3.6 基于OHCI的USB主机 —— OHCI（ED结构说明）

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ED结构说明

节选自OHCI的规范。

Name

HC

Access

Description

FA

R

FunctionAddress

This is the USB address of the function containing the endpoint that this ED controls

EN

R

EndpointNumber

This is the USB address of the endpoint within the function

D

R

Direction

This 2-bit field indicates the direction of data flow (IN or OUT.)  If neither IN nor OUT is specified, then the direction is determined from the PID field of the TD. The encoding of the bits of this field are:

 

 

Code

Direction

 

 

 

00b

Get direction From TD

 

 

 

01b

OUT

 

 

 

10b

IN

 

 

 

11b

Get direction From TD

 

 

 

 

 

S

R

Speed

Indicates the speed of the endpoint: full-speed (S = 0) or low-speed (S = 1.)

K

R

sKip

When this bit is set, the HC continues on to the next ED on the list without attempting access to the TD queue or issuing any USB token for the endpoint

F

R

Format

This bit indicates the format of the TDs linked to this ED.  If this is a Control, Bulk, or Interrupt Endpoint, then F = 0, indicating that the General TD format is used.  If this is an Isochronous Endpoint, then F = 1, indicating that the Isochronous TD format is used.

MPS

R

MaximumPacketSize

This field indicates the maximum number of bytes that can be sent to or received from the endpoint in a single data packet

TailP

R

TDQueueTailPointer

If TailP and HeadP are the same, then the list contains no TD that the HC can process. If TailP and HeadP are different, then the list contains a TD to be processed.

H

R/W

Halted

This bit is set by the HC to indicate that processing of the TD queue on the endpoint is halted, usually due to an error in processing a TD.

C

R/W

toggleCarry

This bit is the data toggle carry bit.  Whenever a TD is retired, this bit is written to contain the last data toggle value (LSb of data Toggle field) from the retired TD.  This field is not used for Isochronous Endpoints

HeadP

R/W

TDQueueHeadPointer

Points to the next TD to be processed for this endpoint.

NextED

R

NextED

If nonzero, then this entry points to the next ED on the list