Information Technology Reference
In-Depth Information
requests received from the host, and passes data between devices and the
host controller.
A USB host can be a desktop or notebook computer, a handheld, or any
embedded system that contains host-controller hardware and software. To
communicate with mass-storage devices, the host must have a driver that
supports the protocols defined for USB's mass-storage class.
A USB device contains USB device-controller hardware and a microcontrol-
ler, CPU, or other intelligent hardware. As Chapter 1 explained, some
devices contain a microcontroller with an on-chip USB device controller,
while other devices use a microcontroller or CPU that interfaces to a USB
controller on a separate chip. The hardware that implements the low-level
USB protocols in the device controller is called the serial interface engine
(SIE). Program code in a USB device is typically firmware stored in non-vol-
atile memory. Some devices manage USB communications entirely in hard-
ware and require no programming for the USB communications.
A USB device can connect to a host's root hub or to an external hub. The
device can have a standard USB series-B or mini-B receptacle, a vendor-spe-
cific connector, or a permanently attached USB cable. The upstream
(toward the host) end of the device's cable has a series-A plug that attaches to
a host or hub or a mini-A plug that attaches to an On-The-Go device. Fig-
ure 2-1 shows the different plug types.
An On-The-Go (OTG) device is a special kind of USB device that can func-
tion as a limited-capability host or as a device. An On-The-Go device has a
mini-AB receptacle that can accept a mini-A plug or a mini-B plug. An
example of a USB On-The-Go device is a camera that can function as a
mass-storage device that stores images that PCs can access via USB and as a
host that sends images to a USB printer.
Host Responsibilities
A USB host manages power and communications on the bus. The USB host
has these responsibilities:
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