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TS3USB221EVM: TS3USB221EVM

Gary Charlton
Prodigy 35 points
Part Number: TS3USB221EVM
Other Parts Discussed in Thread: TS3USB221

Hello,

Please could you help.

I have designed and built a USB 2.0 4 port multiplexer using the TS3USB221.

Having followed all the guidelines for 90 ohm differential strip line design we find that the USB ports are unable to operate at 480Mb.

The design effectively has 2 TS3USB221 in series with each other.

I suspect my PCB layout so we are about to test the theory by cascading two TS3USB221 EVM modules.

One of the  issues that was noted with the EVM is that the USB signal layer also has a ground plane around the differential trace.

All the design guides indicate this is incorrect (. Also, maintain a minimum keep-out area of 30 mils to any other signal throughout the length of the trace.)

Is this the issue? (My PCB does not have ground plane on USB signal layer)

  • 0
    TI__Guru**** 177161 points

    Hi

    The benefit of having a ground plane on the same layer as the high speed differential signal trace is that it reduces the radiating magnetic field of the high speed differential signal trace. But it is best to keep the distance between the high speed differential trace and the ground plane equal to at least twice the trace width distance. This minimizes the capacitance of the plane which can alter the characteristic impedance of the trace.

    Do you see USB operating at 480MHz when cascading the two EVMs together? 

    When cascading multiple switches, signal bandwidth, Ron, total capacitance all need to be taken into account, please take a look at this e2e post: https://e2e.ti.com/support/archive/internal/int-switches-multiplexers/f/397/t/87105?tisearch=e2e-sitesearch&keymatch=ts3usb221%252525252520cascade.

    Did you also take a look of the USB2 eye diagram?

    Do you mind if I can take a look at your layout?

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 35 points

    Many thanks David for your response.

    In answer to your questions

    We have a USB type b connector as an input. The signal is then routed through 2 ts3usb221 to a USB type A connector with a USB 2.0 stick.

    We can read and write data to the USB stick without issue.

    If the USB 2.0 stick is changed to a bootable USB stick the host PC refuses to boot from it.

    We have tried an evaluation board with single ts3usb221 and the pc boots from the stick every time.

    This leads me to believe that the PC attempts to enumerate at USB 2.0 full speed and either the losses in the switches are too great or my PCB layout is at fault.

    Unfortunately I only have access to a 100MHz scope so no chance of an eye diagram or even seeing the activity on the +D or -D lines.

    I would be really grateful if you could look at my layout and give me some pointers.

    The first prototype PCB was 4 layer 1.6 fr4.

    Logic signals  and USB power on top layer ,Vcc  3v3 on 2nd layer, USB gnd 3rd layer and USB sig on bottom layer.

    No splits in the USB gnd, no stubs and routed as short as possible.

    What's the best way to share my layout?

    Again many thanks.

    Gary

  • 0 in reply to Gary Charlton
    TI__Guru**** 177161 points

    Gary

    Would you please accept my friendship request and then you can send me the layout file using the private message.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 35 points

    Hi David,

    Friendship accepted and files sent by private message.

    Regards,

    Gary

  • 0 in reply to Gary Charlton
    TI__Guru**** 177161 points

    Support provided through private message, close this thread.

    Thanks

    David