Trace port PCB design guidelines

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Purpose

On-Chip Trace signals are transmitted at high frequencies and are subject to signal distortion. High signal distortion can prevent a trace tool from capturing the trace stream correctly.

This document gives guidelines on how to design the PCB to minimize high frequency effects.

Recommended Measures

The CPU should be soldered on the PCB. Any socket can reduce the signal integrity dramatically.

All trace port lines on the PCB should be as short as possible (max ~2,5 cm),

Traces should run on the same layer, or layers with the same impedance.

Preferred layer impedance is 50 Ohm.

Mictor ground pins should be connected directly to PCB’s GND plane.

Trace clock should be serially terminated by 47 Ohm resistor as close as possible to the driver. The value of the resistor may be changed depending on driver characteristics.

Trace clock should be clean of crosstalk – if possible with double distance to closest nets.

Trace clock should have only point-to-point connection – any stubs should be avoided.

It is strongly recommended also for other (data) lines to be point-to-point only. If any stubs are needed, they should be as short as possible, when longer are required, there should be a possibility to optionally disconnect them (e.g. by jumpers).

Trace port data bus inner crosstalk is not so important, but it is critical to isolate the whole bus from other signals (including from the trace port clock).

Example High Frequency Effects

The following examples show how the length of trace lines is reflected in signal integrity and consequently in functionality. One of typical evaluation boards was used, where the CPU seats on the upper piggyback board, which is optionally fit to lower, larger measurement board using a high quality inter-board connectors.

The Line Calibration Result shows a view at the signal from the trace tool perspective. A trace tool can adjust the sampling position by shifting the sampling voltage and sampling delay in reference to the clock. The dotted sampling points indicate a stable signal, and the X positions indicate a changing (unstable) signal.

The Oscilloscope shows the same picture measured by the oscilloscope. The yellow line is the trace clock and blue lines the trace data. The dark area within the signal is called the data eye – sampling within this region yields a reliable trace recording.

Trace lines with short stubs

The size of the data eye is large and the tool has no problems in recording the trace.





Trace Line Calibration result


Measured by oscilloscope

Trace lines with longer stubs (over connector to other board)

By passing through a connector, the data eye shrinks. The auto calibration can still find a safe sampling position, but any additional obstacle (CPU socket, longer PCB lines,…) could shrink the data eye to a point where no safe sampling point could be established.





Trace Line Calibration result


Measured by oscilloscope