USB 2.0 may not be "High Speed"; USB 3.0 may not be "SuperSpeed"

Hi Max, the band is ELP (have you ever listened to any of Emerson's earlier band, "The Nice"? Incredible stuff there too...)

I'm mainly writing to vent about one of my pet peeves when discussing USB. You did the same thing that just about everybody else does, but it is inaccurate:

      High Speed USB = USB 2.0
             but...
      USB 2.0 <> High Speed

The point is that a USB device can be USB 2.0 compliant but not support High Speed. This is a very important distinction. When I was working on an embedded USB driver for an ARM-based embedded system, I discovered this point in the USB specifications. It is important because I have found lots of USB devices marketed simply as USB 2.0 that do not support USB High Speed, especially a lot of the really cheap USB products (which I can't resist sometimes). The product specifically has to say "High Speed" and use this logo:

Note the "Hi-SPEED" at the top. In fact, there was a time when a single Low Speed or Full Speed device plugged into a USB hub would cause the link between the host and the hub to have to drop down to the slower speed and stay there for all transactions. I believe USB hubs from the past few years have implemented a more complex speed switching algorithm so that you can access a High Speed USB flash drive in a hub at 480 Mbps while the mouse plugged into the same hub can talk at Low Speed without affecting the flash drive data transfer speed.

I have not looked at the USB 3.0 spec, but I expect it will also support all speeds. There will be devices that can say they are USB 3.0 but not support SuperSpeed. For example, there is no reason a USB 3.0 compliant mouse has to send its data at 4.8 Gbps, nor should it be expected to. Thus, when I talk about USB, I try to mention the speed names, (Low Speed, Full Speed, High Speed, and now SuperSpeed) and not the version numbers.

I know, it's kind of picky, but we are engineers after all. If we don't split hairs, who will? ;)

Who indeed? :-) Actually, this point is very well taken; I learned something useful here and I will be sure to speak more precisely in the future...

Questions? Comments? Feel free to email me – Clive "Max" Maxfield – at max@techbites.com). And, of course, if you haven't already done so, don't forget to Sign Up for our weekly Programmable Logic DesignLine Newsletter.

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