基于OHCI的USB主机 —— OHCI（TD结构）

TD数据结构定义如下：

typedef struct _AT91S_UHP_TD {     volatile unsigned int Control ;     volatile unsigned int CBP ;     volatile unsigned int NextTD ;     volatile unsigned int BE ; } AT91S_UHP_TD , * AT91PS_UHP_TD ;

General Transfer Descriptor

Transfers for control, bulk, and interrupt all use the same format for their Transfer Descriptor (TD).  This General TD is a 16-byte, host memory structure that must be aligned to a 16-byte boundary.

General Transfer Descriptor Format

	3		2	2	2	2	2	2		2	2	1	1											0		0
	1		8	7	6	5	4	3		1	0	9	8											3		0
Dword 0	CC			EC		T		DI			DP		R	―
Dword 1	Current Buffer Pointer (CBP)
Dword 2	Next TD (NextTD)																							0
Dword 3	Buffer End (BE)

General TD Format

Note :   In Dword0, there are fields that are read/write by the HC.  The unused portion of this Dword (indicated by ‘―’ ) must either not be written by Host Controller or must be read, and then written back unmodified.  The Host Controller Driver should not modify any portion of the TD while it is accessible to the HC.