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

Coleman, Dave (dave.coleman@intel.com)
Mon, 29 Mar 1999 15:29:00 -0800

The star topology may be useful, and can work, depending upon the SI and
timing needs vs. driver strength and edge rate. It comes down to amount of
imbalance of branch lengths tolerable (edge rate dependent) vs. driver
strength (effective impedance "seen" wrt incident vs. reflective switching).

I would consider using the star if incident wave switching is achieved and
yields less max flight time than a daisy chain - also should yield tighter
spread between min and max flight time for this same case (in which daisy
chain will have much wider spread between min and max) - Only would be held
back by layers/routability.

Also, can possibly do a star without termination - whereas a daisy chain
probably would need end termination for incident wave switching results
without ringback problems.

-----Original Message-----
From: Shayle Hirschman [mailto:shayle@mho.net]
Sent: Monday, March 29, 1999 2:51 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : RE: [SI-LIST]: Long bus or star?

D.C. Sessions wrote:
>
>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.
>

D.C.,

Right, of course, with psec rise times. I'm more used to a couple of ns of
rise time, such as which Chris was asking about. In that case, 4 to 6 inch
legs of a star may be possible, but only worth the effort if the equivalent
length sum of a daisy-chain adds undesirable delay or skew.

I guess the question I'm posing is: Would, in that case, 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?

Shayle

>> 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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