Motorola MPC823e Reference Manual page 772

Communication Processor Module
19. Write 0x00000000 to the GSMR_L to configure the CTS2 and CD2 pins to
automatically control transmission and reception (DIAG field). Normal operation of the
transmit clock is used. Notice that the transmitter (ENT) and receiver (ENR) are not
enabled yet.
20. Write 0x00000030 to the GSMR_L to enable the SCC2 transmitter and receiver. This
additional write ensures that the ENT and ENR bits are enabled last.
Note: After 5 bytes are transmitted, the TX buffer descriptor is automatically closed.
The receive buffer is automatically closed after 16 bytes are received. Any data
received after 16 bytes causes a busy (out-of-buffers) condition since only one
RX buffer descriptor is prepared.
16.9.22 The SCCx in Ethernet Mode
The Ethernet IEEE 802.3 protocol is a widely used LAN based on the carrier-sense multiple
access/collision detect (CSMA/CD) approach. Ethernet and IEEE 802.3 protocols are very
similar and can coexist on the same LAN. They are referred to synonymously as Ethernet
in this manual, unless specifically noted. Ethernet and IEEE 802.3 frames are based on the
frame structure illustrated in Figure 16-105.
START
FRAME
PREAMBLE
DELIMITER
7 BYTES 1 BYTE
NOTE: The LSB of each octet is transmitted first.
The frame begins with a 7-byte preamble of alternating ones and zeros. Since the frame is
Manchester encoded, the preamble gives receiving stations a known pattern on which to
lock. The start frame delimiter signifies the beginning of the frame and follows the preamble.
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. However, this
addressing has never been widely used in the industry.
The next field is the type field in Ethernet and the length field in IEEE 802.3. 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 unique from any type fields used in
Ethernet. This has limited the length of the data portion of the frame to 1,500 bytes and the
total frame length to 1,518 bytes. The last 4 bytes of the frame are the frame check
sequence (FCS), which is the standard 32-bit CCITT-CRC polynomial used in many other
protocols.
16-318
FRAME LENGTH IS 64–1518 BYTES
SOURCE TYPE/
DEST
LENGTH
ADDR ADDR
6 BYTES 6 BYTES 2 BYTES
Figure 16-105. Ethernet Frame Format
MPC823e REFERENCE MANUAL
FRAME
CHECK
DATA
SEQUENCE
46–1500 BYTES 4 BYTES
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