Xilinx 1000BASE-X - Manual (Page 212)

212 www.xilinx.com Ethernet 1000BASE-X PCS/PMA or SGMII v9.1
UG155 March 24, 2008
Appendix D: 1000BASE-X State Machines R
Start of Frame Encoding
The Even Transmission Case
Figure D-1 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.
It is important to note that the encoding of Idle periods /I2/ is constructed from a couple
of code groups—the /K28.5/ character (considered the even position) and the /D16.2/
character (considered the odd position). In this example, the assertion of the gmii_tx_en
signal of the GMII occurs in the even position. In response, the state machines insert a Start
of Packet code group /S/ following the Idle (in the even position). This is inserted in place
of the first byte of the frame preamble field.
Figure D-1: 1000BASE-X Transmit State Machine Operation (Even Case)
gmii_txd[7:0]
gmii_tx_en
gmii_tx_er
preamble
SFD
PCS Transmit Engine Encoding
preamble
SFD
tx_code_group I2 I2 I2 S