
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.
rx_code_group I2 I2 I2 S preamble
SFD
PCS Receive Engine Decoding
gmii_rxd[7:0] | preamble |
gmii_rx_dv
gmii_rx_er
SFD
Figure D-2: 1000BASE-X Reception State Machine Operation (Even Case)
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.
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