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USB layout - matched length

Milan Zelenka
Intellectual 870 points

Hello,
SPRAAR7A of January 2013 reads as follows:

Route DP/DM traces close together for noise rejection on differential signals, parallel to each other and within two mils in length of each other (start the measurement at the chip package boundary, not to the balls or pins).

I am not quite sure how to interpret the latter part of the statement in parentheses. Could you please confirm that I am supposed to match 2 mils in length measured at the edge of device body while ignoring 30-mil pitch of the BGA balls?

Thanks,
Milan  

  • 0
    TI__Guru**** 176186 points

    Milan

    The measurement of DP and DM trace length starts at the chip package boundary, you do not need to take the DP/DM trace length inside the package into account. Ideally, you want to make sure DP and DM have equal length. But if there is a diffrence between DP and DM trace length, the max difference is 2mils.

    Thanks

    David 

  • 0 in reply to David (ASIC) Liu
    Intellectual 870 points

    David,
    typical matched length req for Ethernet front end is 50 mils. I suspect 2 mils for USB might a typo. Could you please double check that?

    Also, I still do not fully understand why you ask for 2 mils matched length while ignoring 30mil pitch of DP+ and DP- BGA balls.

    Thanks,
    Milan

     

        

  • 0 in reply to Milan Zelenka
    TI__Guru**** 176186 points

    Milan

    2mil is the correct requirement for DP/DM. The DP/DM trace length are compensated and matched within the device package, this is why we can ignore the 30mil pitch and start the measurement from the device package boundary.

    Thanks

    David