PCIe interlane length matching


I’m designing a board and using UPHY RX/TX 12 ~ 19 as a x8 bus.

Similarly, I’m also using UPHY RX/TX 22,23,10,11 as a x4 bus.

Is there any requirement for length matching between the lanes in the same bus?

Usually we have requirements for length matching all TXs and all RXs in the same bus, but I can’t find them in the Design Guide.

Best regards.

You are supposed to have read both the PCI Express Base Specification Revision 4.0 and the PCI Express Card Electromechanical Specification Revision 4.0. The latter says in section 4.7.5 “Lane-to-Lane Skew”:

Skew System Board: Value 1.25 ns: Estimates about a 7-inch trace length delta on FR4 boards.

7 inch delta is hard to reach.

For add-in cards the allowable skew is 0.35ns or 2 inch.

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