PCI Express supports lane reversal for the lanes.

Can REFCLK+ and REFCLK- also be swapped? That is, connect REFCLK+ from one device to REFCLK- on the other?

  • \$\begingroup\$ Do you mean polarity reversal? Also iy would likely be highly device dependent so without the device you mean it's hard to say. \$\endgroup\$
    – Justme
    Apr 2, 2023 at 19:50
  • \$\begingroup\$ The polarity swap is just a 180 degree phase offset. \$\endgroup\$ Apr 12, 2023 at 6:11

1 Answer 1


The polarity of the reference clock can be inverted (p/n swapped).

In fact PCIe devices don't actually need a common reference clock, any 100MHz clock within a set +/-300ppm tolerance will do (+/-100ppm for Gen 5). The receiver serdes blocks include FIFOs to compensate for phase shift and frequency drift between lanes and the reference clock, along with skip packets for aligning data streams in the protocol.

The polarity of the lanes themselves can also be swapped (support is required by the spec)

As a side note, "lane reversal" is not to do with polarity, but rather swapping the order of the lanes - e.g. for a x4 link you connect lane 3 to 0, 2 to 1, 1 to 2, and 0 to 3. This is an optional feature, with no guarantee of support. It will work as long as at least one device (endpoint or upstream) supports it, but no link can be formed if neither support the feature.

  • \$\begingroup\$ Awesome, this will simplify the layout in my design a lot! Thanks! \$\endgroup\$
    – Greg
    Apr 5, 2023 at 0:09

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