Freescale Semiconductor MPC8260 - Manual (Page 785)

MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
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 PowerQUICC II
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 6 Bytes 2 Bytes 46–1500 Bytes 4 Bytes
Frame Length is 64–1518 Bytes
NOTE: The lsb of each octet is transmitted first.