I have a board with differential pairs.
USB is fine, 9 sorts the skew out.
But for Gigabit LAN and SSD - Rpi CM4 - the multiple differential pair lengths need to be set, I can do that with 8 with setting the length
My question is, do I need to do a 9 on the pairs before I do an 8?
Thanks
Andy.
Can’t help you, but now I understand the reference to 8 & 9 . . .
Normally you’ll tune the skew after you tune the length of your differential pair, as the skew comes normally from the fact that you mostly won’t have the same amount of left/right bends, which would make the length of the signals unequal.
But if you tune the skew after won’t the lengths then be incorrect?
I would really like a definitive answer as I an ready to order boards…
tuning the skew shouldn’t make the pair longer, as you (in theory) just match the shorter wire in the pair with the longer (and length matched) one. obviously you should check at some point that the longer wire is actually length matched with the other pairs.
Yes, that does sound sensible.
I will give it another try…
Yes, looks OK - I thought it was probably the way to do it but it is my first time doing this particular thing so confirmation is nice 
Thanks all!
no need to go too crazy on the matching, honestly unless its a 28 Gbps PAM pair… just do your best.
a few mm is not going to kill it. It’s all about rise times. if the mismatch is less than a third of a rise time, dont worry.
think about how you might route it to get equal left and right bends… that can improve life.
Gigabit LAN, generally just ensure the clk is longer than the others…
Great, another of life’s great mysteries…
Thanks
Andy.
This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.