Xilinx Zynq-7000, Design Manual

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Zynq-7000 PCB Design Guide www.xilinx.com 47
UG933 (v1.8) November 7, 2014
Chapter 4: SelectIO Signaling
topography is point-to-point or multi-point defines important aspects of the interface that
determine which termination strategies are appropriate and which are not.
Bidirectional Point-to-Point Topographies
The simplest bidirectional topography is point to point. That is, there are two transceivers
connected by a transmission line. Because bidirectional interfaces need to operate equally
well in both directions, symmetry of the topography is desirable. While asymmetrical
topographies can be designed with reasonably good signal integrity, the easiest way to
ensure good signal integrity is to keep the topography symmetrical. Thus any termination
used on one side of the link should also be used on the other side of the link. Series
termination ( Figure 4-8 ) is rarely appropriate for bidirectional interfaces as incoming
signals are attenuated by the series resistor of the receiving transceiver. Parallel termination
( Figure 4-7 ) almost always achieves better signal levels at both receivers.
Controlled-impedance drivers, whether crudely controlled in the form of a weak LVCMOS
driver or adaptively controlled in the form LVDCI or HSLVDCI, also can have good results as
shown in Figure 4-9 , Figure 4-10 , and Figure 4-11 (implemented with a low-drive strength
LVCMOS driver). Always use IBIS simulation to determine the optimal termination resistor
value, V
TT
voltage level and VRN/VRP reference resistor values for these terminations.
X-Ref T a rget - Fig u re 4-7
Figure 4-7: Parallel Terminated Bidirectional Point-to-Point Topography
X-Ref T a rget - Fig u re 4- 8
Figure 4-8: Series Terminated Bidirectional Point-to-Point Topography: Not
Recommended
UG933_c4_07_031711
V
TT
V
TT
Z
0
= 50
Ω
R
P
= Z
0
= 50
Ω
R
P
= Z
0
= 50
Ω
R
O
= 25
Ω
R
O
= 25
Ω
UG933_c4_08_031711
R
S
= Z
0 –
R
0 =
25
Ω
R
S
= Z
0 –
R
0 =
25
Ω
Z
0
= 50
Ω
R
O
= 25
Ω
R
O
= 25
Ω
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