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This question has been asked a couple of times, with some interresting answers but I need to go further in depth. My device is a Corsair Void Pro USB. I want to change its USB A connector to a USB C connector, and it must work on all my devices (PC and mobile). The reason for this conversion is that USB A is bulky, so is the USB A to USB C adaptor, and most of my devices have USB C receptacles. Those that don't, I'll just use a minuscule converter (shown below).

I used this specific connector:

USB C male connector USB C male connector

I followed this popular Youtube video (and adapted it for my specific connector) and it worked on my Google Pixel Slate. But when using an adapter to convert USB C back to USB A (for my PC), the device worked when the USB C connector was inserted in one way but not the other (i.e. it's not reversible). Here's the adapor:

USB C to USB A adaptor

WARNING: Any adaptor with a USB-C receptacle such as this one is potentially unsafe according to the USB spec. See MacGuffin's anwser.

So I figured I had to wire both the A and the B side of the USB C connector (that required some micro-soldering directly on the connector leads). Now the connector works when inserted both ways in the adaptor. Here's the schematic:

USB C schematic

The device works fine on my Pixel Slate, on a Chromebook and on my PC. But not on my phone (Google Pixel 3), no matter the way it's connected! It kind of starts for 2 or 3 seconds on a Google Pixel 1 but then it stops. However, here's a silly cascade of adaptors that just works:

USB C connector -> USB C female to USB A male -> USB A female to USB C male -> Phone

Silly cascade of adaptors

The white adaptor is the one that came with a Google Titan security key set.

Now the question: what's missing from my connector so my device works on my phone? (Or what's so special about Google's USB A to USB C adaptor?)

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  • \$\begingroup\$ I think those breakouts may have issues with tying CC1 and CC2 together. \$\endgroup\$
    – Passerby
    Mar 15, 2021 at 15:31
  • \$\begingroup\$ They're not. R1 and R2 traces are both tied to Vcc on one side, and to CC1 and CC2 on the other (pullup config). Also, the resistor that comes with the board is tied to the opposit CC. That does not seem right, but that does not affect my design anyway. I just removed the resistor and used the traces for my own pulldowns. \$\endgroup\$
    – Jerther
    Mar 15, 2021 at 15:49
  • \$\begingroup\$ So is that schematic an accurate depiction of how you have it wired? And I may be wrong but I'm guessing that the white adapter has resistors for a UFP, 5.1k to VBus/5V on both cc1 and cc2 as well as the pull downs for dfp. \$\endgroup\$
    – Passerby
    Mar 15, 2021 at 16:21
  • \$\begingroup\$ You could sacrifice the adapter to check. \$\endgroup\$
    – Passerby
    Mar 15, 2021 at 17:04
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    \$\begingroup\$ You are testing your modification against an adapter that violates the spec. Don't do that. To work around the lack of "flippability" when tested against the nonstandard adapter with USB-C/female to USB-A/male you had to violate the spec, because the adapter violates the spec. The connector was fine before and then you broke it by connecting A6 to B6 and A7 to B7. Two wrongs don't make a right. Stop using the adapter that was made broken from the factory. I realize that the USB adapters that follow the spec are bulky and inconvenient but they won't start fires when plugged together. \$\endgroup\$
    – MacGuffin
    Jul 28, 2022 at 15:26

2 Answers 2

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A big problem here is that you are using an adapter that violates the USB spec. A male USB-C should never be adapted to a male USB-A. Doing so not only violates the spec but creates a fire hazard.

Just so I'm clear it is this adapter that violates the spec. enter image description here

Please stop using this adapter before you injure yourself or others. Smash it to pieces so it is no longer a threat to people or hardware. Again, it is a fire hazard, that is why the USB spec prohibits it.

Edit to add:

The document that spells out the USB-C wiring specifications is called "USB Type-C Cable and Connector Specification" or something similar and can be found on the USB.org website. Here's a link but the last time I tried to link to the document the link broke soon after, so those interested may simply have to search for the document on their own. https://www.usb.org/sites/default/files/USB%20Type-C%202.1%20Release%2020220316_0.zip

In section 2.2, titled "USB Type-C Receptacles, Plugs and Cables", there is the following text: "USB Type-C receptacle to USB legacy adapters are explicitly not defined or allowed. Such adapters would allow many invalid and potentially unsafe cable connections to be constructed by users."

If the question is how to correctly wire a USB-C receptacle to a USB-A plug then the answer is there is no correct way to do so, that is because the people that wrote the spec did not define a correct wiring. If the question is how to safely wire a USB-C receptacle to a USB-A plug then the answer is that is impossible without also defining the intended outcome, and that defined outcome is not contradicted in the spec. There's more than one way to wire such an adapter and get something that "works", but not all of them are consistent with preventing unsafe uses of such adapters.

I've seen people argue that they've used adapters like that in the photo and not seen any damage done or undesired behavior. I have no doubt of that. The issue is that such an adapter has the potential of causing hardware damage and bodily harm if someone was not aware of when and where it was safe to use. Because it would take a very lengthy explanation on the internal workings of the USB protocols I won't even attempt to explain why it is not safe here and now. I would hope that people take the warnings of the people that wrote the spec as sufficient to not use these adapters and destroy any they currently possess. It is better to intentionally destroy a $10 adapter than use these adapters to unintentionally destroy a $1000 computer.

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  • \$\begingroup\$ Could you point to the exact USB spec that this adapter violates, and explain more? \$\endgroup\$
    – Jerther
    Jul 24, 2022 at 23:06
  • \$\begingroup\$ Thanks for pointing it out. The adaptor is quite irrelevant in the whole matter as I asked about the USB-C plug connections, and the answer also does not involve the adaptor. However, I take the warning seriously, and I'll update the question. Oh and the adaptor is worth like 10¢ ;) \$\endgroup\$
    – Jerther
    Jul 28, 2022 at 12:36
  • \$\begingroup\$ @Jerther The adapter is the entire reason for the question, it is far from irrelevant. You point out in the question that the original USB-C modification works fine in every computer that is used except the PC with the "dodgy" adapter. In that case the modified device is not "flippable", working in one orientation but not the other. If you didn't use the dodgy adapter then there's no problem to solve. Or did I miss something? \$\endgroup\$
    – MacGuffin
    Jul 28, 2022 at 16:25
  • \$\begingroup\$ I merely wrote about the adapter to add information that might lead to a solution. A clue if you will. The problem was that I couldn't build a custom USBC male connector on my device and have it work on all my host devices; my phone specifically. In hindsight it just brings confusion, and focus away from the real matter I had in mind. So I could very well edit the adapter out from the question and plainly ask "How to properly wire this dongle". \$\endgroup\$
    – Jerther
    Jul 28, 2022 at 20:06
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WARNING: This wiring I've come up with was to make my USB-C plug flippable in a USB-C to USB-A adaptor. Such adaptor is potentially unsafe, so use this answer at your own risk.

Well, turns out only the data connections have to be hooked on both A and B sides, and only one CC is needed (A5 or B5, does not matter since the connector is reversable):

Working schematics

Kind of makes sense since in USB2 configuration, CC2 (B5) becomes VCONN:

USB C Pinout in USB2 configuration

Tested on:

  • Google Pixel 1
  • Google Pixel 3
  • Google Pixel Slate
  • ASUS Chromebook C434
  • PC (with dodgy adapter)
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  • \$\begingroup\$ The wiring described violates the USB-C specification. The USB-C specification describes any adapter with a USB-C receptacle as "potentially unsafe". See section 2.2 of the USB type-C cable and connector specification. \$\endgroup\$
    – MacGuffin
    Jul 26, 2022 at 18:42
  • \$\begingroup\$ I read through the spec. This answer involves no receptacle. In the original post, I show the device has a USB-C plug. Is it the red symbol in the schematic that's misleading? \$\endgroup\$
    – Jerther
    Jul 28, 2022 at 12:33
  • \$\begingroup\$ Your answer is unclear on where you are making these connections. In the USB type-C spec any USB-C plug is supposed to leave B6 and B7 open. It appears you are making a modification because the plug is not "flippable" in the USB-C receptacle to USB-A plug adapter. The modification appears to be made to accommodate an adapter that breaks the spec, and by making the modification the plug now breaks the spec. If we imagine this modification to be an internalized USB-C to USB-A adapter assembly then by connecting the data pins as described you violated the spec. \$\endgroup\$
    – MacGuffin
    Jul 28, 2022 at 15:12
  • \$\begingroup\$ Oh I understand your point of view now. The dodgy adapter misled me to make a dodgy wiring. I'm sorry it took so long. Well I can't recall if the first wiring I did was working on my phone. Would you say the wiring in the video I mentionned is correct and the device should work with the phone as it does now? \$\endgroup\$
    – Jerther
    Jul 28, 2022 at 20:15
  • \$\begingroup\$ The video shows the "correct" wiring in that it is demonstrated to work. One problem is that USB is a high speed protocol and so will be very sensitive to wire length and impedance, requiring tools and precision that was not shown in the video to prevent RF interference and/or unreliable behavior. I suspect a lot of destroyed hardware was edited out of the video. I'd think that modifying USB-A devices to USB-C is a high risk and low reward activity. It's too easy to cause expensive damage trying this, that is unless one has the proper tools. The proper tools will be expensive. \$\endgroup\$
    – MacGuffin
    Jul 28, 2022 at 23:56

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