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

Shayle Hirschman (shayle@mho.net)
Mon, 29 Mar 1999 16:50:48 -0600

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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>

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