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AM335x : PCB Trace length between USB connector and PHY

Younès Gahnem
Intellectual 300 points

Hello,

I saw the application note "High Speed Differential Signal Routing" which was very helpful.

You recommend a minimum spacing between high speed differential pairs (keep out layer).

But I didn't see any recommended specifications for the USB trace length between the USB Phy (microcontroller) and the USB port.

Do you recommended a max. PCB Trace length ? Or is it not so important ?

Thank you in advance,

Regards

Younes

  • 0
    TI__Guru**** 393215 points

    Hi Younes,

    This must be traced with 90Ohm differential impedance. It's good practice to keep these traces away from other signal and power traces, and to have a solid uninterrupted ground plane under them. Length isn't of great importance - after all USB cables are quite long themselves.

  • 0 in reply to Biser Gatchev-XID
    Intellectual 300 points
    Thank you again Bisher.

    Regards
    younes
  • 0
    TI__Mastermind 27720 points
    Hello Younes,
    I'm happy you found the application note useful.

    You used the term PHY with the term microcontroller in parentheses and used AM335x in your title so I wanted to make sure we are all on the same page here.

    Some of our SoCs use an external (discrete) USB PHY. In these cases, Biser is correct in that the overall trace length from the PHY to the connector is not of great importance as the USB specification allows for up to 4ns of delay from the transceiver to the associated USB2.0 port, but it should still be kept as short as practical to reduce EMI. Needless to say, I would not recommend anything *close* to the 24" of trace this 4ns value typically permits.

    Our newer devices, such as the AM335x, have integrated the USB PHY so the SoC-to-USB port trace length is much more critical. In these cases, please refer to Appendix A of the aforementioned app note for specifics on what the trace length requirements are. As always in high-speed system design, shorter is better for these traces.

    -DK