I'm doing PCB layout for a USB 3.0 connection and I'm curious about if there's a length match requirement between the unidirectional super speed pairs and the bidirectional high speed pair. This isn't for a specific USB 3.0 driver or receiver, but what is considered good practice for operation.
For clarity I'm going to refer to the bidirectional high speed lines as BI+ and BI- and the unidirectional super speed lines as TX+, TX-, RX+ and RX-. What I have seen so far is listed below:
- TX+, TX- lines should be within 5 mils (0.13 mm) of each other.
- RX+, RX- lines should be within 5 mils (0.13 mm) of each other.
- BI+, BI- lines should be within 50 mils (1.25 mm) of each other.
- The TX pair and RX pair don't have any length requirement with respect to each other.
The length matching requirements in pairs make sense, but I'm not confident that the TX pair and RX pair don't have a length mismatch requirement for all devices. I'm also not sure if the BI pair has any length requirement with respect to the TX and RX pairs. Are there any general rules of thumb to follow for matching the lengths of pairs to other pairs to avoid errors?