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參數(shù)資料
| 型號： | μPD9210FGC-7EA |
| 廠商： | NEC Corp. |
| 英文描述： | USB Host Controller(USB主控制器) |
| 中文描述： | USB主機(jī)控制器（的USB主控制器） |
| 文件頁數(shù)： | 35/95頁 |
| 文件大?。?/td> | 423K |
| 代理商： | ΜPD9210FGC-7EA |
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CHAPTER 3 DESCRIPTION OF USB HOST CONTROLLER FUNCTIONS
35
3.4 Transfer Descriptors
TDs (Transfer Descriptors) are used by the HC to indicate the buffer for the data that is sent to or from an
endpoint. TDs are divided into two types: General TDs and Isochronous TDs. General TDs are used by interrupt,
control, and bulk endpoints while Isochronous TDs are used for handling isochronous transfers.
For both General and Isochronous TDs, buffers ranging from 0 bytes to 8,192 bytes can be indicated. Also, the
data buffer described by one TD can be divided into two pages. This enables the elimination of various problems,
such as problems related to forced physical connection of buffers or transfer of surplus data.
When the HCD appends a TD, the TD indicated by TailP is linked to the new TD, and TailP is then changed to
indicate the appended TD. Therefore, the appended TD is always added to the end of the TD queue. The HC
services the TD asynchronously in relation to servicing performed by the host processor. Consequently, when it is
necessary to switch from the TD queue to something else, the HC’s endpoint TD queue servicing must be suspended
to avoid problems from occurring due to this switch. Suspension of TD servicing is achieved when the HCD sets the
Skip bit in the ED to be deleted.
3.4.1 General transfer descriptor format
General TDs are used for control, bulk, or interrupt transfers, and they must always be allocated in 16-byte units to
system memory.
Figure 3-9. General TD Format
0
3
1
2
8
0
0
Dword 0
Dword 1
Dword 2
Dword 3
2
7
2
6
2
5
2
4
2
3
2
1
2
0
1
9
1
8
R
0
3
CC
–
EC
T
DI
DP
Current Buffer Pointer (CBP)
Next TD (NextTD)
Buffer End (BE)
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