Information Technology Reference
In-Depth Information
supports a file system can also create, read, write to, and delete files on its
own.
OTG Devices and Conventional Hosts
A USB host in an embedded system can be a conventional USB host or an
On-The-Go device. A system that never functions as a USB device must
function as a conventional host. If the system must function as a USB host
and device at the same time, the system must contain separate SIEs for the
USB host and device functions. The SIEs can be on a single chip or different
chips. If the system functions as both a USB host and device but not both at
the same time, the system can be an OTG device.
Conventional hosts and OTG hosts have different requirements in some
areas. Support for external hubs is required in a conventional host and
optional in an OTG device. A conventional host must provide 500 mA per
port (or 100 mA if battery powered), while an OTG device needs to provide
just 8 mA per port unless a supported peripheral requires more. A conven-
tional host must provide bus power at all times, while an OTG device can
switch off bus power when unneeded.
General Host Functions
The host enumerates each device to learn about its capabilities. To enumer-
ate a device, a host typically issues the following standard USB requests:
Set Address. To set the device's address on the bus.
Get Descriptor (device). To read the device descriptor.
Get Descriptor (configuration). To read the configuration descriptor and
subordinate descriptors, including the interface and endpoint descrip-
tors.
Set Configuration. To configure the device and enable communications.
The host can also request any string descriptors the device supports, includ-
ing the descriptor containing the serial number.
Figure 11-1 shows bus events and host requests directed to a newly attached
USB flash drive on a Windows XP host. The host requests some descriptors
multiple times and resets the bus after the first Get Descriptor request. This
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