I’m trying to write a custom design rule to enforce a specific, nonzero skew between different traces on my PCB. The reason for the nonzero constraint is that my PCB needs to compensate for the skew introduced by another board.
Let’s say I want to enforce that net /one is 2 mm shorter than net /two. I tried this rule:
(version 1)
(rule NonzeroSkew
(constraint skew (min 1.9mm) (opt 2.0mm) (max 2.1mm))
(condition "A.NetName == '/one' || A.NetName == '/two'")
)
If the two tracks /one and /two have length 9 mm and 10 mm (wrong skew), I get the following two DRC errors:
Error: Skew between tracks out of range (rule 'NonZeroSkew' min skew 1.9000 mm; actual -1.000 mm; target net length 10.0000 mm (from /two); actual 9.0000 mm)
Pad 2 [/one] of NT1 on F.Cu
Error: Skew between tracks out of range (rule 'NonZeroSkew' min skew 1.9000 mm; actual 0.000 mm; target net length 10.0000 mm (from /two); actual 10.0000 mm)
Pad 2 [/two] of NT3 on F.Cu
If the two tracks have length 9 mm and 10 mm (correct skew), I should get no DRC error. Instead, I still get one error:
Error: Skew between tracks out of range (rule 'NonZeroSkew' min skew 1.9000 mm; actual 0.000 mm; target net length 10.0000 mm (from /two); actual 10.0000 mm)
Pad 2 [/two] of NT3 on F.Cu
See also screenshot below:
My understanding is that, with my custom rule above, KiCad enforces that both nets /one and /two have a skew of 2 mm with respect to the longest trace. But that’s not possible, since one of the two traces will be the longest and will always have a skew of 0 mm.
Basically, I have two questions:
- Is this the expected behavior, or is this a bug?
- Is there a way of modifying my custom rule to achieve the goal of enforcing a nonzero skew between two traces, without spurious errors?