MPC8280 datasheet(789/1386 Pages) FREESCALE

789 / 1386 page

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

25-1

Chapter 25

SCC Ethernet Mode

The Ethernet IEEE 802.3 protocol is a widely used LAN protocol 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.

Figure 25-1 shows Ethernet and IEEE 802.3 frame structure.

Figure 25-1. Ethernet Frame Structure

The frame begins with a 7-byte preamble of alternating ones and zeros. Because the frame is Manchester

encoded, the preamble gives receiving stations a known pattern on which to lock. The start frame delimiter

follows the preamble, signifying the beginning of the frame. The 48-bit destination address is next,

followed by the 48-bit source address. Original versions of the IEEE 802.3 specification allowed 16-bit

addressing, but this addressing has never been widely used and is not supported.

The next field is the Ethernet type field/IEEE 802.3 length field. The type field signifies the protocol used

in the rest of the frame and the length field specifies the length of the data portion of the frame. For Ethernet

and IEEE 802.3 frames to coexist on the same LAN, the length field of the frame must always be different

from any type fields used in Ethernet. This limits the length of the data portion of the frame to 1,500 bytes

and total frame length to 1,518 bytes. The last 4 bytes of the frame are the frame check sequence (FCS), a

standard 32-bit CCITT-CRC polynomial used in many protocols.

When a station needs to transmit, it checks for LAN activity. When the LAN is silent for a specified period,

the station starts sending. At that time, the station continually checks for collisions on the LAN; if one is

found, the station forces a jam pattern (all ones) on its frame and stops sending. Most collisions occur close

to the beginning of a frame. The station waits a random period of time, called a backoff, before trying to

retransmit. Once the backoff time expires, the station waits for silence on the LAN before retransmitting,

which is called a retry. If the frame cannot be sent within 15 retries, an error occurs

10-Mbps Ethernet transmits at 0.8 µs per byte. The preamble plus start frame delimiter is sent in 6.4 µs.

The minimum 10-Mbps Ethernet interframe gap is 9.6 µs and the slot time is 52 µs.

25.1

Ethernet on the MPC8280

Setting GSMR[MODE] to 0b1100 selects Ethernet. The SCC performs the full set of IEEE 802.3/Ethernet

CSMA/CD media access control and channel interface functions.

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 64–1518 Bytes

NOTE: The lsb of each octet is transmitted first.

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