
R
Appendix D: 1000BASE-X State Machines
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.
gmii_txd[7:0] | preamble |
gmii_tx_en
gmii_tx_er
SFD
PCS Transmit Engine Encoding
tx_code_group I2 I2 I2 S preamble
SFD
Figure D-1: 1000BASE-X Transmit State Machine Operation (Even Case)
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| UG155 March 24, 2008 |