USB3 pairs connected together on USB-C connector? - Page 1
Forum section: electronics / projects

author: LukeW
author_rank: Frequent Contributor
timestamp: June 18, 2023, 03:19:08 am
content: I know on USB2 systems it's common to just connect together both data differential pair pins at the USB-C connector, i.e. both D+ pins are connected together and both D- pins are connected together, and this seems to work fine. But is the same approach correct for USB3? Will it cause stub reflection issues or cause the transceivers to misbehave?For USB3 implemented on a USB-C connector, there are two copies of the TX differential pair and two copies of the RX differential pair.A 24-pin USB type C connector is connected to a CYUSB3013 USB3 interface (which needs both the USB2 pair and the USB3 lines).Should they be connected together? Like attached image.Or, if only one of them is connected, one of the transceivers will not be connected at any given time (which may change depending on cable orientation) and will the USB link "know" which transmitter and receiver to use?

author: ataradov
author_rank: Super Contributor
timestamp: June 18, 2023, 04:08:29 am
content: I think you will need an active switch. There are 4 possible combinations of the cable flips, and some will flip RX and TX pairs. So, there is no way this would work.Ok, I'm very confused. No, it is impossible to flip them. But It does not look like there are any reference designs without an active switch.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: June 18, 2023, 04:24:03 am
content: It's only two combinations, and yes, this needs a switch.e.g.https://e2e.ti.com/blogs_/b/analogwire/posts/how-to-select-the-right-usb-type-c-signal-switches

author: ataradov
author_rank: Super Contributor
timestamp: June 18, 2023, 04:30:35 am
content: It can be flipped two ways on one side and two ways on the other side. There are total of 4 combinations. Logically/electrically it would work with direct connection. But with the way connectors are pinned out, it is not possible to have short traces between them, so active muxes appear to be absolutely necessary.And USB spec says that for single-lane implementation you need "the functional equivalent of a switch".Also, there is this hackhttps://richardg867.wordpress.com/2020/02/29/usb-c-done-cheap/- ASUS connected one side of the connector to one host port and the other side of the connector to the other host port. So, depending on the orientation device enumerates on a different port.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: June 18, 2023, 05:28:34 am
content: What am I missing? You either hook up lane 1 (RX1 & TX1) or lane 2 (RX2 & TX2). What are the other 2 options?

author: ataradov
author_rank: Super Contributor
timestamp: June 18, 2023, 05:38:31 am
content: From one side it is correct. My comment was about the both sides + a cable. It is really irrelevant here, the comment was made when I though that it would not work even logically.On one side it is just a 1:2 mux for RX and TX.I also tried to find some "cost optimized" products from China that would skip the mux, but even they don't do this, so I'd take it as "it is not going to work at all".

author: Marsupilami
author_rank: Frequent Contributor
timestamp: June 18, 2023, 06:17:58 am
content: Ahh ok, now it makes more sense.TUSB542 doesn't seem overly complicated, and it's ~$5. Which is not nothing, but I don't think it's worth an unwanted science project.

author: ataradov
author_rank: Super Contributor
timestamp: June 18, 2023, 06:28:18 am
content: With TUSB542 you would also need an IC/MCU that handles CCx lines and sets the direction.There are ICs that take CCx lines and negotiate the direction automatically. HD3SS3220 is one of them, and it is ~$3.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: June 18, 2023, 06:32:14 am
content: Quote from: ataradov on June 18, 2023, 06:28:18 amEven better!

author: ataradov
author_rank: Super Contributor
timestamp: June 18, 2023, 06:35:49 am
content: Quote from: wraper on June 18, 2023, 06:29:28 amUSB2 just goes to one of the hosts. There is no need for the same device to talk on USB2 and USB3 at the same time.He does the experiment later in the article. USB3 jumps across ports and USB2 enumerates on one of them.

author: ataradov
author_rank: Super Contributor
timestamp: June 18, 2023, 06:37:31 am
content: Quote from: Marsupilami on June 18, 2023, 06:32:14 amAnd the section with routing recommendations at the end of the datasheet is an interesting read. They don't even want to have stubs left by the vias. If you want a via to an internal layer, you have to back-drill it. Talk about expensive.