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« Return to Thread: USB 2.0 device through a USB 1.1 hub
Re: USB 2.0 device through a USB 1.1 hub
by Alan Stern
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On Wed, 22 Oct 2008, Tim Roberts wrote:
> > Furthermore, you can't rely on the "other speed config" descriptor to
> > tell you what speed a USB 2.0 device is running at. If it's running at
> > full speed then the "other speed config" descriptor will be for a
> > high-speed configuration, and vice versa, if it's running at high speed
> > then the "other speed config" descriptor will be for a full-speed
> > configuration.
> >
>
> But that's clearly sufficient. As long as you can tell the difference
> between them, you just ask for the current configuration descriptor. If
> the one you get is your high-speed descriptor, then you are running
> high-speed. If the one you get is your full-speed descriptor, then you
> are running full-speed.
If you can tell the difference then you don't need to use the "other
speed config" descriptor at all.
The real question is how can you tell the difference? It depends on
the device, obviously, but in general full-speed and high-speed configs
tend to be pretty similar. The most notable distinction, as I pointed
out earlier, is the size of the Bulk endpoint maxpacket values.
However, if somebody is concerned with only one specific device, things
become simpler. Compare the two descriptors for the two difference
speeds, looking for any distinguishing characteristic. Then use that
characteristic to tell which speed the current config descriptor is
for.
Alan Stern
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> > Furthermore, you can't rely on the "other speed config" descriptor to
> > tell you what speed a USB 2.0 device is running at. If it's running at
> > full speed then the "other speed config" descriptor will be for a
> > high-speed configuration, and vice versa, if it's running at high speed
> > then the "other speed config" descriptor will be for a full-speed
> > configuration.
> >
>
> But that's clearly sufficient. As long as you can tell the difference
> between them, you just ask for the current configuration descriptor. If
> the one you get is your high-speed descriptor, then you are running
> high-speed. If the one you get is your full-speed descriptor, then you
> are running full-speed.
If you can tell the difference then you don't need to use the "other
speed config" descriptor at all.
The real question is how can you tell the difference? It depends on
the device, obviously, but in general full-speed and high-speed configs
tend to be pretty similar. The most notable distinction, as I pointed
out earlier, is the size of the Bulk endpoint maxpacket values.
However, if somebody is concerned with only one specific device, things
become simpler. Compare the two descriptors for the two difference
speeds, looking for any distinguishing characteristic. Then use that
characteristic to tell which speed the current config descriptor is
for.
Alan Stern
-------------------------------------------------------------------------
This SF.Net email is sponsored by the Moblin Your Move Developer's challenge
Build the coolest Linux based applications with Moblin SDK & win great prizes
Grand prize is a trip for two to an Open Source event anywhere in the world
http://moblin-contest.org/redirect.php?banner_id=100&url=/
_______________________________________________
Libusb-devel mailing list
Libusb-devel@...
https://lists.sourceforge.net/lists/listinfo/libusb-devel
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