Overview of high-speed design

Failure to observe high-speed design rules when designing a target system containing an ARM® Embedded Trace Macrocell (ETM) trace port can result in incorrect trace data being captured. You must give serious consideration to high-speed signals when designing the target system.

The signals coming from an ETM trace port can have very fast rise and fall times, even at relatively low frequencies. For example, a signal with a rise time of 1ns has an effective knee frequency of 500MHz and a signal with a rise time of 500ps has an effective knee frequency of 1GHz (fknee = 0.5/Tr).

Note

These principles apply to all of the trace port signals, but special care must taken with TRACECLK.

You must make the following considerations for high-speed design:

Avoid stubs

Stubs are short pieces of track that tee off from the main track carrying the signal to, for example, a test point or a connection to an intermediate device. Stubs cause impedance discontinuities that affect signal quality and must be avoided.

Special care must therefore be taken when ETM signals are multiplexed with other pin functions and where the PCB is designed to support both functions with differing tracking requirements.

Minimize signal skew (balancing PCB track lengths)

You must attempt to match the lengths of the PCB tracks carrying the trace port signals from the ASIC to the Mictor connector to within approximately 0.5 inches (12.5mm) of each other. Any greater differences directly impact the setup and hold time requirements.

Minimize crosstalk

Normal high-speed design rules must be observed. For example, do not run dynamic signals parallel to each other for any significant distance, keep them spaced well apart, and use a ground plane and so forth. Particular attention must be paid to the TRACECLK signal. If in any doubt, place grounds or static signals between the TRACECLK and any other dynamic signals.

Use impedance matching and termination

All PCB tracks carrying trace port signals must be impedance matched to approximately 50Ω. Series termination is highly recommended on all high-speed signals.

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