Zynq-7000 PCB Design Guide
46
UG933 (v1.8) November 7, 2014
Chapter 4:
SelectIO Signaling
The main transmission line should be kept as short as possible. Lengths up to 20 inches or
more are practical for most I/O standards as long as precise trace impedance is maintained
and crosstalk sources are avoided. The lengths of interim segments of the main
transmission line need not be equal. Their relative lengths can be arbitrary. Receivers at
different points along the main transmission line receive the signal with varying amounts of
delay, but all signal rise times are similar.
Stubs stretching from the main transmission line to the individual receivers must be kept as
short as possible
. The longer these stubs become, the more corrupted the received
waveforms are. Simulation and measurement are required to assess signal integrity at the
individual receivers.
lists example I/O interface types that can be used with the unidirectional
multi-drop topography.
LVTTL and LVCMOS do not specify any canonical termination method. Parallel termination
at the end of the long t-line is an appropriate termination method.
Bidirectional Topography and Termination
The two basic subsets of bidirectional topographies are point-to-point and multi-point. A
point-to-point topography has two transceivers (driver and receiver sharing one device
pin), while a multi-point topography can have many transceivers. Whether or not a
X-Ref T
a
rget - Fig
u
re 4-6
Figure 4-6:
Basic Multi-Drop Topography
Output
Main Transmission Line
Input 4
2 x Z
0
= 100
Ω
2 x Z
0
= 100
Ω
V
CCO
Input 2
length < 8mm
Z
0
= 50
Ω
Z
0
= 50
Ω
Z
0
= 50
Ω
Z
0
= 50
Ω
Input 3
length < 8mm
Z
0
= 50
Ω
Z
0
= 50
Ω
Z
0
= 50
Ω
Z
0
= 50
Ω
Input 1
length < 8mm
UG933_c4_06_031711
Table 4-2:
Example I/O Interface Types for Unidirectional Multi-Drop I/O Topographies
LVTTL
LVCMOS
HSTL
SSTL
