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Configuration Descriptor

A configuration descriptor describes information about a specific device configuration (see Table 4). The descriptor describes

the number of interfaces provided by the configuration.

Table 4. Configuration Descriptors

Offset

Field

Size

Value

Default

Description

0

bLength

1

Number

09

Size of this descriptor in bytes.

1

bDescriptorType

1

Constant

02

CONFIGURATION.

2

wTotalLength

2

Number

002E

Total length of data returned for this configuration. Includes the

combined length of all descriptors (configuration, interface,

endpoint, and class or vendor specific) returned for this

configuration.

4

bNumInterfaces

1

Number

01

Number of interfaces supported by this configuration.

5

bConfigurationValue

1

Number

01

Value to use as an argument to Set Configuration to select this

configuration.

6

iConfiguration

1

Index

00

Index of string descriptor describing this configuration.

7

bmAttributes

1

Bitmap

A0

Configuration characteristics

D7

Bus Powered

D6

Self Powered

D5

Remote Wakeup

D4..0 Reserved (reset to 0)

A device configuration that uses power from the bus and a local

source sets both D7 and D6. The actual power source at runtime

may be determined using the Get Status device request.

If a device configuration supports remote wakeup, D5 is set to 1.

8

MaxPower

1

mA

C8

Maximum power consumption of USB device from the bus in this

specific configuration when the device is fully operational.

Expressed in 2 mA units (i.e., 50 = 100 mA).

Note: A device configuration reports whether the configuration is

bus-powered or self-powered. Device status reports whether the

device is currently self-powered. If a device is disconnected from its

external power source, it updates device status to indicate that it is

no longer self-powered.

A device may not increase its power draw from the bus, when it

loses its external power source, beyond the amount reported by its

configuration.

If a device can continue to operate when disconnected from its

external power source, it continues to do so. If the device cannot

continue to operate, it fails operations it can no longer support. Host

software may determine the cause of the failure by checking the

status and noting the loss of the device’s power source.