USB Device Development----USB Descriptor Overview (LIBUSB library usage) _LIBUSB

Speaking of USB devices, we have to mention a variety of descriptors (descriptors), in general, descriptors are as follows:

1: Device Descriptor (Device descriptors)

2: Configuration descriptor (Configuration descriptors)

2: Interface Descriptor (Interface descriptors)

3: Endpoint descriptor (Endpoint descriptors)

A USB device has only one device descriptor, the device descriptor defines how many configurations the device has, each configuration descriptor corresponds to the configuration descriptor, and the configuration descriptor defines how many interfaces are in the configuration, each interface has a corresponding interface descriptor. The interface descriptor also defines how many endpoints the interface has, each endpoint corresponds to an endpoint descriptor, and the endpoint descriptor defines the endpoint size, type, and so on. From this we can see that the relationship between the USB descriptor is one layer, the last layer is the device descriptor, the following is the configuration descriptor, the following is the interface descriptor, and the following is the endpoint descriptor. When the descriptor is obtained, the device descriptor is obtained, and then the configuration descriptor is obtained, and the configuration descriptor, interface descriptor, and endpoint descriptor are read back one at a time, depending on the configuration set length in the configuration descriptor. It may also have access to device serial numbers, vendor strings, product strings, and so on.

The detailed relationship is shown in the following illustration:

Below we look at the detailed definition of these descriptors from the software perspective (the following definition is taken from the window system, the Linux naming is slightly different, the member content is consistent)

The device descriptor structure is as follows:

typedef struct _USB_DEVICE_DESCRIPTOR {
  uchar  blength;                  The descriptor structure Body size (18 bytes)
  uchar  bDescriptorType;          The descriptor type (fixed in the structure is 0x01)
  USHORT Bcdusb;                   USB version number
  Uchar  bdeviceclass;             Device class code (by USB official distribution)
  Uchar  bdevicesubclass;          The subclass code (officially allocated by USB)
  Uchar  bdeviceprotocol;          Device protocol code (distributed by USB official)
  Uchar  bMaxPacketSize0;          The maximum packet size of endpoint 0 (valid size is 8,16,32,64)
  USHORT Idvendor;                 Manufacturer number (by USB official distribution)
  USHORT idproduct;                Product number (manufacturer allocation)
  USHORT Bcddevice;                Equipment Factory number
  Uchar  imanufacturer;            Device manufacturer String Index
  uchar  iproduct;                 Product Description String index
  uchar  iserialnumber;            Device serial number string index
  uchar  bnumconfigurations;       The number of configurations that can be supported at current speeds
} usb_device_descriptor, *pusb_device_descriptor;

The configuration descriptor structure is as follows:

typedef struct _USB_CONFIGURATION_DESCRIPTOR {
  uchar  blength;                  The descriptor structure body size
  uchar  bdescriptortype;          The descriptor type (fixed in the structure is 0x02)
  USHORT wtotallength;             All data sizes returned by this configuration
  uchar  bnuminterfaces;           The number of interfaces for this configuration
  uchar  bconfigurationvalue;      The parameter value required by the set_configuration command
  uchar  iconfiguration;           The index value of the string describing the configuration
  uchar  bmattributes;             The choice of power supply mode
  Uchar  maxpower;                 Equipment from the bus to extract the maximum current
} usb_configuration_descriptor, *pusb_configuration_descriptor;

The interface descriptor structure is as follows:

typedef struct _USB_INTERFACE_DESCRIPTOR {
  uchar blength;                   The descriptor structure size
  Uchar bDescriptorType;           The type number of the interface descriptor (0x04)
  Uchar Binterfacenumber;          The number of the interface is
  Uchar balternatesetting;         The alternate interface descriptor number
  Uchar bnumendpoints;             The number of end points used by the interface, excluding endpoints 0
  Uchar Binterfaceclass;           Interface type
  Uchar Binterfacesubclass;        Connection type
  Uchar Binterfaceprotocol;        Interfaces follow the protocol
  Uchar IInterface;                Describes the string index value of the interface
} usb_interface_descriptor, *pusb_interface_descriptor;

The endpoint descriptor structure is as follows:

typedef struct _USB_ENDPOINT_DESCRIPTOR {
  uchar  blength;                 Endpoint descriptor byte number size (7 bytes)
  uchar  bDescriptorType;         Endpoint Descriptor type number (0x05)
  uchar  bendpointaddress;        Endpoint address and input and output attributes
  Uchar  bmattributes;            The transport type attribute of the endpoint
  USHORT wmaxpacketsize;          The maximum packet size of end registration and hair
  uchar  binterval;               Host query endpoint time interval
} usb_endpoint_descriptor, *pusb_endpoint_descriptor;

In addition to the above four descriptors, there is a string descriptor that, for a USB device, is not necessarily implemented (when not implemented, the descriptor should be set to 0 to indicate that the feature is not implemented).

typedef struct _USB_STRING_DESCRIPTOR {
Uchar Blength; Number of String Descriptor bytes
Uchar bDescriptorType; String descriptor type Number (0X03)
               WCHAR Bstring[1]; Unicode string
} usb_string_descriptor, *pusb_string_descriptor;

Reprint: http://blog.csdn.net/saloon_yuan/article/details/7837492

Resources:

1, Libusb open Source Library related

2. USB Device Development----Non-flooding design based on LIBUSB