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1) Transfer Description Processing
Transfer Descriptor processing is the fundamental operation performed by a Host Controller.
The service flow for servicing a Transfer Descriptor is shown in Figure 18-7 below. The rest of
this section describes the steps necessary for completing service of a Transfer Descriptor.
SERVICE
TRANSFER
DESCRIPTOR
Calculate PACKET
Addr and Size
GTD
Perform SOF check
Time available?
Execute USB
Transaction
YES
Execute USB
Transaction
Write PACKET
to memory
Status Writeback
TD Complete?
Retire TD
NO
FINISHED
YES
NO
PID = OUT?
Read PACKET
from memory
Perform SOF check
YES
NO
Time available?
YES
NO
Compare
Number with
Frame in ED
Frame - Number
> N?
Frame - Number
< 0?
ITD
NO
NO
(
Frame - Number
provides Offset)
YES
(Error)
YES
(Early)
Figure 18.7 Transfer Description Service Flow
Содержание HD64465
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