Series resistors on USB2 data lanes


We are designing our own carrier for Xavier. We are having some concern/questions on USB2 series resistors.

  1. Do we need to add series resistors (18 - 33 ohms) on USB2 data lanes to match up with the impedance of transmission line (PCB trace)? Or is USB2 driver on Xavier SoM/SoC able to automatically adjust USB2 source impedance to match up with transmission line?

  2. We are able to put Xavier in force recovery mode on our carrier, and able to flash Xavier. However, data throughput is super slow. Note that we only have the USB2 portion (using USB0_N/P) on USB-C port on our carrier. But I am pretty sure this is NOT the cause of this super slow data speed issue. Because we are using a USB2 only USB-C cable, and data speed when flashing on devkit carrier was fast. We have 33 ohm series resistors on USB2 (USB0_N/P). Would this be the cause of slow throughput issue? When in force recovery mode, the USB2 of Xavier will be in USB Device mode. ], In OEM DG, “7.1.1 USB 2.0 Design Guidelines”, is this apply to both Host and Device mode? Or Host mode only? If it is Host mode only, where can I find the guide for Device mode?

Best regards.

Hi, why do you add the series resistors? There is no such request for USB lines. The impedance is guaranteed by layout and routing. You can refer to the routing requirements of product design guide for detail info.

Hi @Trumany

Thanks for your reply.
I believe resistors are used to “tune” the transmission line. On a properly tuned transmission line, the signals will travel happily from A to B with minimum interference and distortion.

One more additionally question on USB3 UPHY lanes. Do we need to add series resistors on USB3 UPHY lanes (UPHY_TX1/RX1)?


No need if layout is good, as you can see in the P2822 schematic in DLC.


Yes, you are right. After we take out these series resistors and shorted with wires, all issues are resolved.
Thanks for the help.


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