3
\$\begingroup\$

Looking at laying out a PCB that will utilize PCIE. I've read that you want to keep intra-pair trace length to 5mils max and that inter-pair trace length matching is not terribly important. The current plan was to keep all Tx pairs within 100mils and all Rx pairs within 100mils, but what I'm not sure about is how close to the same length the Tx pairs, Rx pairs, and Clock pair have to be. I read one site that said that up to a 3" difference is acceptable but that seems rather large to me.

\$\endgroup\$
1
  • 3
    \$\begingroup\$ Rx and Tx are rather independent. Texas Instruments has pretty good documentation of PCB design when it comes to high speed interfacing. \$\endgroup\$
    – user103380
    Jan 29, 2020 at 19:35

1 Answer 1

4
\$\begingroup\$

From Wikipedia

While the lanes are not tightly synchronized, there is a limit to the lane to lane skew of 20/8/6 ns for 2.5/5/8 GT/s so the hardware buffers can re-align the striped data

The allowed skew between the databytes in one direction is 6ns for 8 GT/s. Rx and Tx length matching is not critical as there is wide allowed duration. Assume it as a full duplex communication. How much exactly depends on the data link layer but definitely okay with 3 inch or so. Assuming 160ps per inch of propogation delay, the the 3 inch propogation delay will be about 0.5 ns. (Less than 2 ns)

  • Most important is to match and route the intra differential pairs well.
\$\endgroup\$
1
  • \$\begingroup\$ Thanks, this was very helpful. \$\endgroup\$
    – vhailor
    Feb 13, 2020 at 0:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.