Xilinx Zynq-7000, Design Manual

Page: 42 / 77

Zynq-7000 PCB Design Guide www.xilinx.com 42
UG933 (v1.8) November 7, 2014
Chapter 4: SelectIO Signaling
The SelectIO standards can be used in countless topographies depending on the
requirements of the system. SelectIO drivers and receivers adhering to a standard (SSTL,
LVCMOS, etc.) either can be used according to the letter of the standard (published by a
standards body such as EIA/TIA or JEDEC) or they can be mixed and matched with drivers or
receivers from another standard or hybrid I/O. An I/O standard specification might define
something as limited as the V
IL
and V
IH
of the receiver, or it might define every aspect of the
interface, including driver impedance and slew rate, PCB trace length and topography, value
and position of passive termination, the maximum input capacitance of a receiving device,
and even the maximum number of receivers.
It is up to the designer to apply the standard in question to the system in which it is
working. There are many decisions to make with respect to topographies and termination,
which determine the signal integrity of the interface. It is of utmost importance that the
signal integrity of each interface be verified through both simulation and measurement.
Termination generally refers to impedance-matching or impedance-compensating devices
that are used to maintain signal integrity in an interface. While many types of elements can
be used as terminators (such as, resistors, capacitors, diodes), this discussion is limited to
resistive termination. In general, capacitor and diode termination techniques are more
complicated.
Unidirectional Topographies and Termination
The two basic subsets of unidirectional topographies are point-to-point and multi-drop. A
point-to-point topography has one driver and one receiver, while a multi-drop topography
has one driver and many receivers. Whether or not a topography is point-to-point or
multi-drop defines important aspects of the interface that determine which termination
strategies are appropriate and which are not.
Unidirectional Point-to-Point Topographies
The simplest unidirectional topography is point-to-point. That is, there is one driver and
one receiver. Termination, if present, can consist of parallel termination at the receiver
( Figure 4-1 ), series termination at the driver ( Figure 4-2 ), or a controlled-impedance driver
( Figure 4-3 and Figure 4-4 ). Always use IBIS simulation to determine the optimal resistor
values, V
TT
voltage level, and VRN/VRP reference resistors for these terminations.
X-Ref T a rget - Fig u re 4-1
Figure 4-1: Parallel-Terminated Unidirectional, Point-to-Point Topography
Z
0
= 50
Ω
UG933_c4_01_031711
R
P
= Z
0
= 50
Ω
V
TT
R
O
= 25
Ω
Send Feedback