The USB Mass Storage Class - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 50

Table 3-2: The configuration descriptor has information about the device's power
requirements.
Byte Field Description

0 bLength Descriptor size in bytes. Always 09h.

1 bDescriptorType The constant CONFIGURATION (02h).

2 wTotalLength The number of bytes in the configuration descriptor and all

of its subordinate descriptors.

4 bNumInterfaces The number of interfaces in the configuration.

5 bConfigurationValue Identifier for Set Configuration and Get Configuration

requests. Use 01h for the first configuration.

6 iConfiguration Index of string descriptor for the configuration. Set to 00h if

there is no string descriptor.

7 bmAttributes Self/bus power and remote wakeup settings.

8 bMaxPower The amount of bus power the device requires, expressed as

(maximum milliamperes / 2).

iConfiguration. Index to a string descriptor that contains a string describ-
ing the configuration. This value is zero if there is no configuration string
descriptor.

bmAttributes. Bit 6 = 1 if the device is self-powered or zero if bus-powered.
Bit 5 = 1 if the device supports the remote wakeup feature, which enables a
suspended USB device to tell its host that the device wants to communicate.
A USB device must enter the Suspend state if there has been no bus activity,
including Start-of_Frame markers, for 3 milliseconds. If a suspended device
requires action from the host, a device with remote wakeup enabled can
request the host to resume communications. Mass-storage devices typically
don't support remote wakeup. Bits 0..4 must equal 0. Bit 7 must equal 1.
bMaxPower. Specifies how much bus current a device requires. The bMax-
Power value equals one half the number of milliamperes requested. If a
device requires 200 milliamperes, bMaxPower = 100. The maximum cur-
rent a device can request is 500 milliamperes. If the requested current isn't
available, the host can refuse to configure the device. Some battery-powered
hosts and all bus-powered hubs supply only 100 milliamperes per port. To
enable a device to operate entirely from bus power when attached to these
hosts and hubs, bMaxPower must equal 32h or less.