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List Control Block
1) Priority
The list priorities are modified as endpoints are serviced and at periodic intervals. In each
frame, an interval of time is reserved for processing of items in the Control and Bulk lists. This
interval is at the beginning of each frame. The Host Controller Driver limits this time by
writing HcPeriodicStart with a bit time after which periodic transfers (Interrupt and
Isochronous) have priority for use of the bus. During periodic list processing, the Interrupt list
specific to the current frame is serviced before the Isochronous list. When processing of the
periodic lists ends, processing of the Control and Bulk lists resumes. Figure 18-2 shows the
priority among periodic lists and non-periodic lists within a single frame.
FRAME Boundary
Non-periodic lists
Interrupt List
Non-periodic lists
FrameRemaining > PeriodicStart
Isochronous List
Periodic List Processing Started
FRAME Boundary
FrameRemaining < PeriodicStart
FrameRemaining = PeriodicStart
Periodic List Processing Complete
SECTION 2
SECTION 1
SECTION 3
SOF Token
NOTE: FrameRemaining and PeriodicStart are fields in the
HcFmRemaining and HcPeriodicStart Operational
registers.
Figure 18.2 List Priority within a USB Frame
Summary of Contents for HD64465
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