Triple Speed Ethernet designs using SGMII PCS and 1000BASE-X protocol will receive wrong number of preamble bytes.
For 1000BASE-X protocol, the encoding of Idle periods /I2/ is constructed from 2 code groups: /K28.5/ character at even position and /D16.2/ at odd position.If gmii_tx_en of GMII assertion occurs at odd position, the transmit state machine is unable to immediately insert a Start-Of-Packet code group /S/ as the Idle character must be completed. Therefore, the /S/ character will be inserted after the /I2/ is completed. In this scenario, /D16.2/ of /I2/ is inserted in the place of the first byte of preamble field, and /S/ at the second byte of preamble field.During reception of this frame, the receive state machine will replace /S/ with a preamble byte. However, the first preamble byte of the original transmit GMII frame is not replaced with preamble byte (it was replaced with /D16.2/ character to complete the Idle sequence during transmit operation). This scenario results a single byte of preamble loss across the system.The IEEE 802.3-2008 specification explains this issue in clause 126.96.36.199.1: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 PCSs transmit state diagram (see Figure 365). The 1000BASE-X PCS receive function replaces the /S/ code-group with an octet of preamble. The third octet of preamble is the first octet passed through the 1000BASE-X PHY without modification.