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

D. C. Sessions ([email protected])
Thu, 25 Mar 1999 09:38:12 -0800

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 have a hard time believing this. With any process since
about 3 micron CMOS you have to go to quite a bit of trouble
to slow down edges this much. With processes only a little
behind the curve (say, 600 or 500 nm ) the edges are likely
to be more in the 500-600ps range.

What do the V/T tables in the IBIS model say?

> 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?

Unless the driver has VERY well-behaved output impedance and
you balance the branches VERY well and your driver stub is
VERY short you're going to have to terminate anyway.

D. C. Sessions
[email protected]

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