIST]: Long bus or star?

D. C. Sessions (dc.sessions@vlsi.com)
Mon, 29 Mar 1999 13:39:42 -0700

Shayle Hirschman wrote:
>
> How about if each leg of the star is short enough to not be considered a
> transmission line relative to the signal's rise and fall time?
>
> Would reflections then add, acceptably, while the signal is still
> transitioning? Or would this lead to too large an amplitude due to multiple
> simultaneous reflections from each leg?

How do you propose to keep several legs ALL less than 12mm long?
(at least 60ps/cm, risetime of less than 300ps, line length
Tr/4 => 1.25cm) Note that this is a foolishly optimistic
calculation.

> At 10:58 AM 3/29/99 -0800, you wrote:
> >>Chris Bobek wrote:
> >
> >
> >>I have an address bus that is 75ohm, and 19" long. There are 8 loads >that
> >>are daisy-chained along the trace. The edge from the driver (which
> >>is at one end) is 3ns.
> >
> >>I was wondering if I would be better off with a star pattern, where >each
> leg is short (less than 4"). Would this work and not require
> >>any termination?
> >
> >
> >>Thank you very much,
> >
> >
> >Star patterns are a nightmare. There are so many reflections in
> >the system that it easily becomes unstable. If the star is not
> >perfectly balanced your signal integrity will degrade rapidly.
> >Just the process variations for each agent can be enough to degrade
> >your signal integrity to an unacceptable level. Furthermore, if
> >you have a GTL-like bus a star wont work because it would require
> >too many pull-up resistors. Star topologies can be made to work,
> >but not without a lot of pain. They should be avoided for a high
> >speed bus unless all other options have failed.
> >
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> >
>
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-- 
D. C. Sessions
dc.sessions@vlsi.com

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