Information Technology Reference
In-Depth Information
attached USB devices. If the attached USB device is a function, then attachment notifications
will be dispatched by USB software to interested host software.
Removal of USB devices
When a USB device has been removed from one of its ports, the hub automatically disables
the port and provides an indication of device removal to the host. Then the host removes
knowledge of the USB device. If the removed USB device is a hub, the removal process
must be performed for all of the USB devices that were previously attached to the hub. If the
removed USB device is a function, removal notifications are sent to interested host software.
USB host: hardware and software
The USB host interacts with USB devices through the host controller. The host is responsible
for the following:
•
Detecting the attachment and removal of USB devices
•
Managing control flow between the host and USB devices
•
Managing data flow between the host and USB devices
•
Collecting status and activity statistics
•
Providing a limited amount of power to attached USB devices
USB system software on the host manages interactions between USB devices and host-based
device software. There are five areas of interactions between USB system software and de-
vice software, they are:
•
Device enumeration and configuration.
•
Isochronous data transfers.
•
Asynchronous data transfers.
•
Power management.
•
Device and bus management information.
Whenever possible, USB software uses existing host system interfaces to manage the above
interactions. For example, if a host system uses Advanced Power Management (APM) for
power management, USB system software connects to the APM message broadcast facility
to intercept suspend and resume notifications.
9.2.5 USB host controller registers
VID (vendor identification register)
Address offset
00-01h
Default value
8086h
Attribute
Read only
The VID register contains the vendor identification number. This register, along with the
device identification register, uniquely identifies any PCI device. Writes to this register have
no effect. Bit description 15:0 vendor identification number. This is a 16-bit value assigned
to Intel.
