MPC8540 datasheet(630/1302 Pages) FREESCALE

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MPC8540 PowerQUICC III Integrated Host Processor Reference Manual, Rev. 1

14-4

Freescale Semiconductor

Three-Speed Ethernet Controllers

The elements of an Ethernet frame are as follows:

•

Preamble— The 7-byte preamble of alternating ones and zeros used for receiver timing

synchronization (each byte containing the value 0x55).

•

Start frame delimiter (SFD)—A sequence of 0xD5 (10101011 because the bit ordering is

lsb first) indicates the beginning of the frame.

•

48-bit destination address (DA)—The first bit identifies the address as an individual

address (0) or a group address (1). The second bit is used to indicate whether the address is

locally-defined (1) or globally-defined (0).

•

48-bit source address (SA)— (Original versions of the IEEE 802.3 specification allowed

16-bit addressing, which has never been used widely.)

•

Ethernet type field/IEEE 802.3 length field—The type field signifies the protocol (for

example, TCP/IP) used in the rest of the frame. The length field specifies the length of the

data portion of the frame. For both Ethernet and IEEE 802.3 frames to exist on the same

LAN, the length field must be unique from any type fields used in Ethernet. This limitation

requires that a type field be identified by a decimal number equal to or greater than 1536

(0x0600) but less than 65535 (0xFFFF). If the number, however, is between 0 and 1,500

(0x0000 through 0x05DC) then this field indicates the length of the MAC client data. The

range from 1,501 to 1,535 (0x5DD through 0x5FF) was intentionally left undefined.

•

Data and padding—Padding is optional. It is only needed if the data is smaller than 46

octets (one octet = one byte) to ensure the minimum frame size of 64 octets as specified in

the IEEE 802.3 standard. In 802.3x the first two octets of the data field are used as opcode

(OP) (pause = 0x0001) and the second two octets are used to transmit a pause time (PT)

parameter (pausetime = 0x0000 for on and 0xFFFF for off). In addition, a third two-octet

field can be used for an extended pause control parameter (PTE). Because the use of these

fields varies with the protocol used, the ability to examine them and report their content can

significantly accelerate Ethernet frame processing.

•

Frame-check sequence (FCS)—Specifies the standard 32-bit cyclic redundancy check

(CRC) obtained using the standard CCITT-CRC polynomial on all fields except the

preamble, SFD and CRC.

Figure 14-4 provides additional details of the Ethernet/IEEE 802.3 frame structure.

MPC8540 Datasheet(PDF) 630 Page - Freescale Semiconductor, Inc