Spinpoint M8U-Internal Product Manual REV 3.4
65
INSTALLATION
■
bmRequestType
This bitmapped field identifies the characteristics of the specific request. In particular, this field identifies the
direction of data transfer in the second phase of the control transfer. The state of the
Direction
bit is ignored
if the
wLength
field is zero, signifying there is no Data stage.
The USB Specification defines a series of standard requests that all devices must support. These are
enumerated in Table 6-6. In addition, a device class may define additional requests. A device vendor may
also define requests supported by the device.
Requests may be directed to the device, an interface on the device, or a specific endpoint on a device. This
field also specifies the intended recipient of the request. When an interface or endpoint is specified, the
wIndex
field identifies the interface or endpoint.
■
bRequest
This field specifies the particular request. The
Type
bits in the
bmRequestType
field modify the meaning of
this field. This specification defines values for the
bRequest
field only when the bits are reset to zero,
indicating a standard request (refer to Table 6-6).
■
wValue
The contents of this field vary according to the request. It is used to pass a parameter to the device, specific to
the request.
■
wIndex
The contents of this field vary according to the request. It is used to pass a parameter to the device, specific to
the request.
The
wIndex
field is often used in requests to specify an endpoint or an interface. Figure 6-30 shows the
format of
wIndex
when it is used to specify an endpoint.
Figure 6-31: wIndex Format when Specifying an Endpoint
The
Direction
bit is set to zero to indicate the OUT endpoint with the specified
Endpoint Number
and to one
to indicate the IN endpoint. In the case of a control pipe, the request should have the
Direction
bit set to zero
but the device may accept either value of the
Direction
bit.
Figure 6-31 shows the format of
wIndex
when it is used to specify an interface.
Figure 6-32: wIndex Format when Specifying an Interface
■
wLength
This field specifies the length of the data transferred during the second phase of the control transfer. The
direction of data transfer (host-to-device or device-to-host) is indicated by the
Direction
bit of the
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