Xilinx 1000BASE-X manual End of Frame Encoding, Preamble Shrinkage

End of Frame Encoding

R

Preamble Shrinkage

As previously described, a single byte of preamble can be lost across the 1000BASE-X system (the actual loss occurs in the 1000BASE-X PCS transmitter state machine).

•There is no specific statement for this preamble loss in the IEEE 802.3-2002specification; the preamble loss falls out as a consequence of the state machines (see figures 36-5 and 36-6 in the IEEE 802.3-2002specification).

•IEEE 802.3ah-2004does, however, specifically state in clause 65.1.3.2.1:

“NOTE 1 – The 1000BASE-X PCS transmit function replaces the first octet of preamble with the /S/ code-group or it discards the first octet and replaces the second octet of preamble with the /S/ code-group. This decision is based upon the even or odd alignment of the PCS’s transmit state diagram (see Figure 36-5).“

End of Frame Encoding

The Even Transmission case

Figure D-5illustrates 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 response to the deassertion of gmii_tx_en, an End of Packet code group /T/ is immediately inserted. The even and odd alignment described in “Start of Frame Encoding” persists throughout the Ethernet frame. If the /T/ character occurs in the even position (the frame contained an even number of bytes starting from the /S/ character), then this is followed with a single Carrier Extend code group /R/. This allows the /K28.5/ character of the following Idle code group to be aligned to the even position.

Note: The first Idle to follow the frame termination sequence will be a /I1/ if the frame ended with positive running disparity or a /I2/ if the frame ended with negative running disparity. This is illustrated as the shaded code group.

gmii_txd[7:0]	FCS
gmii_tx_en
gmii_tx_er

PCS Transmit Engine Encoding

tx_code_group FCS T R I1/I2 I2 I2 I2

Figure D-5:1000BASE-X Transmit State Machine Operation (Even Case)

Ethernet 1000BASE-X PCS/PMA or SGMII v9.1 www.xilinx.com 215

UG155 March 24, 2008