[SI-LIST] Re: Query regarding different type of Routing topologies
- From: "Reams, William" <William.Reams@xxxxxxxxxxxx>
- To: 'Karthik P' <karthik_package@xxxxxxxxx>
- Date: Fri, 28 Jan 2011 08:59:24 -0700
Let's tackle the star vs. daisy chain part of the question with a quick
trade-off study:
If routing a multidrop bus as a 'star' topology with all legs being equal
length, what would the signal from one point on the star look like at all the
receive points? On the other hand, if the leg lengths are 'badly' mismatched
(i.e. lots of different length stubs), what would the signal look like at the
receive points? Short answer, it can look pretty good. So from an SI
standpoint, a star topology on something like a PCI bus can be desirable IF the
physical constraints allow for it.
However, if you want a star topology, you'll need a via field at the center of
the star from which all the legs extend. From the above PCI example, that can
take up quite a bit of space (board cost!) for the via field. For a system with
physical constraints such as a PC with multiple slots, that would also require
quite a bit more board space (more cost!) to 'zigzag' your traces in order to
match lengths of the legs. But with the length constraints in the PCI spec, one
can have an acceptable SI solution in minimal space and thus the reason that a
PCI bus in a PC is daisy chained.
So there's a first pass at the design trade-offs of the two topologies. A daisy
chained PCI bus works for example in the slotted PC environment (low cost)
where in an embedded environment (bus completely under your control) the star
topology might be the better solution. So the normal SI answer: it depends.
Examine your design situation and you'll begin to understand when one works
better than the other as well as why specs like PCI get written the way they do.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Karthik P
Sent: Friday, January 28, 2011 8:31 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Query regarding different type of Routing topologies
Hi All,
Kindly let me know the different type of routing topologies used in PCB. For
Instance, advantage and disadvantage over star and daisy chain routing and how
to choose.
In DDR3, we use fly-by-topology routing. How it differ from DDR2 routing.
ThanksKarthik
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