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CDC EEM Subclass Overview

CDC EEM Subclass Overview

Nov 25, 2020

Overview

A CDC EEM device is composed of the following endpoints:

  • A pair of Bulk IN and OUT endpoints.

Table - CDC EEM Subclass Endpoints Usage describes the usage of the different endpoints:

Table - CDC EEM Subclass Endpoints Usage

Endpoint

Direction

Usage

Bulk In

Device-to-host

Send Ethernet frames and commands to host.

Bulk OUT

Host-to-device

Receive Ethernet frames and commands from host.

 

CDC EEM Messages

CDC EEM defines a header that is prepended to each message sent to / received from the host. The CDC EEM header has a size of two bytes. A USB transfer on the Bulk endpoints can contain multiple EEM messages. An EEM message can also span on multiple USB transfers.

Table - CDC EEM Message Format describes the content of a CDC EEM message.

Table - CDC EEM Message Format

Bytes

0..1

2..N

Content

Header

Payload (optional)

 

Table - CDC EEM Header Format describes the content of a CDC EEM header.

Table - CDC EEM Header Format

Bit

15

14 .. 0

Content

bmType

Depends on bmType value

 

  • bmType represents the message type, either a regular Ethernet frame or a CDC EEM specific command.

Table - CDC EEM Data Header Format describes the content of a CDC EEM data message header.

Table - CDC EEM Data Header Format

Bit

15

14

13 .. 0

Content

bmType (0)

bmCRC

Length of Ethernet frame

  • bmCRC indicates if the CRC was calculated on the Ethernet frame. If not, CRC is set to 0xDEADBEEF.

 

Table - CDC EEM Command Header Format describes the content of a CDC EEM command message header. Note that the EEM commands provide USB local link management. This management is opaque to the network stack.

Table - CDC EEM Command Header Format

Bit

15

14

13 .. 11

10 .. 0

Content

bmType (1)

bmReserved (0)

bmEEMCmd

bmEEMCmdParam

  • bmEEMCmd represents the command code to execute. 

  • bmEEMCmdParam contains command data. Fomat depends on the bmEEMCmd.

For more information on CDC EEM messages format, see "Universal Serial Bus Communications Class Subclass Specification for Ethernet Emulation Model Devices" revision 1.0. February 2, 2005, section 5.1.