Cirrus Logic EP93xx - Manual (Page 330)

9-28 DS785UM1
Copyright 2007 Cirrus Logic
1/10/100 Mbps Ethernet LAN Controller
EP93xx User’s Guide
9
9
9
Figure 9-14. Multiple Fragments Per Transmit Frame
In the example shown in Figure 9-14 , one frame is transmitted from three fragments. The
MAC starts the frame by acquiring the medium and transmitting the preamble. Then, the
fragments 0, 1, 2 are transmitted in order for a total of 446 bytes (39 + 388 + 19). Since the
CRC bit in the first frame fragment is clear, the HW appends the 4 byte CRC. Thus, 4 more
bytes are added to the frame for the CRC making the total frame length 450 bytes. Finally, the
MAC sends the end-of-frame.
The CMD field is 4 bits. Only the AF bit is valid. The other fields are reserved.
9.2.3.9 Transmit Descriptor Format
Transmit Descriptor Format - First Word
Definition:
Transmit Descriptor, first word. Contains the base address of the data buffer.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
TBA
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
TBA
register sizes are in bits,
and shown in parentheses ().
T x B u fA d r 0 ( 3 2 )
T x D B u f A d r 1 ( 3 2 )
T x D B u f A d r 2 ( 3 2 )
Data Fragment 0 Fragment 0
Length
89 bytes
Example: Fragments 0, 1, 2 make-up one complete frame.
Data Fragment 1
Fragment 1
Length
388 bytes
Data Fragment 2
Fragment 2
Length
19 bytes
EOF = 0
EOF = 0
EOF = 1
Buffer
Length 0 (12)
Buffer
Length 0 (12)
Buffer
Length 0 (12)
CMD
(4)
CMD
(4)
CMD
(4)
Buffer
Index 0 (15)
Buffer
Index 0 (15)
Buffer
Index 0 (15)
EOF
EOF
EOF