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 differential pairs route on
one side of the footprint, while Receiver differential pairs route through the other side.
Figure 18. PCI Express Midbus Routing Example
2.5
PCB Layer Stackup Considerations
Determining the PCB layer stackup is one of the most important steps in designing and implementing a
system. The PCB layer 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 sufficient 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 layer 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 or stripline.
Microstrip traces have only one reference plane, and therefore, represent traces on the outer layers (top
and bottom layers) 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 8632-AA Quick Start Hardware Design Guide, Version 1.1
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© 2007 PLX Technology, Inc. All Rights Reserved.
