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

Jon Keeble (j.keeble@fairlightesp.com.au)
Thu, 01 Apr 1999 10:42:06 -0800

>John --
>
>What a great idea to series source each leg at the hub! Very, very nice.
>
>Shayle
>
Seeing as you were so nice about that idea, how about this one.

If there are many arms to the star, then even halving the load seen by
the
driver might still be
too much.

Active termination might be the answer: this involves receiving with an
invertor, and feeding back the output of the invertor to its input with
a
resistor, say 3Zo.

This has the effect of moving the transmission line back toward the
opposite
level, reducing the swing (current step) required of the source.

In other words, the idea is to trade off voltage swing for impedance
matching.

This relies on the switching point being at around half rail. Most 3.3v
logic seems to switch very close to 1.5v, with not a lot of variation
with
temperature. So reducing the swing from 3v to 1v should be quite
acceptable.

A related idea is recommended by Analog devices for the RDn and WRn
lines on
groups of multiple SHARC DSPs (the opposite problem, where a good signal
needs to be generated at one SRAM from one of say 4 SHARCs).

This can be found in Chapter 11 of the SHARC USers Manual, available at
www.analog.com

Jon Keeble

One way to get a weak source connected to multiple loads

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