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S3C2500B
The main transmission state machine
The main transmission state machine implements the remaining MAC layer protocols. If there is data to be
transferred, if the inter-frame gap is valid, and if the MII is ready (that is, if there are no collisions and no CRS in
full-duplex mode), the transmitter block then transmits the preamble followed by the SFD.
After the SFD and preamble are transmitted, the block transmits 64-byte data, regardless of the frame length,
unless short transmission is enabled. This means that if the frame is less than 64-byte, it will pad the LLC data
field with zeros. It will also append the CRC to the end of the frame, if CRC generation is enabled.
If there is any collision during this first 72-byte time (8-byte preamble and SFD, and 64-byte frame), the main
transmission state machine stops the transmission and transmits a jam pattern (32-bit 1's). It then increments the
collision attempt counter, returns control to the back-off state machine, and re-transmits the frame when the
back-off time has elapsed and the gap time is valid.
If there are no collisions, the transmitter block transmits the rest of the frame. At this time (that is, after the first
60- byte have been transmitted without collisions), the main transmission state machine lets the BDMA engine
overwrite the frame. After it transmits the first 64-byte, the transmitter block transmits the rest of the frame,
appending the CRC to the end. Parity errors, FIFO errors, or more than 16 collisions will force the transmission
state machine to abort the frame (no retry) and to transmit the next frame.
Tx_clk
Tx_en
TxD [3:0]
Crs
Col
P
R
E
A
Figure 7-7. Timing for Transmission without Collision
M
B
L
E
ETHERNET CONTROLLER
7-41
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