MPC8280 datasheet(1235/1386 Pages) FREESCALE

1235 / 1386 page

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

36-1

Chapter 36

Fast Ethernet Controller

The Ethernet IEEE 802.3 protocol is a widely-used LAN based on the carrier-sense multiple

access/collision detect (CSMA/CD) approach. Because Ethernet and IEEE 802.3 protocols are similar and

can coexist on the same LAN, both are referred to as Ethernet in this manual, unless otherwise noted.

Ethernet/IEEE 802.3 frames are based on the frame structure shown in Figure 36-1.

Figure 36-1. Ethernet Frame Structure

The elements of an Ethernet frame are as follows:

•

7-byte preamble of alternating ones and zeros.

•

Start frame delimiter (SFD)—Signifies the beginning of the frame.

•

48-bit destination address.

•

48-bit source address. 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 used in the rest of

the frame, such as TCP/IP; the length field specifies the length of the data portion of the frame. For

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 requirement limits the length of the data portion of the frame

to 1,500 bytes and, therefore, the total frame length to 1,518 bytes.

•Data

•

Four-bytes frame-check sequence (FCS), which is the standard, 32-bit CCITT-CRC polynomial

used in many protocols.

When a station needs to transmit, it waits until the LAN becomes silent for a specified period (interframe

gap). When a station starts sending, it continually checks for collisions on the LAN. If a collision is

detected, the station forces a jam signal (all ones) on its frame and stops transmitting. Collisions usually

occur close to the beginning of a frame. The station then waits a random time period (backoff) before

attempting to send again. When the backoff completes, the station waits for silence on the LAN and then

begins retransmission on the LAN. This process is called a retry. If the frame is not successfully sent within

15 retries, an error is indicated.

Preamble

Start Frame

Data

Delimiter

Destination

Address

Type/

Length

Source

Address

Frame Check

Sequence

7 Bytes

1 Byte

6 Bytes

2 Bytes

46–1500 Bytes

4 Bytes

Frame Length is 64–1,518 Bytes

Note: The lsb of each octet is transmitted first.

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