Xilinx LOGICORE UG144 - Manual (Page 48)

48 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
The transmission timing depicted in Figure 5-7 shows the GEMAC core configured to have
the FCS field passed in by the client. In this case, it is the responsibility of the client to
ensure that the frame meets the Ethernet minimum frame length requirements as the
GEMAC core will not perform any padding of the payload. See
“Configuration Registers,”
on page 78 .
Client Underrun
Figure 5-8 illustrates the timing of an aborted transfer. An example of this situation is a
FIFO connected to the client interface that empties before a frame transfer is complete.
When the client asserts tx_underrun during a frame transmission, the GEMAC core
inserts an error code to corrupt the current frame, and then falls back to idle transmission.
It is the responsibility of the client to re-queue the aborted frame for transmission.
When an underrun occurs, tx_data_valid may be asserted on the clock cycle after the
tx_underrun
assertion to request a new transmission.
Figure 5-7: Frame Transmission with Client-supplied FCS
gtx_clk
tx_data[7:0]
tx_data_valid
tx_ack
tx_underrun
DA SA DATA FCS L/T
Figure 5-8: Frame Transmission with Underrun
gtx_clk
tx_data[7:0]
tx_data_valid
tx_ack
tx_underrun
DA SA DATA L/T