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Chapter 1
1-3-2 POWER DISTRIBUTION
The host can supply power to USB devices that are directly connected to the host. USB
devices may also have their own power supplies. USB devices that use power from the
cable are called bus-powered devices. Bus-powered device can draw a maximum of 500
mA from the host. USB devices that have alternative source of power are called self-
powered devices.
1-4 DEVICE STRUCTURE AND ENUMERATION
Before the host application can communicate with a device, the host needs to understand
the capabilities of the device. This process takes place during device enumeration. After
enumeration, the host can assign and load a specific driver to allow communication
between the application and the device.
During enumeration, the host assigns an address to the device, reads descriptors from the
device, and selects a configuration that specifies power and interface requirements. In order
for the host learns about the device’s capabilities, the device must provide information
about itself in the form of descriptors.
This section describes the device logical organization from the USB host’s point of view.
1-4-1 USB DEVICE STRUCTURE
From the host point of view, USB devices are internally organized as a collection of
configurations, interfaces and endpoints.
CONFIGURATION
A USB configuration specifies the capabilities of a device. A configuration consists of a
collection of USB interfaces that implement one or more USB functions. Typically only one
configuration is required for a given device. However, the USB specification allows up to
255 different configurations. During enumeration, the host selects a configuration. Only one
configuration can be active at a time. The device uses a configuration descriptor to inform
the host about a specific configuration’s capabilities.
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