USB 2.0 over USB 3.0 - Page 1
Forum section: electronics / projects

author: cblackburn
author_rank: Newbie
timestamp: October 27, 2021, 02:12:46 pm
content: Hi All,I have a bit of a strange requirement and, unfortunately, whenever a requirement is strange it tends not to have ready made silicon you can buy off the shelf!I have a situation where I have a single USB 3.0 (assume gen 1 for simplicity) uplink. Note that I donothave a USB 2.0 uplink on that port. On the downstream I need to plug in several USB 3.0 and USB 2.0 devices. Simple me assumed that this would be just fine as the USB 2.0 packets would just be tunnelled over the nice new shiny USB 3.0 interface right? Right?! Sadly not, it seems that all manufacturers (and standards) have assumed that the USB 2.0 stack runs completely in parallel to the USB 3.0 stack and basically doesn't interact in any way.So, can anyone think of a way of "tunnelling" the USB 2.0 packets over the USB 3.0 interface"? Preferably in a way which will just work with an upstream computer without having to be split apart before entering the PC? Ideally I'dlovesome sort of ready to go silicon IC or even a ready made hub that I can just buy which would do it. Alas, I've not found anything.Hopefully you guys will have some bright ideas which can help meThanksChris

author: langwadt
author_rank: Super Contributor
timestamp: October 27, 2021, 02:30:18 pm
content: doesn't seem like it would be USB by any standard so why would it exist?

author: oPossum
author_rank: Super Contributor
timestamp: October 27, 2021, 02:36:41 pm
content: There are good reasons that capability is not part of the USB 3 specifications.Despite that this part exists...https://www.via-labs.com/product_show.php?id=96I strongly recommend not using this part or anything else from VIA/VLA.

author: voltsandjolts
author_rank: Unknown
timestamp: October 27, 2021, 02:42:56 pm
content: Not sure what you mean exactly but is a USB2 hub what your after?

author: cblackburn
author_rank: Newbie
timestamp: October 27, 2021, 03:31:22 pm
content: Indeed, this solution is very non standard. The upstream port is being forwarded over a long distance by modulating the data onto fiber optics and then demodulating at the far end. Unfortunately this part of the solution does not support USB 2.0 and only modulates/demodulates the USB 3.0 port.Obviously, one option would be to change the fiber extender part of the solution but us engineers don't always have influence over the whole stack!

author: oPossum
author_rank: Super Contributor
timestamp: October 27, 2021, 03:38:01 pm
content: If you want an OTS solution...http://www.everprotech.com/view/107.htmlIt is made for use with fiberoptic USB 3 cables. Uses the VL670/671.

author: voltsandjolts
author_rank: Unknown
timestamp: October 27, 2021, 04:00:08 pm
content: Sorry, I'm still a bit confused.What's wrong with a hub, like one of the SS hubs that has USB3 and USB2 ports on it?Or does this have to be a silicon solution to embed into your proprietary device?

author: magic
author_rank: Super Contributor
timestamp: October 27, 2021, 04:51:09 pm
content: USB2.0 hubs don't perform any buffering and "speed translation". When the host talks to a full speed device, it sends full speed data which are passed by the hub in real time to the appropriate port.If USB3.0 works the same way and I dunno but it probably does, you won't be able to communicate with USB2.0 devices through a hub if its upstream link only supports SuperSpeed.

author: cblackburn
author_rank: Newbie
timestamp: October 27, 2021, 06:05:05 pm
content: The best way I can think to explain the problem is using the graphic from this website:-https://www.mouser.co.uk/new/microchip/microchip-usb5532b-controller/. I have also attached it to this post for prosperity (I know these websites have a habit of changing over time).As you can see in the block diagram, despite being a "USB 3.0 Hub with backwards compatibility" what itactuallyis is a USB 3.0 hub and a USB 2.0 hub mashed onto the same silicon die. All of the USB 3.0 data flows through the USB 3.0 hub controller and all of the USB 2.0 data flows through a completely separate USB 2.0 hub controller. To be fair to Microchip here, this is the recommended design pattern from the USB Implementers Forum. They are doing nothing wrong or unusual by using this design.One of the side effects of this design pattern is that data has to flow separately from USB 2.0 devices and USB 3.0 devices. The hub assumes that an upstream port will be provided for both USB 2.0 and USB 3.0. In my case, unfortunately, this assumption is not valid as USB fiber modulators (almost always) use only the USB 3.0 channel to move data and do not provide the USB 2.0 channel. This means that the USB extender systems are not standards compliant and hence have all sorts of issues.I'm looking for a solution which can encapsulate the USB 2.0 packets over the USB 3.0 channel so that I can work around my issue. The solution from oPossum looks particularly good, I've requested a quite form the manufacturer.

author: voltsandjolts
author_rank: Unknown
timestamp: October 27, 2021, 07:50:51 pm
content: Ah, got it.Thanks for the explanation, I wasn't aware of that 'gotcha'.This fibre extenderclaims to be USB3-2-1 compatible so it might fix your problem, but little technical detail is given so, hmm. And maybe you want to keep and make use of the extender system you already have. Best of luck!

author: voltsandjolts
author_rank: Unknown
timestamp: October 28, 2021, 10:49:48 am
content: I see thatLindy have a "USB2 adapter"for their USB3 fibre extenders, but it's out of stock.Couldn't find any info on what chipset it uses.Edit: Ah, just noticed this looks suspiciously like the device oPossum linked in reply #5 yesterday. Call me slowEdit2:Open hardware for VL670/671which has links to device datasheets and notes on bugs.