Friday, August 20, 2004
Device Descriptors
The device descriptor of a USB device represents the entire device. As a result a USB device can only have one device descriptor. It specifies some basic, yet important information about the device such as the supported USB version, maximum packet size, vendor and product IDs and the number of possible configurations the device can have. The format of the device descriptor is shown below.
->The bcdUSB field reports the highest version of USB the device supports. The value is in binary coded decimal with a format of 0xJJMN where JJ is the major version number, M is the minor version number and N is the sub minor version number. e.g. USB 2.0 is reported as 0x0200, USB 1.1 as 0x0110 and USB 1.0 as 0x0100.
->The bDeviceClass, bDeviceSubClass and bDeviceProtocol are used by the operating system to find a class driver for your device. Typically only the bDeviceClass is set at the device level. Most class specifications choose to identify itself at the interface level and as a result set the bDeviceClass as 0x00. This allows for the one device to support multiple classes.
->The bMaxPacketSize field reports the maximum packet size for endpoint zero. All devices must support endpoint zero.
->The idVendor and idProduct are used by the operating system to find a driver for your device. The Vendor ID is assigned by the USB-IF.
->The bcdDevice has the same format than the bcdUSB and is used to provide a device version number. This value is assigned by the developer.
->Three string descriptors exist to provide details of the manufacturer, product and serial number. There is no requirement to have string descriptors. If no string descriptor is present, a index of zero should be used.
->bNumConfigurations defines the number of configurations the device supports at its current speed.
->The bcdUSB field reports the highest version of USB the device supports. The value is in binary coded decimal with a format of 0xJJMN where JJ is the major version number, M is the minor version number and N is the sub minor version number. e.g. USB 2.0 is reported as 0x0200, USB 1.1 as 0x0110 and USB 1.0 as 0x0100.
->The bDeviceClass, bDeviceSubClass and bDeviceProtocol are used by the operating system to find a class driver for your device. Typically only the bDeviceClass is set at the device level. Most class specifications choose to identify itself at the interface level and as a result set the bDeviceClass as 0x00. This allows for the one device to support multiple classes.
->The bMaxPacketSize field reports the maximum packet size for endpoint zero. All devices must support endpoint zero.
->The idVendor and idProduct are used by the operating system to find a driver for your device. The Vendor ID is assigned by the USB-IF.
->The bcdDevice has the same format than the bcdUSB and is used to provide a device version number. This value is assigned by the developer.
->Three string descriptors exist to provide details of the manufacturer, product and serial number. There is no requirement to have string descriptors. If no string descriptor is present, a index of zero should be used.
->bNumConfigurations defines the number of configurations the device supports at its current speed.
