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USB_ENDPOINT_DESCRIPTOR structure

The USB_ENDPOINT_DESCRIPTOR structure is used by USB client drivers to retrieve a USB-defined endpoint descriptor.

Syntax


typedef struct _USB_ENDPOINT_DESCRIPTOR {
  UCHAR  bLength;
  UCHAR  bDescriptorType;
  UCHAR  bEndpointAddress;
  UCHAR  bmAttributes;
  USHORT wMaxPacketSize;
  UCHAR  bInterval;
} USB_ENDPOINT_DESCRIPTOR, *PUSB_ENDPOINT_DESCRIPTOR;

Members

bLength

Specifies the length, in bytes, of this descriptor.

bDescriptorType

Specifies the descriptor type. Must be set to USB_ENDPOINT_DESCRIPTOR_TYPE.

bEndpointAddress

Specifies the USB-defined endpoint address. The four low-order bits specify the endpoint number. The high-order bit specifies the direction of data flow on this endpoint: 1 for in, 0 for out.

bmAttributes

The two low-order bits specify the endpoint type, one of USB_ENDPOINT_TYPE_CONTROL, USB_ENDPOINT_TYPE_ISOCHRONOUS, USB_ENDPOINT_TYPE_BULK, or USB_ENDPOINT_TYPE_INTERRUPT.

wMaxPacketSize

Specifies the maximum packet size that can be sent from or to this endpoint.

bInterval

The bInterval value contains the polling interval for interrupt and isochronous endpoints. For other types of endpoint, this value should be ignored. This value reflects the device's configuration in firmware. Drivers cannot change it.

The polling interval, together with the speed of the device and the type of host controller, determine the frequency with which the driver should initiate an interrupt or an isochronous transfer. The value in bInterval does not represent a fixed amount of time. It is a relative value, and the actual polling frequency will also depend on whether the device and the USB host controller operate at low, full or high speed.

If either the host controller or the device operates at low speed, the period of time between interrupt transfers (also known as the polling "period") is measured in units of 1 millisecond frames, and the period is related to the value in bInterval as indicated the following table:

Value of bIntervalPolling Period (1-millisecond frames)Interrupt
0 to 158Supported.
16 to 3516Supported.
36 to 25532Supported.
> 255Polling intervals > 255 are forbidden by the USB specification.

 

For devices and host controllers that can operate at full speed, the period is measured in units of 1 millisecond frames, and the period is related to the value in bInterval as indicated the following table:

Value of bIntervalPolling Period (1-millisecond frames)Interrupt Isochronous
11Supported.Supported.
2 to 32Supported.Supported.
4 to 74Supported.Supported.
8 to 158Supported.Supported.
16 to 3116Supported.Not supported.
32 to 25532Supported.Not supported.
> 255Polling intervals > 255 are forbidden by the USB specification.

 

For devices and host controllers that can operate at high speed, the period is measured in units of microframes. There are eight microframes in each 1 millisecond frame. The period is related to the value in bInterval by the formula Period = 2 ** (bInterval - 1), as indicated the following table:

Value of bIntervalPolling Period (microframes)Interrupt Isochronous
11Supported.Supported.
22Supported.Supported.
34Supported.Supported.
48Supported.Supported.
516Supported.Not supported.
632Supported.Not supported.
7 to 25532Supported.Not supported.
> 255Polling intervals > 255 are forbidden by the USB specification.

 

The mappings in the preceding tables between periods and polling intervals are valid in Windows 2000 and later operating systems.

Requirements

Header

Usb100.h (include Usb100.h)

See also

UsbBuildGetDescriptorRequest
_URB_CONTROL_DESCRIPTOR_REQUEST
USB Structures

 

 

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