Xilinx LOGICORE UG144 - Manual (Page 42)

42 www.xilinx.com 1-Gigabit Ethernet MAC v8.5 User Guide
UG144 April 24, 2009
Chapter 5: Using the Client Side Data Path R
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Client-Supplied FCS Passing
If the GEMAC core is configured to pass the FCS field to the client (see “Configuration
Registers,” on page 78 ), this is handled as shown in Figure 5-3 . In this case, any padding
inserted into the frame to meet Ethernet minimum frame length specifications will be left
intact and passed to the client.
Even though the FCS is passed up to the client, it is also verified by the GEMAC core, and
rx_bad_frame
asserted if the FCS check fails.
VLAN Tagged Frames
Figure 5-4 illustrates the reception of a VLAN tagged frame (if enabled). The VLAN frame
is passed to the client so that the frame may be identified as VLAN tagged. This is followed
by the Tag Control Information bytes, V1 and V2. More information on the interpretation
of these bytes may be found in IEEE 802.3-2005 standard.
Figure 5-3: Frame Reception with In-Band FCS Field
gmii_rx_clk
rx_data[7:0]
rx_data_valid
rx_good_frame
rx_bad_frame
DA SA DATA L/T FCS
Figure 5-4: Reception of a VLAN Tagged Frame
gmii_rx_clk
rx_data[7:0]
rx_data_valid
rx_good_frame
rx_bad_frame
DA SA DATA L/T
81 00V1V2
VLAN
tag