Xilinx 1000BASE-X - Manual (Page 215)

Ethernet 1000BASE-X PCS/PMA or SGMII v9.1 www.xilinx.com 215
UG155 March 24, 2008
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 802.3-2002
specification; the preamble loss falls out as a consequence of the state machines (see
figures 36-5 and 36-6 in the IEEE 802.3-2002 specification).
•
IEEE 802.3ah-2004 does, 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-5 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 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.
Figure D-5: 1000BASE-X Transmit State Machine Operation (Even Case)
gmii_txd[7:0]
gmii_tx_en
gmii_tx_er
FCS
FCS I2 I2 I2 T R I1/I2
tx_code_group
PCS Transmit Engine Encoding