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Spinpoint M8U-Internal Product Manual REV 3.4
75
INSTALLATION
6.3.6.4.2 Device Descriptor
A device descriptor describes general information about a USB device. It includes information that applies
globally to the device and all of the device’s configurations. A USB device has only one device descriptor. A
high-speed capable device that has different device information for full-speed and high-speed must also have
a device_qualifier descriptor.
The DEVICE descriptor of a high-speed capable device has a version number of 2.0 (0200H). If the device is
full-speed only or low-speed only, this version number indicates that it will respond correctly to a request for
the device_qualifier desciptor (i.e., it will respond with a request error).
The bcdUSB field contains a BCD version number. The value of the bcdUSB field is 0xJJMN for version
JJ.M.N (JJ – major version number, M – minor version number, N – sub-minor version number), e.g., version
2.1.3 is represented with value 0x0213 and version 2.0 is represented with a value of 0x0200.
The bNumConfigurations field indicates the number of configurations at the current operating speed.
Configurations for the other operating speed are not included in the count. If there are specific configurations
of the device for specific speeds, the bNumConfigurations field only reflects the number of configurations for
a single speed, not the total number of configurations for both speeds.
If the device is operating at high-speed, the bMaxPacketSize0 field must be 64 indicating a 64 byte maximum
packet. High-speed operation does not allow other maximum packet sizes for the control endpoint (endpoint
0).
All USB devices have a Default Control Pipe. The maximum packet size of a device’s Default Control Pipe
is described in the device descriptor. Endpoints specific to a configuration and its interface(s) are described in
the configuration descriptor. A configuration and its interface(s) do not include an endpoint descriptor for the
Default Control Pipe. Other than the maximum packet size, the characteristics of the Default Control Pipe are
defined by this specification and are the same for all USB devices.
The bNumConfigurations field identifies the number of configurations the device supports. Table 6-11 shows
the standard device descriptor.
Table 6-11: Standard Device Descriptor