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USB 2.0 Host Controller—Intel
Frame to B-Frame alignment requires that the queue head be reachable from
consecutive periodic frame list locations. System software cannot build an efficient
schedule that satisfies this requirement unless it uses FSTNs.
Structure of EHCI Periodic Schedule Utilizing Interrupt Spreading" on page 457
illustrates the general layout of the periodic schedule.
Figure 69.
General Structure of EHCI Periodic Schedule Utilizing Interrupt Spreading
The periodic frame list is effectively the leaf level a binary tree, which is always
traversed leaf to root. Each level in the tree corresponds to a 2
efficiently manage periodic bandwidth on the USB by spreading interrupt queue heads
that have the same poll rate requirement across all the available paths from the frame
list. For example, system software can schedule eight poll rate eight queue heads and
account for them once in the high-speed bus bandwidth allocation.
When an endpoint is allocated an execution footprint that spans a frame boundary, the
queue head for the endpoint must be reachable from consecutive locations in the frame
list. An example would be if 8
the interface, to get 8
8
. It would then have to move 4
0b
4
. This upsets the integrity of the binary tree and disallows the use of the spreading
0
technique.
FSTN data structures are used to preserve the integrity of the binary-tree structure and
enable the use of the spreading technique.
FSTNs" on page 458
requirements for using FSTNs.
The following queue head fields are initialized by system software to instruct the host
controller when to execute portions of the split-transaction protocol.
August 2006
Order Number: 306262--, Revision: 004US
®
®
IXP45X and Intel
IXP46X Product Line of Network Processors
where such an endpoint. Without additional support on
0b
reachable at the correct time, software would have to link 8
0b
defines the hardware and software operational model
Intel
and everything linked after into the same path as
1
"Host Controller Operational Model for
®
®
IXP45X and Intel
IXP46X Product Line of Network Processors
Figure 69, "General
B4512-01
N
poll rate. Software can
1
Developer's Manual
457
to
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