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

Chris Bobek (cbobek@cadence.com)
Mon, 29 Mar 1999 14:33:49 -0800

>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.

Where did you get 300ps for the risetime? I was using 3ns.

Here's a situation: Assume you have a trace that's 20" long. Obviously,
this needs to be terminated for a 3ns edge. For the sake of argument,
assume you calculate a "short" trace to be 3". That is, you can get by
without termination (not counting ringing) for that "short" trace.

Now, instead of laying out the board with this 20" trace that is daisy
chained to each device, you supply a separate path to each device from the
point-source that are each 3" long with no termination. Is this
configuration considered "bad" or undesireable?

What I realize here is that you will have a reflection that hits one of the
devices at the end of the 3" trace. It will reflect all the way back to the
other 4 devices, which are each 6" away. Because this length is considered
"long" it will cause problems, no?

So, is everyone basically saying that you should always *try* to daisy chain
devices, no matter how long the chain is? (Incidentally, I made a decision
to keep the daisy chain and terminate it).

Thank you all for your help,

Chris

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