Author Topic: USB 3.0 USB C Reversible Connection , Why mux is needed?  (Read 1296 times)

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Offline RoadRunnerTopic starter

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USB 3.0 USB C Reversible Connection , Why mux is needed?
« on: July 09, 2020, 08:52:31 pm »
I am working on a project targeting to achieve USB 3.0 USB C reversible connection, After searching on web i found that quite a few schematic shows solution using of USB 3.0 Mux.

While USB 2.0 the just short both of D+ together and also D- together.  As shows in a reference schematic.  For usb 3.0 a mux is used.

My guess is using mux, Routing of Ghz range signal will be easier.
My question is this just for better routing for USB3.0 lines or there a specific reason for Having a dedicated mux.
Because i guess if short RX2+ to RX1+ and so on, It should work?

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Online langwadt

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Re: USB 3.0 USB C Reversible Connection , Why mux is needed?
« Reply #1 on: July 09, 2020, 09:28:45 pm »
the same reason that before the gigabit ethernet standard you needed crossover ethernet cables

usb 3.0 use a pair for each direction, the two ends have to agree on which pair each end transmits on and swap so rx goes to tx

usb 2.0 does both directions on a single pair
 

Offline jbb

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Re: USB 3.0 USB C Reversible Connection , Why mux is needed?
« Reply #2 on: July 09, 2020, 10:22:56 pm »
As I understand it, the USB SuperSpeed signals are some GHz and need to be routed really carefully to maintain signal integrity.

Simply tying the superspeed pairs together will create a transmission line stub and impair the signal integrity. This could result in failure to pass certification tests, or unreliable link behaviour or maybe total failure.

If you can get by with USB Hi Speed (480 Mb/s) then life is easier...
 


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