2.4
Midbus Routing
Midbus footprints can be placed into the routing path, to provide an interface to various protocol analyzers, as well
as provide a location to probe a signal using oscilloscopes. Transmitter pairs route on one side of the footprint,
while Receiver signals route through the other side.
Figure 9. PCI Express Midbus Routing Example
2.5
PCB Stackup Considerations
Determining the PCB stackup is one of the most important steps in designing and implementing a system. The
PCB stackup should be determined prior to board routing, because it will determine the trace width and spacing
requirements necessary to achieve a particular characteristic impedance and differential impedance. After the
stackup is known, the trace width can be selected. For a single-ended signal, this is enough to determine the
characteristic impedance of that trace. For differential signals, the last step is to determine the separation
between the positive and negative conductors, to achieve the needed differential impedance.
Additionally, a PCB stackup can determine the power supply de-coupling scheme for a device. Parallel plane
capacitance exists between a PCB’s DC power and ground planes. PCB reference planes have an insignificant
amount of series inductance; therefore, their effective frequency range is much higher than that of
discrete capacitors.
PCB traces can be implemented as one of two types of transmission lines – microstrip and stripline. Microstrip
traces have only one reference plane, and therefore, represent traces on the outer layers (top and bottom layer)
of a PCB. Stripline traces have two reference planes and are implemented using inner routing layers. Typically,
stripline traces are only available for PCBs with six or more layers. Microstrip and stripline traces each have their
own properties, which must be weighed when determining which type of trace to use.
PEX 8618 Quick Start Hardware Design Guide – Version 1.2
Copyright © 2009 by PLX Technology, Inc. All rights reserved.
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