[Solved] Differential pair: what is the difference between the "Tune Skew" measurement and the "Routed Length" mesurement?

[JPR75]

Hi,

I have a differential pair I0_P and I0_N which I want to adjust the skew. On the status bar, I can read that the routed length are IO_P = 30.0625 mm and IO_N = 29.9039 mm which give a skew of 0.1586 mm:

But the “Tune Skew of a Differential Pair” tool gives a skew of 3.1493 mm:

I know that the “Routed Length” takes in account my board stack up which is 0.305 mm, but I don’t understand the skew given by the “Tune Skew of a Differential Pair” tool.
Anyone knows how this tool measure the skew?

KiCad version 8.04 ; Windows 10

Thank you

[JPR75]

The problem was that one of my segment was not correctly connected to the center of a via.
Both measurements are equal now…

[paulvdh]

There is more to it. A single via would not have resulted in an over 3mm difference.
There is another current issue for this length tuning:

Trouble understading DRC criteria for length tuning

This is a known issue - the net inspector, interactive router, and DRC algorithms use slightly different algorithms to calculate trace lengths: Skew tuner disagrees with DRC (#18123) · Issues · KiCad / KiCad Source Code / kicad · GitLab We’re just figuring out the best way to unify these so they all agree.

[JamesJ]

Yes there are a number of issues with the algorithms.

[paulvdh]

This afternoon I’ve been reading https://docs.kicad.org/8.0/en/getting_started_in_kicad to see if there are chapters I can add something useful to, and it also states a similar discrepancy.

https://docs.kicad.org/8.0/en/pcbnew/pcbnew.html#differences_between_net_inspector_and_length_tuner