Freescale Semiconductor MPC8313E Family Reference Manual page 1038

Universal Serial Bus Interface
Figure 16-54 illustrates the general layout of the periodic schedule.
Periodic
Frame List
Figure 16-54. 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
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 8 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
such an endpoint. Without additional support on the interface, to get 8
software would have to link 8
same path as 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.
hardware and software operational model 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.
• SplitXState. This is a single bit residing in the Status field of a queue head
is used to track the current state of the split transaction.
• Frame S-mask. This is a bit-field where-in system software sets a bit corresponding to the
microframe (within an H-Frame) that the host controller should execute a start-split transaction.
This is always qualified by the value of the SplitXState bit in the Status field of the queue head.
For example, referring to
indicating that if the queue head is traversed by the host controller, and the SplitXState indicates
Do_Start, and the current microframe as indicated by FRINDEX[2–0] is 0, then execute a
start-split transaction.
MPC8313E PowerQUICC II Pro Integrated Processor Family Reference Manual, Rev. 3
16-96
•
•
•
N
poll rate. Software can efficiently manage periodic bandwidth
to 8 . It would then have to move 4
1 0b
Section 16.5.7, "Periodic Frame Span Traversal Node (FSTN),"
Figure 16-53, case one, the S-mask would have a value of 0b0000_0001
Linkage repeats every 8 for
remainder of frame list
Level 8 Level 4
Level 2
8
7
8
6
4
3
8
5
4
8
2
4
8
4
3
1
8
2
4
0
8
1
8
0b
8
0
and everything linked after into the
1
Level 1
(Root)
2
1
• • •
1
0
2
0
reachable at the correct time,
0b
defines the
(Table 16-55). This bit
Freescale Semiconductor
were
0b