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Trace Line Calibration shows a view at the signal from the trace tool perspective.

 

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 sits on the upper piggyback board, which is optionally fit to lower, larger measurement board using a high quality inter-board connectors.

 

 

 

Indicators in the Trace Line Calibration table

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 auto calibration logic decides automatically on Recommended values (shown as “R”). But in cases of huge noise this might not be possible and the user has to set these values manually in Current input boxes (shown as “o” in the graph).

 

 

 

 

Oscilloscope measurements

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 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.

 

 

 

Perform Trace Line Calibration test

Trace port Calibration test is required due to different trace line lengths. Refer to this chapter for step-by-step instructions.