Netclass differential pair clearance?

[floating-io]

I’m looking for a bit of clarity as to the the meaning of “Clearance” in a netclass as it applies to differential pairs. The “obvious” (heh) assumption would be that when routing a differential pair, the various parameters would apply like this:

It appears, though, that the clearance param may actually apply to the individual tracks within the pair, which is somewhat counter-intuitive. And if that is the case, then how do you specify the clearance around the whole pair so that your pours are properly voided where necessary?

Will someone please unconfuse me?

(Am using 5.99 nightlies btw, in case that matters here…)

[paulvdh]

I just did a few simple experiments and both DP_Width and DP_Gap are used as would be expected from your screenshot.

I do not understand this:

@floating-io

It appears, though, that the clearance param may actually apply to the individual tracks within the pair, which is somewhat counter-intuitive.

Clearance is a property that gives a distance between two different items. I set the clearance of a differential pair to 2mm (just to make it obvious) and regenerated the zone around it, and the clearance between the diff pair and the zone is indeed 2mm (after re-generating the zone with [F8] of course.)

However, when I set the clearance of the diff. pair to 0.111mm and regenerate the zone, then it keeps a clearance of 0.508mm, because that is what the (default) clearance of the zone itself is set to.

[floating-io]

Did you run DRC on it?

If I set clearance > gap, the DRC will complain because the pair’s traces are too close together relative to the clearance.

[paulvdh]

Oh, a DRC issue. Why didn’t you say so in your first post?
Hint: http://catb.org/~esr/faqs/smart-questions.html

So I ran DRC and it complains a lot.
Then I simplified my test and it complained a bit less

In Pcbnew / File / Board Setup / Design Rules / Net Classes I made a “diffferr” netclass with a 0,888mm Clearance and 0.666mm DP Gap. and routed a diff. track.

The first 6 messages are just for the presumed “clearance violation”, but the last is a “Differential pair gap out of range” and it suggest that the value I entered for clearance (The distinguisable 0.888) as being the gap set in the design rules.

** Drc report for /home/paul/projects/kicad/aaaaaaaaaaaaaaaaaaaa/aaaaaaaaaaaaaaaaaaaa.kicad_pcb **
** Created on 2021-03-20 13:59:10 **

** Found 7 DRC violations **
[clearance]: Clearance violation (netclass 'diffferrr' clearance 0,8880 mm; actual 0,6660 mm) Severity: error
   @(101,3700 mm, 66,3500 mm): Track [/asdf_P] on F.Cu, length 1,2898 mm
   @(102,2820 mm, 64,7220 mm): Track [/asdf_N] on F.Cu, length 17,6360 mm
[clearance]: Clearance violation (netclass 'diffferrr' clearance 0,8880 mm; actual 0,6660 mm) Severity: error
   @(119,9180 mm, 64,7220 mm): Track [/asdf_N] on F.Cu, length 1,2898 mm
   @(102,2820 mm, 65,4380 mm): Track [/asdf_P] on F.Cu, length 17,6360 mm
[clearance]: Clearance violation (netclass 'diffferrr' clearance 0,8880 mm; actual 0,6660 mm) Severity: error
   @(102,2820 mm, 65,4380 mm): Track [/asdf_P] on F.Cu, length 17,6360 mm
   @(102,2820 mm, 64,7220 mm): Track [/asdf_N] on F.Cu, length 17,6360 mm
[clearance]: Clearance violation (netclass 'diffferrr' clearance 0,8880 mm; actual 0,6660 mm) Severity: error
   @(119,9180 mm, 65,4380 mm): Track [/asdf_P] on F.Cu, length 1,2898 mm
   @(119,9180 mm, 64,7220 mm): Track [/asdf_N] on F.Cu, length 1,2898 mm
[clearance]: Clearance violation (netclass 'diffferrr' clearance 0,8880 mm; actual 0,6660 mm) Severity: error
   @(101,3700 mm, 63,8100 mm): Track [/asdf_N] on F.Cu, length 1,2898 mm
   @(102,2820 mm, 65,4380 mm): Track [/asdf_P] on F.Cu, length 17,6360 mm
[clearance]: Clearance violation (netclass 'diffferrr' clearance 0,8880 mm; actual 0,6660 mm) Severity: error
   @(102,2820 mm, 64,7220 mm): Track [/asdf_N] on F.Cu, length 17,6360 mm
   @(119,9180 mm, 65,4380 mm): Track [/asdf_P] on F.Cu, length 1,2898 mm
[diff_pair_gap_out_of_range]: Differential pair gap out of range (netclass 'diffferrr' minimum gap: 0,8880 mm; actual: 0,6660 mm) Severity: error
   @(102,2820 mm, 65,4380 mm): Track [/asdf_P] on F.Cu, length 17,6360 mm
   @(102,2820 mm, 64,7220 mm): Track [/asdf_N] on F.Cu, length 17,6360 mm

** Found 0 unconnected pads **

** Found 0 Footprint errors **

** End of Report **

I’ll post this now here. (14:04 local time)
My next step is to search on gitlab for this. If it’s not mentioned yet I’ll make a bug report for it (I have a nice test project now for it)

I’ll be back in 10 minutes or so…

14:25. It took me a bit longer…
I have created a new issue for this:

Differential pair gap out of range fault by DRC. (#7975) · Issues · KiCad /...

Please add some comments if you think the error report is incomplete.

[floating-io]

Nah, you pretty much covered it, thanks. :). Saved me the trouble.

[paulvdh]

Most of the work was in creating the test project and to figure out and verify what was going on, and when I saw that the number for clearance was reported as a gap, it was clear to make an issue out of it.

Also:
Developers are KiCad’s most precious resource. Please do not add too many “casual conversations” on gitlab, but keep it to text that is actually relevant to fixing stuff, feature requests etc. Every second they spend reading your post, they are not improving source code. The user forum is a better place for such remarks and friendly conversations are welcome here.