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Universal Serial Bus Interface
MCF5253 Reference Manual, Rev. 1
24-62
Freescale Semiconductor
24.8.6.1
Queue Head Horizontal Link Pointer
The first DWord of a Queue Head contains a link pointer to the next data object to be processed after any
required processing in this queue has been completed, as well as the control bits defined below.
This pointer may reference a queue head or one of the isochronous transfer descriptors. It must not
reference a queue element transfer descriptor.
24.8.6.2
Endpoint Capabilities/Characteristics
The second and third DWords of a Queue Head specifies static information about the endpoint. This
information does not change over the lifetime of the endpoint. There are three types of information in this
region:
•
Endpoint Characteristics. These are the USB endpoint characteristics including addressing,
maximum packet size, and endpoint speed.
•
Endpoint Capabilities. These are adjustable parameters of the endpoint. They effect how the
endpoint data stream is managed by the host controller.
•
Split Transaction Characteristics. This data structure is used to manage full- and low-speed data
streams for bulk, control, and interrupt via split transactions to USB2.0 Hub Transaction
Translator. There are additional fields used for addressing the hub and scheduling the protocol
transactions (for periodic).
The host controller must not modify the bits in this region.
Table 24-55. Queue Head DWord 0
Bit
Name
Description
31–5 QHLP Queue Head Horizontal Link Pointer. This field contains the address of the next data object to be processed in
the horizontal list and corresponds to memory address signals [31–5], respectively.
4–3
–
Reserved. These bits must be written as zeros.
2–1
Typ
This field indicates to the hardware whether the item referenced by the link pointer is an iTD, siTD or a QH. This
allows the host controller to perform the proper type of processing on the item after it is fetched.
00 iTD (isochronous transfer descriptor)
01 QH (queue head)
10 siTD (split transaction isochronous transfer descriptor)
11 FSTN (frame span traversal node)
0 T
Terminate.
1 Last QH (pointer is invalid).
0 Pointer is valid.
If the queue head is in the context of the periodic list, a one bit in this field indicates to the host controller that this
is the end of the periodic list. This bit is ignored by the host controller when the queue head is in the
Asynchronous schedule. The software must ensure that queue heads reachable by the host controller always
have valid horizontal link pointers.
Содержание MCF5253
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