USB-C passthrough (including CC line) not making connection

Question

I'm trying to make a USB-C switcher.

I'm using a rotary switch (center) to have one input USB-C receptacle switch between two output receptacles. This is so I can plug my keyboard into the input and two computers into the two outputs, and use a knob to select which computer the keyboard is attached to without unplugging any cables.

Here is my circuit diagram:

The VBUS, D+, and D- lines of the two output receptacles are switched through the rotary switch and connected to the input receptacle. Pins CC1 and CC2 of each Output receptacle are joined, and then switched through the rotary switch and connected to the CC1 and CC2 pins of the input receptacle. A common ground plane is shared between all three receptacles and isn't switched by the rotary switch.

Here's the PCB I designed:

I thought I could join together the CC pins (since there is only one CC line in USB-C cables) and pass the CC line through the switch to allow for native USB-C power delivery negotiation between each host and the keyboard. But when I soldered up the boards, that didn't work. No connection was made no matter what cables or plug orientation I used. It also didn't help to unplug one cable so only two cables were plugged into the device, one input and one output. I have verified with a multimeter that my solder connections on the board are sound.

My question is: why isn't passing through the VBUS, D+, D-, and CC lines of a USB-C cable, with a common ground, allowing USB connections?

I have a few ideas:

• Maybe the fact that I'm sharing ground between all three receptacles isn't good, and is messing up various things.
• Maybe the fact that B8 and A8 aren't passed through is causing some issue? But I've learned these are USB-C SuperSpeed lines and I think it should work fine to leave them unconnected. (I don't need high-speed data since I just want this to work for a keyboard.)

Thanks for the help!

Answer 1

The CC pin connections are not correct.

Sure, there is only one CC wire in the cable itself.

The other CC pin that is not wired 7n the cable is pulled down to ground via a resistor by the cable, in the connector, which enables the host to detect which way the connector is plugged in to host.

The cable wiring will then connect the remaining CC pin to the device, to detect if something is connected, and what type of device is connected.

The device (keyboard) will also detect cable orientation from which pin is connected to correct resistance by cable and which pin is connected to host.

Only when correct cable and device resistance signatures are detected, the host will turn the standard 5V VBUS on to power the device.

So having short-circuited the CC pins, the host can't detect cable orientation or device type so it will not turn 5V VBUS on.

Since your device is a keyboard, it is highly unlikely it needs to communicate USB PD, but even if it would like to communicate USB PD, nothing happens as the keyboard won't receive even 5V from host.

Answer 2

This board ought to work for a keyboard usecase, but there's a situation where this might not work.

It's indeed likely your problem is CC-related, but not in a way that anyone has correctly identified. With a 'smart' USB-C port (aka, most of the native USB-C ports), you only get 5V out of it when the port detects a pulldown out of certain range of values on one of its two CC pins. Even then, this circuit ought to work in the simplest usecase, as I don't see why the downstream devices wouldn't just expose the (typical, other values possible) 5.1K pulldown through CC seamlessly.

Unless you're using emarker-equipped cables for connecting any of the parts circuit, that is. The emarker is a cable-end-embedded IC, connected to the pin opposite of the CC pin, referred to as "VCONN". When two CC pins are connected together, the pulldown-connected CC pin and VCONN-bearing pin will be joined together, with the emarker IC consuming some current, and making the effective resistance no longer be (typical) 5.1K. This is what happened with Raspberry Pi 4.

My recommendation - test your board with a USB-A to USB-C cable, plugged into your PC's USB-A port. That is a surefire way to verify such a board, since there's no CC detection involved at any point, VBUS supply is non-conditional with such a cable. Otherwise, if you only have C-C cables, test if you actually get 5V on VBUS, given you're using a 'smart' port (most of them, save for USB A male-C female adapters). If not, this is most certainly a CC-related issue.

If a USB A-C cable works, see if your board works using the cheapest C-C cables possible, USB 2.0 only kind of deal, make sure they're not "100W". Such cables are +- guaranteed to be emarker-less.

While USB 2.0 does technically require impedance-matched pairs, it's well known for working through wet string, as they say - even more so for a keyboard, which is likely to use 12Mbps. See if you actually get USB errors - on Linux, sudo dmesg -Hw shows them in realtime.

Answer 3

To summarise and conclude:

It's not possible to create a USB-C pass-through device and remain compliant. To remain compliant, you would have to act as a hub.

The reason it's not possible is that USB-C cables are either EMCA (Electronically Marked Cable Assembly) or not. For EMCA cables, one CC line is passed-through, and the other (called Vconn) is pulled down (800Ω ~ 1.2kΩ) and terminated. Optionally Vconn is also used to power an EMCA controller (E-Marker) so the cable can communicate its capabilities.

For non-EMCA cables, one CC line is passed through like before, but the other is not connected.

So if you join up CC1 and CC2 like you've done, it will work for non-EMCA cables, but not EMCA cables. If instead you pass CC1 and CC2 straight through, it will work for non-EMCA cables, and even if one of them is EMCA, in one orientation only. Two EMCA cables will still fail because the two EMCA devices will clash in either orientation.

Other than implementing a hub, or only using non-EMCA cables, the other option is to use a non-standard "extension cable". These cables have both CC lines passed through. Typically they're female on one end, so your Out A and Out B would need to be male, but that way you could get away either a EMCA or non-EMCA cable on IN.

Whatever way you slice it, pass-through is non-compliant. USB-C is not your father's USB...