Dropped TX_AXI_TVALID Before End of Frame
Streaming data means continuous DATA bytes from the start of frame (SOF) until the end of
frame (EOF). This means that TX_AXI_TVALID should remain asserted from
SOF until EOF. In Cut Through mode, a dropped
TX_AXI_TVALID during streaming data breaks the continuity of the
streaming data. When this occurs, the XGMII TX outputs an error frame. Refer to Figure 1.
TX_AXI_TVALID = 0,
the entire column of TX AXI signals (TDATA, TKEEP, TLAST, and TUSER) is deemed as
invalid. Figure 1 illustrates how TX_AXI_TVALID defines the start of frame
(SOF).TX_AXI_TVALID (Cut Through Mode
Only)
However, in Store Forward mode, the Ethernet 10G MAC core stores the entire frame
before it starts transmitting. Based on the storing mechanism shown in Figure 2, a dropped TX_AXI_TVALID indicates that the whole column of
TX_AXI_* signals (TDATA, TKEEP, TLAST, and TUSER) is not stored in
the TXFIFO. Upon TX_AXI_TLAST, i.e., once the entire frame is stored,
the Ethernet 10G MAC core starts transmitting
the Ethernet frame at the XGMII TX interface. Hence, a dropped
TX_AXI_TVALID does not warrant an error frame during
Store Forward mode.