Freescale Semiconductor P&E Tracelink, User Manual, Page: 20 / 39

Tracelink User Manual 17
4.4.1 Avoid impedance discontinuities
Stubs are printed circuit board tracks that branch off from the main track and are
usually formed when placing test points or connecting multiple components to the
same net. Stubs should be avoided at all costs as even short stubs can cause serious
signal integrity issues.
Vias are a source of impedance change and the number of vias used should be
minimized.
Keep track widths constant throughout the signal path.
4.4.2 Minimize skew
Try to match the length of all printed circuit board tracks used to carry the trace
signals.
4.4.3 Ground plane
A solid ground plane will ensure a good return path and minimize noise. Avoid having
breaks in the ground plane if possible. Ideally the ground plane should be situated in
the layer next to the signal layer carrying the trace signals.
4.4.4 Minimize crosstalk
If the design has multiple signals running parallel to each other for long distances,
increase the spacing between them to avoid crosstalk issues. Note that crosstalk can
also occur across layers if two signal layers are adjacent to each other.
4.4.5 Signal termination
To eliminate reflections on the signal path, three impedances must match: the source,
the printed circuit board track, and the load. There are two common termination
schemes to minimize reflection:
• Series termination: A resistor is placed in series with the signal as close
as possible to the source. The resistor value plus the output impedance of
the source should equal the printed circuit board track impedance.
• Parallel termination: A resistor is connected between the signal and
ground. This resistor is placed as close as possible to the load (eg. the
debug header). The value of the resistor is equal to the printed circuit
board track impedance.