Other Parts Discussed in Thread: HD3SS3220, 3220UFP-DGLEVM
Hi
I am working on system design with the following blocks. It contains TUSB8020B to work as a hub to two secondary USB subsystems which can sink up to 5V, 2.5A in total. The system doesn't need a PD negotiation for higher voltages above 5V or any Alt Modes. The system on whole can be compared in functionality with a USB 3.1 Hub with a fixed USB Type C cable coming out on the left end(which will be plugged into a PC, This USB plug is flippable by a user). The device is only powered via the same single USB C cable from the PC(No external power).
The UFP side of TUSB8020B is where I am having some issues. Since it's the early stages of the design, the green-coloured blocks are things which I have full control over in terms of changes. Every item including the cable design can be customised.
My initial thought was to have it exactly as per the block diagram above with a USB Type C receptacle 1 inside the device which will mate with USB type C Plug 2. I am shorting the D+/D- pins on the receptacle PCB end. Since it's low speed if think that should be OK. The overall issue with this arrangement is that USB Type C receptacle 1 would give 4 pairs of Super speed differential signals and TUSB8020B can accept only 2 pairs. I have the following options
Option 1:
Short the 2 pairs of superspeed lines on the PCB after receptacle 1 and feed those 2 pairs to TUSB8020. I am sure this is frowned upon as it will create stubs and it's not advised for USB 3.1 speeds. Is there any documentation which mentions what can be a stub length of USB 3.1 speed if I ever need to go to this option(as a last resort)?
Option 2:
Feed 4 pairs of SuperSpeed lines to a controller+mux like HD3SS3220 and feed the output 2 pairs of the mux to TUSB8020 and let the mux handle SS line selection. This I believe is the ideal solution. But I would not want to go with this option(unless it's the only one) as it does add to the BOM cost and I am hardly using any of its full features. Is there a similar or cheaper alternative which can work for my use case as a sink? Also since the device role is fixed, I am not looking for any communications to this controller to set different modes if any.
Option 3:
Since Plug 2 and Receptacle 2 are permanently fixed inside the device. Can I leave 2 pairs of SS lines unconnected? For the other 2 pairs, I can route to TUSB8020. I think this will work but it might affect the flip-ability of Plug 1 by the user at the PC end. Can someone confirm that this will fail if the user connects Plug 1 in the opposite orientation?
It will be great if someone more experienced in USB design can weigh in on the pros and cons of these 3 options. If there is any other option which can be explored, I am happy to hear that also.
Thanking you in advance.
