Xilinx 1000BASE-X - Manual (Page 213)

Ethernet 1000BASE-X PCS/PMA or SGMII v9.1 www.xilinx.com 213
UG155 March 24, 2008
Start of Frame Encoding R
Reception of the Even Case
Figure D-2 illustrates the reception of the in-bound code-group stream, received either
serially using the RocketIO transceiver, or in parallel across the TBI, and translation of this
code-group stream into the receiver GMII. This is performed by the PCS Receive Engine.
The Start of Packet code group /S/ is replaced with a preamble byte. This results in the
restoration of the full preamble field.
The Odd Transmission Case
Figure D-3 illustrates the translation of GMII encoding into the code-group stream
performed by the PCS Transmit Engine; this stream is transmitted out of the core, either
serially using the RocketIO transceiver, or in parallel across the TBI.
In this example, the assertion of the gmii_tx_en signal of the GMII occurs in the odd
position; in response, the state machines are unable to immediately insert a Start-Of-Packet
code group /S/ as the Idle character must first be completed. The Start of Packet code
group /S/ is therefore inserted (in the even position) after completing the Idle. This results
in the /D16.2/ character of the Idle /I2/ sequence being inserted in place of the first byte
of the preamble field, and the Start-Of-Packet /S/ being inserted in place of the second
byte of preamble as illustrated.
Figure D-2: 1000BASE-X Reception State Machine Operation (Even Case)
gmii_rxd[7:0]
gmii_rx_dv
gmii_rx_er
preamble
SFD
preamble
SFD
rx_code_group I2 I2 I2 S
PCS Receive Engine Decoding