Tim Newsom escreveu:
> I know everyone says that they cant be a usb host. So I am going to
> ask a stupid question and maybe someone with more experience can fill
> me in on the technical reasons for the answers....
> Based on the pinouts is it just the lack of power for vbus that keeps
> a USB device from pretending to be a master? I know that USB 2.0
> allows some general use cases where a slave can be a master. I
> remember reading somewhere that the gumstix would never be a master
> device, but it would seem that all you need to do is provide a GPIO
> controlled +5v to the vbus and an appropriate driver and all would be
> well. Am I way off the mark there? Has someone already fixed this
> problem when I wasn't looking? I have a need to make the gumstix
> control some devices and I would like to use the USB bus to make it
A true usb host is way much more complicated device than a device. This
is proposital, so that the hard part is at the host and the devices
could be as simple as possible (in fact, there are usb devices which are
implemented on old microcontrollers without any provision for it, just
Realizing that, it has been proposed an intermeadiate solution, which is
called an OTG (http://www.usb.org/developers/onthego/), which is a set
of software standards to allow a minimal host to accept a limited range
of devices. An example is pict-bridge, which allows for instance a
printer to host a usb camera. It's by no means a complete usb host,
tough some devices are compatible with it by design.
On the pxa255 there's no host nor otg provision, on the pxa 27x I think
there are full host and full devices.
There are philips arm7 microcontrollers (lpc 211x if I'm not mistaken)
that presents dual-mode USB 2.0 ports (OTG or device).
Depending on your needs, you could solve it with OTG specs, otherwise,
specially if you need a host to support plugging generic usb devices of
all sorts to it, then you need a full host.
There is a compact-flash usb host thing that some people used to connect
to gumstix, I haven't tried it, but as far as I know, that's the only
functional thing providing full usb host capabilities to gumstix devices