Information

Universal Serial Bus Interface
MPC8308 PowerQUICC II Pro Processor Reference Manual, Rev. 1
13-108 Freescale Semiconductor
usually also maps to one high-speed isochronous split transaction. The exception to this rule is the
H-Frame boundary wrap cases mentioned above.
The siTD data structure describes at most, one frame's worth of high-speed transactions and that
description is strictly bounded within a frame boundary. Figure 13-58 illustrates some examples. On the
top are examples of the full-speed transaction footprints for the boundary scheduling cases described
above. In the middle are time-frame references for both the B-Frames (HS/FS/LS Bus) and the H-Frames.
On the bottom is illustrated the relationship between the scope of an siTD description and the time
references. Each H-Frame corresponds to a single location in the periodic frame list. The implication is
that each siTD is reachable from a single periodic frame list location at a time.
Figure 13-58. siTD Scheduling Boundary Examples
Each case is described below:
• Case 1: One siTD is sufficient to describe and complete the isochronous split transaction because
the whole isochronous split transaction is tightly contained within a single H-Frame.
• Case 2a, 2b: Although both INs and OUTs can have these footprints, OUTs always take only one
siTD to schedule. However, INs (for these boundary cases) require two siTDs to complete the
scheduling of the isochronous split transaction. siTDX is used to always issue the start-split and
the first N complete-splits. The full-speed transaction (for these cases) can deliver data on the
full-speed bus segment during microframe 7 of H-Frame
Y+1
, or microframe 0 of H-Frame
Y+2
. The
complete splits are scheduled using siTD
X+2
(not shown). The complete-splits to extract this data
must use the buffer pointer from siTD
X+1
. The only way for the host controller to reach siTD
X+1
from H-Frame
Y+2
is to use siTD
X+2
's back pointer.
Software must apply the following rules when calculating the schedule and linking the schedule data
structures into the periodic schedule:
• Software must ensure that an isochronous split-transaction is started so that it will complete before
the end of the B-Frame.
076543210 07654321 217654
B-Frame
Y
B-Frame
Y+1
B-Frame
Y+2
B-Frame
Y–1
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H-Frame
Y
H-Frame
Y+1
3
H-Frame
Y+2
H-Frame
Y–1
Case 1 Case 2a Case 2b
siTD
X
siTD
X+1
Back Pointer
Full-Speed Transaction