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If a device is designed with a USB-C port that's intended for use exclusively in an alt mode such as Displayport or PCIe rather than for use as a general-purpose USB port, then the hardware to support the USB 3 protocol for that port would be a waste of money.

Does the spec require USB 3 support even if that support will be unused? Or can USB 3 be omitted, and just (relatively cheap) USB 2 provided for the billboard device class and for negotiating the alt mode configuration for the four twisted pairs that would normally be used for USB 3?

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  • \$\begingroup\$ Using a USB port for non USB compatible Device seems like the worst thing to do in terms of end user not plugging the wrong thing in, assuming consumer non technical end users \$\endgroup\$
    – Passerby
    Nov 5, 2014 at 19:01
  • \$\begingroup\$ @Passerby - Indeed, that was the motivation for the Billboard Device Class. The idea is that even if the device doesn't generally use USB for anything, it at least has enough USB support so that when plugged into a host that's expecting a USB device, the device appears as a USB device and can announce what it actually is. As I said below in a comment to PkP, it would seem just as good to put a sticker on the device saying "this is not a USB device; please read the manual", but at least the engineers recognized the problem that you pointed out, and they made a solution that's probably adequate. \$\endgroup\$
    – Porthem
    Nov 5, 2014 at 21:33
  • \$\begingroup\$ The Alt Mode spec at least ensures that nothing will be damaged; the worst case is that nothing would happen. The Billboard class avoids even that harmless worst case, by ensuring that something happens. For example, if USB-C replaces the Displayport connector so your future external monitor has only a USB-C connector on it, and you plug it into a USB port on your laptop that doesn't support Displayport Alt Mode, then your laptop will display a message, "that device requires a Displayport Alt Mode host port". But plug it into a headless server, and there is no other monitor for the message. ;) \$\endgroup\$
    – Porthem
    Nov 5, 2014 at 21:53
  • \$\begingroup\$ Hmmm, I'm thinking that if the CC pin is used to communicate the alternative mode information, shouldn't it suffice? Surely all USB-C hosts support the CC pin? Or are they pushing the USB-C connector for USB 1.1 and USB2.0 hosts also? How do you feel about Billboard spec 1.0 clause 2.2.1 "If Port Pair successfully enter into Modal Operation (as defined by [USBPD])then the Device Container is not required to expose the Billboard Device." How about that DP cable example (extremetech.com/wp-content/uploads/2014/09/…) that doesn't have USB at all? \$\endgroup\$
    – PkP
    Nov 6, 2014 at 16:19
  • \$\begingroup\$ @PkP - USB-C is intended to replace all previous USB connectors for all versions of USB for all host and device types. USB-C cables have one signaling pair dedicated to USB 2,and USB-C connectors have 4 contact pads (due to connector reversibility) for that signaling pair. If the devices do not successfully enter into an alt mode, then billboard device exposure is required, within one second IIRC; the purpose of the billboard device is to ensure that something is there in case the alt mode is unsuccessful. As I wrote before, your DP cable example shows that billboard class does seem silly. \$\endgroup\$
    – Porthem
    Nov 7, 2014 at 1:12

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From what I can read from the USB Type C Specification release 1.0 and the USB Power Delivery Specification, I'm feeling that I might go as far as to say that if you don't need the USB functionality in your device, then you might not need to provide any USB device circuitry at all.

The USB-C spec chapter 5.1.1 says that the alternate mode is selected by USB Power Delivery ("PD") signaling. And taking a look at the USB Power Delivery specification chapter 5.8.3 says that in USB-C the PD messages are sent with baseband BMC signaling on the CC line, not in the USB data lines. Furthermore, 5.8.1 says that the bitrate is 300kbps. This would hint to the direction that more simple circuitry than a USB device or host port would be necessary to carry the PD messages needed to select the alternative mode.

If you need USB, consider this snippet from tomshardware:

USB Type-C is compatible with previous iterations of USB, at least in that it supports all prior protocols from USB 2.0 onward, including the drive stack, capabilities and infrastructure.

Based on this, for any USB functionality, I would say that it's perfectly ok even to use ancient 12 mbps USB circuitry, if it's fast enough for your design. 12 Mbps 'Full speed' and even 1.5 Mbps 'slow speed' are covered by the USB 2.0 specification.


[Edit] Please see the comments below for discussion about the "USB Billboard Device" interface which may be required, seemingly for the purpose of showing "This device doesn't support USB" types of messages via USB. If such an interface is required, then even low-speed (1.5 Mbps) should be enough. Further clarification from more experts is welcome.

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  • \$\begingroup\$ Right, I was mistaken about the lines used for alt mode configuration; USB is needed only for providing the billboard class. But the USB-C spec section 5.1 (right above 5.1.1 that you referenced) says "All hosts and devices (except chargers) using a USB Type-C receptacle shall expose a USB interface. In the case where the host or device optionally supports Alternate Modes ... Where no equivalent USB functionality is implemented, the device shall provide a USB interface exposing a USB Billboard Device Class used to provide information to identify the device". It appears USB is mandatory. \$\endgroup\$
    – Porthem
    Nov 3, 2014 at 8:22
  • \$\begingroup\$ It does appear that slow and cheap USB 2 would be fine for the billboard, even 1.5Mbps as you said. The spec allows USB 2 devices on USB-C that don't use the four USB 3 signaling pairs, but I can't find in the spec where it says one way or the other about whether a device (or host) that does use those four signaling pairs for an alt mode is allowed to omit USB 3. Section 5.1.2.2 talks about a mandatory "safe state" for transitioning from USB to an alt mode, which seems to imply that there must be USB support on those four signaling pairs (which must be USB 3) to be transitioned from. \$\endgroup\$
    – Porthem
    Nov 3, 2014 at 8:41
  • \$\begingroup\$ Hmm... consider one of the example pictures such as this one: extremetech.com/wp-content/uploads/2014/09/… which shows a USB-C DisplayPort cable with no USB, just the small box titled "ID" that goes to the CC line. I think it sort of implicates that you don't need the USB. Any ideas? PS. I just realized that I have no clue as to what is "billboard"... :| \$\endgroup\$
    – PkP
    Nov 3, 2014 at 9:34
  • \$\begingroup\$ Billboard Device Class is a dummy USB device class just to make a device that normally operates only in an alt mode show up and announce what it is when plugged into a regular USB host that doesn't support the alt mode. The purpose is to prevent user-unfriendly silent errors, where you plug a device into your computer and nothing happens, with no explanation of why it isn't working. It's analogous to "This site requires Javascript" written in plain HTML on a Javascript-only website so you don't get an entirely blank page when you visit the site with Javascript turned off. \$\endgroup\$
    – Porthem
    Nov 3, 2014 at 10:10
  • \$\begingroup\$ Yes, the billboard class does seem kind of silly, as your converter cable example shows. Instead of including otherwise-unnecessary USB hardware, it would seem cheaper and just as useful to just put a sticker on the device saying "this is not a USB device; please read the manual". But the billboard class is nonetheless in the spec, and apparently mandatory if no other USB device class is supported. \$\endgroup\$
    – Porthem
    Nov 3, 2014 at 10:25

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