Calculate this before Length matching Differential pair

When moving up from low speed design to high speed design, I saw engineers trying to length match USB 2.0 within 0.1mm or even closer to zero.

Yes, zero skew is the best but sometimes due to some reason, the EDA tool might not be able to remove the skew between the differential pair. Let’s take an example of USB 2.0. There is a max allowable skew, typically cited as 7.5 picoseconds for onboard skew, there is a higher skew.

So we should try to minimize it as much as we can to reserve the budget for the cable, so as not to have the product work with some particular USB cable, it should work with all quality of cables.

Let’s translate this 7.5 ps to the difference in the trace length. Assume Er of 4 and we are routing differential micro strip lines. This translates to 1.12mm of allowable skew for USB 2.0. Now when you route your signal, check what’s the maximum allowed skew and what length it translates to.

Add this length into the rule of the EDA tool so you know when you are within the limit.