Confusion with USB C connections and routing - Page 1
Forum section: electronics / projects

author: Northy
author_rank: Regular Contributor
timestamp: October 26, 2020, 05:24:26 pm
content: Hi all,I'm trying to route a USB C connector.I've been a little confused by the connections to it but I think I'm getting there with them now.I've chosen this one:https://www.cuidevices.com/product/interconnect/connectors/usb-connectors/uj31-ch-g1-smt-trhttps://www.cuidevices.com/product/resource/uj31-ch-g1-smt-tr.pdfI'm routing to the Microchip LAN7800https://www.microchip.com/wwwproducts/en/LAN7800As the connector is reversible all the signals obviously cross between the row of SM pins and the row of TH pinsDoes anyone have any tips on routing that out?I'm really trying to keep all the components on the top side of the board, so I was thinking:LAN7800 --> TVSs --> SM pads --> TH pins.I have noticed that the USB port on the LAN7800 doesn't match the standard connector pin out thoughIf I was to via pairs to swap the order I'm guessing it would be best to do it to the USB 2 traces and try and keep the USB 3 super speed pairs without vias?Grateful for any help.Thanks,G

author: thm_w
author_rank: Super Contributor
timestamp: October 26, 2020, 09:25:16 pm
content: Quote from: Northy on October 26, 2020, 05:24:26 pmSure thats probably best. If not put a via in each USB 3 pair and keep the length matched and short.https://e2e.ti.com/support/interface/f/138/t/512449http://ww1.microchip.com/downloads/en/DeviceDoc/Routing%20Checklist%20for%20LAN7800%20SQFN%20Rev%20B.pdfhttps://www.onsemi.com/pub/Collateral/AN-6103-D.PDFAre you using a controlled impedance 4+ layer board?

author: bson
author_rank: Unknown
timestamp: October 26, 2020, 10:00:05 pm
content: I believe USB 3 mandates auto polarity detection, so it should be possible to pin swap to prevent crossing the differential pairs.

author: Northy
author_rank: Regular Contributor
timestamp: October 27, 2020, 10:05:03 am
content: I've just had an 'oh shit' momentI need a mux don't I? I had naively thought that the reversible connection thing of the connector was just done mechanically!G

author: RoadRunner
author_rank: Frequent Contributor
timestamp: October 27, 2020, 10:37:38 am
content: I have made a USB 3.0 board with USB C , Yes you would need a MUX to switch between USB3 lines, I don't know how its done on these specific purpose USB Controllers, You will also need some logic to select between the pairs. On my board selecting which pair works is decided per firmware.Regards

author: Northy
author_rank: Regular Contributor
timestamp: October 27, 2020, 12:30:29 pm
content: Thanks guys.I'm going to try using Ti's HD3SS3212 between the connector and the LAN7800.https://www.ti.com/product/HD3SS3212#product-detailsThe datasheet says the AC coupling 100nF capacitors on the TX pairs can either go between the LAN7800 and the MUX, or the MUX and the connector - but I don't know what's best?Thanks,G

author: Northy
author_rank: Regular Contributor
timestamp: October 27, 2020, 04:41:45 pm
content: I'll try again - having problems with attachments.

author: Northy
author_rank: Regular Contributor
timestamp: October 27, 2020, 04:48:27 pm
content: I can't upload the schematic, it says it fails security checksSo this is where I've got to so far (see above post). I'm building this up as a design block within Eagle to make re-use easier. I've already done the LAN7800 design block.At first literally all the pairs crossed each other, but as this is just a high speed capable mux, I swapped the inputs to it. Should be fine right?Anyway, some of the pairs need to cross, others don't.I think that for USB 3 I'm supposed to keep things matched/symmetrical, so does anyone have any good suggestions on how to move forward with the routing here?Thanks,G

author: Northy
author_rank: Regular Contributor
timestamp: October 27, 2020, 05:11:20 pm
content: Do I need some other chip for the CC1 and CC2 pins?I don't want to take any power form USB, this will be a self powered device.Thanks,G

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 27, 2020, 06:15:40 pm
content: Quote from: Northy on October 27, 2020, 05:11:20 pmNope. For an USB-C device that is configured simply for this use, a simple set of two resistors with the appropriate value between CC1/CC2 and ground is enough.

author: Northy
author_rank: Regular Contributor
timestamp: October 27, 2020, 07:44:24 pm
content: Quote from: SiliconWizard on October 27, 2020, 06:15:40 pmThanks, and just use CC1 and CC2 to switch the MUX?Where do I read up on what resistors etc I need?Thanks,G

author: Northy
author_rank: Regular Contributor
timestamp: October 27, 2020, 09:27:32 pm
content: I'm trying to design a USB 3.1 Upstream Facing Device, but I don't want/need to take any power from the USB cable as the device will be self powered (the port will be for set-up).I'm still not sure what, if anything, I need to do with VCONN. Am I expected to still feed power into VCONN for active cables?I was hoping there would be some way to determine which way up the cable is plugged and use some discrete logic to switch the MUX accordingly, but this doesn't seem to be the case?Thanks,G

author: Northy
author_rank: Regular Contributor
timestamp: October 28, 2020, 03:13:09 pm
content: If anyone other than meis still reading/following this, I've asked Ti for some support/help.I'll update.G

author: jbb
author_rank: Super Contributor
timestamp: October 28, 2020, 09:24:19 pm
content: Ahh, for USB 2 I think you use *separate* pull down resistors on CC1 and CC2, then sense which one gets voltage to work out which way up the cable is.For USB3 super speed there might be some special stuff. You definitely need a multiplexer to flip the SuperSpeed data pairs between ‘right way up’ and ‘other way up’ - you can’t just tie them together because you’ll get transmission line stubs. I don’t know if you need to source VCONN. I don’t know if you need a USB Power Delivery (PD) chip to negotiate use of SuperSpeed pairs.Go to the USB Implementers Forum website and get (no $ needed!) the USB C Power Delivery standard, which should help explain what’s going on.

author: Northy
author_rank: Regular Contributor
timestamp: October 29, 2020, 04:38:04 pm
content: Figure 3 of this document is interesting:http://ww1.microchip.com/downloads/en/Appnotes/00001914B.pdfOne comparator used to switch the direction of the MUX.Now I'm assuming that RX1/TX1 pairs are used when CC1 goes high (well, actually ~0.4V), but trying to make sure that is 100% true is proving quite difficult!G

author: jbb
author_rank: Super Contributor
timestamp: November 03, 2020, 10:13:18 pm
content: The USB C v1.3 standard is quite clear that the CC pin and SuperSpeed pins come as a group, so I think that Microchip app note is fine.It seems:- Sink device doesn’t need to supply Vconn- Cable must be electronicall marked (EMCA) to do SuperSpeed- I think your sink device can do SuperSpeed connection without needing special USB PD chipset etc, in line with that Microchip document. But I’m not certain

author: Northy
author_rank: Regular Contributor
timestamp: November 06, 2020, 08:59:31 am
content: For those playing along at home I'm designing two prototype PCB's to try different ideas before fully committing:Number 1:Magjack -> LAN7800 -> HD3SS3220 (CC Controller and Mux all in one) -> USB Type C connectorNumber 2 build option one:Magjack -> LAN7800 -> HD3SS3212 Mux & LM339 Comparator -> USB Type C connectorNumber 2 build option two:Magjack -> LAN7800 -> HD3SS3212 Mux & TUB321 CC Controller -> USB Type C connectorG