Xilinx LOGICORE UG144 - Manual (Page 47)

1-Gigabit Ethernet MAC v8.5 User Guide www.xilinx.com 47
UG144 April 24, 2009
Transmitting Outbound Frames R
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Transmitting Outbound Frames
Ethernet frames to be transmitted are presented to the client logic on the Transmitter subset
of the Client-Side Interface. For port definition, see
“Transmitter Interface,” on page 26 .
Normal Frame Transmission
Figure 5-6 illustrates the timing of a normal outbound frame transfer. When the client
wishes to transmit a frame, it places the first column of data onto the tx_data port and
asserts a ‘1’ onto tx_data_valid.
When the GEMAC core has read this first byte of data, and in accordance with flow control
requests and interpacket gap requirements, it will assert the tx_ack signal; on the next
and subsequent rising clock edges, the client must provide the remainder of the data for
the frame.
The end of frame is signalled to the GEMAC core by taking tx_data_valid low.
For maximum flexibility in switching and routing applications, the Ethernet frame
parameters (destination address, source address, length/type and optionally FCS) are
encoded within the same data stream that the frame payload is transferred upon, rather
than on separate ports. This is illustrated in the timing diagrams. Definitions of the
abbreviations used in the timing diagrams are defined in
Table 5-1 .
Padding
When fewer than 46 bytes of data are supplied by the client to the GEMAC core, the
transmitter module will add padding up to the minimum frame length. The exception to
this is when the GEMAC core is configured for client-passed FCS; in this case, the client
must also supply the padding to maintain the minimum frame length. See
“Client-
Supplied FCS Passing” for more information.
Figure 5-6: Normal Frame Transmission
gtx_clk
tx_data[7:0]
tx_data_valid
DA SA DATA L/T
tx_ack
tx_underrun