Target Board Design For Tracing With Arm Dstream-St; Overview Of High-Speed Design - ARM DSTREAM-ST System And Interface Design Reference Manual

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4.1

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 or from a Trace Port Interface Unit (TPIU) 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 (

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

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

Avoid stubs

Minimize crosstalk

Use impedance matching and termination

ARM 100893_0100_00_en

= 0.5/Tr

knee

Note

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 trace signals are multiplexed with other pin functions

and where the PCB is designed to support both functions with differing tracking requirements.

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.

All PCB tracks carrying trace port signals must be impedance matched to approximately 50Ω.

Series termination is highly recommended on all high-speed signals.

4 Target board design for tracing with ARM

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DSTREAM-ST

4.1 Overview of high-speed design

4-42

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