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Technical Support
On-Line Manuals
RL-ARM User's Guide (MDK v4)
RL-RTX
RL-FlashFS
RL-TCPnet
RL-CAN
RL-USB
RL-USB for USB Device Applications
RL-USB Device Library
RL-USB Device Features
RL-USB Device Software Stack
RL-USB Device Functions
RL-USB Device Source Files
RL-USB Device Configuration
Audio Device (ADC) Options
Communication Device (CDC) - ACM Options
Human Interface Device (HID) Options
Mass Storage Device (MSC) Options
Create USB Device Applications
Create ADC Applications
Create CDC ACM Applications
Create HID Applications
Create MSC Applications
Create Composite Applications
Test USB Device Applications
Compliance Tests
Test HID Client Application
RL-USB for USB Host Applications
RL-USB Host Library
RL-USB Host Features
RL-USB Host Software Stack
RL-USB Host Functions
RL-USB Host Source Files
RL-USB Host Configuration
Host Controller Driver Selection
Host Controller Driver Configuration
Host Class Driver Selection
Create USB Host Applications
Create USB Host HID Applications
HID_Kbd Example
Create USB Host MSC Applications
Create USB Host Class Driver
RL-USB Host Constants and Structures
Error constants
Speed constants
Transaction Packet Type constants
Transaction Error Type constants
USB Request Block structure (USBH_URB)
Endpoint structure (USBH_EP)
Driver Capabilites structure (USBH_HCI_CAP)
Device Class Instance structure (USBH_DCI)
Host Controller Instance structure (USBH_HCI)
Host Controller Driver structure (USBH_HCD)
USB Concepts
USB Transfer Rates
USB Network
Basic Communication Model
USB Protocol
Control Transfer
Interrupt Transfer
Isochronous Transfer
Bulk Transfer
Descriptors
Device Configuration
Device Descriptor
Configuration Descriptor
Interface Descriptor
Endpoint Descriptor
Device Qualifier Descriptor
Example Programs
Library Reference
Appendix
Interface Descriptor
The Interface Descriptor describes a collection of endpoints. This interface supports a group of pipes that are suitable for a particular task. Each configuration can have multiple interfaces. The interface can be selected dynamically by the USB host. The Interface Descriptor can associate its collection of pipes with a device class, which in turn has an associated class device driver within the host operating system. Typically, the device class is a functional type such as a printer class or mass storage class.
An interface descriptor never includes Endpoint 0 in the number of endpoints. If an interface uses only Endpoint 0, then the field bNumEndpoints must be set to zero.
If no class type has been selected for the device, then none of the standard USB drivers is loaded, and the developer has to provide its own device driver.
| Offset | Field | Size | Value | Description |
|---|---|---|---|---|
| 0 | bLength | 1 | Number | Size of this descriptor in bytes. |
| 1 | bDescriptorType | 1 | Constant | Device Descriptor Type. |
| 2 | bInterfaceNumber | 1 | Number | The number of this interface. Zero-based value, identifying the index in the array of concurrent interfaces supported by this configuration. |
| 3 | bAlternateSetting | 1 | Number | Value used to select this alternate setting for the interface identified in the prior field. Allows an interface to change its settings on the fly. |
| 4 | bNumEndpoints | 1 | Number | Number of endpoints used by this interface (excluding endpoint zero). If this value is zero, this interface only uses the Default Control Pipe. |
| 5 | bInterfaceClass | 1 | Class | Class code (assigned by the USB-IF). A value of zero is
reserved for future standardization. If this field is set to FFH, the interface class is vendor-specific. All other values are reserved for assignment by the USB-IF. |
| 6 | bInterfaceSubClass | 1 | SubClass | Subclass code (assigned by the USB-IF). If the bInterfaceClass field is reset to zero, this field must also be reset to zero. If the bInterfaceClass field is not set to FFH, all values are reserved for assignment by the USB-IF. |
| 7 | bInterfaceProtocol | 1 | Protocol | Protocol code (assigned by the USB). If an interface supports class-specific requests, this code identifies the protocols that the device uses as defined in the device class. If this field is reset to zero, the device does not use a class-specific protocol on this interface. If this field is set to FFH, the device uses a vendor-specific protocol for this interface. |
| 8 | iInterface | 1 | Index | Index of string descriptor describing this interface. |
For example, two device with different interfaces are needed. The
first interface, Interface Zero, has the field
bInterfaceNumber set to zero.
The next interface, Interface One, has the field
bInterfaceNumber set to one and the field
bAlternativeSetting also set to zero (default). It is possible
to define an alternative setting for this device, by leaving the
field bInterfaceNumber set to one, but with the field
bAlternativeSetting is set to one instead of zero.
The first two interface descriptors with bAlternativeSettings equal to zero are used. However, the host can send a SetInterface() request to enable the alternative setting.
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