42
Tsi572 Hardware Manual
May 18, 2012
Integrated Device Technology
www.idt.com
3.3
Tracking Topologies
The tracking topologies required to maintain a consistent differential impedance of 100 ohms to
the signal placed on the transmission line are limited to Stripline and Microstrip types. The
designer must decide whether the signalling must be moved to an outer layer of the board using
a Microstrip topology, or if the signalling may be placed on an inner layer as stripline where
shielding by ground and power planes above and below is possible.
3.3.1
Stripline
The RapidIO buses should be routed in a symmetrical edge-coupled stripline structure in order
to ensure a constant impedance environment. The symmetrical stripline construction is shown in
Figure 7
. This method also provides clean and equal return paths through VSS and VDD from
the I/O cell of the Tsi572 to the adjacent RapidIO device. The use of broadside coupled stripline
construction as shown in
Figure 9
is discouraged because of its inability to maintain a constant
impedance throughout the entire board signal layer.
The minimum recommended layer count of a board design consists of 12 layers. The optimum
design consists of 16 layers. The designer should consider both of these designs and weigh their
associated costs versus performance.
Figure 7: Recommended Edge Coupled Differential Stripline (symmetric when h1=h2)
In order to prevent consuming received eye margin, the =/- track skew of a lane
should be constrained to a maximum of 15pS.
The skew limitation between the shortest lane and longest lane of the RX or TX of a
port is 22 ns.
W
S
h1
h2
t
Power/Ground
plane
Power/Ground
plane
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