Xilinx LOGICORE UG144 - Manual (Page 49)

1-Gigabit Ethernet MAC v8.5 User Guide www.xilinx.com 49
UG144 April 24, 2009
Transmitting Outbound Frames R
-- DISCONTINUED PRODUCT --
VLAN Tagged Frames
Figure 5-9 illustrates transmission of a VLAN tagged frame (if enabled). The handshaking
signals across the interface do not change; however, the VLAN type tag 81-00 must be
supplied by the client to signify that the frame is VLAN tagged. The client also supplies the
two bytes of Tag Control Information, V1 and V2, at the appropriate times in the data
stream. More information on the contents of these two bytes can be found in IEEE 802.3-
2005. For more information about enabling and disabling jumbo frame handling, see
“Configuration Registers,” on page 78 .
Maximum Permitted Frame Length
The maximum legal length of a frame specified in IEEE 802.3-2005 is 1518 bytes for non-
VLAN tagged frames. VLAN tagged frames may be extended to 1522 bytes. When jumbo
frame handling is disabled and the client attempts to transmit a frame that exceeds the
maximum legal length, the GEMAC core will insert an error code to corrupt the current
frame, and the frame will be truncated to the maximum legal length. When jumbo frame
handling is enabled, frames longer than the legal maximum are transmitted error-free. For
more information on enabling and disabling Jumbo frame handling, see
“Configuration
Registers,” on page 78 .
Inter-Frame Gap Adjustment
A configuration bit in the transmitter control register (see “Configuration Registers,” on
page 78 ) allows you to control the length of the inter-frame gap transmitted by the MAC on
the physical interface. If this function is selected, the MAC exerts back pressure on the
client interface to delay the transmission of the next frame until the requested number of
idle cycles has elapsed. The number of idle cycles is controlled by the value on the
tx_ifg_delay
port seen at the start of frame transmission on the client interface.
Figure 5-10 shows the MAC operating in this mode.
Reducing the interframe gap to below the IEEE 802.3-2005 minimum of 12 idles is
supported, but the MAC will transmit an absolute minimum of 4 idles. If the Ethernet
Statistics core is used with the MAC, then accuracy cannot be guaranteed if the interframe
gap adjustment is set to less than 12 idles. However, the tx_statistic_vector and
rx_statistic_vector
values will always remain correct.
Figure 5-9: Transmission of a VLAN Tagged Frame
gtx_clk
tx_data[7:0]
tx_data_valid
tx_ack
tx_underrun
DA SA DATA L/T VLAN
tag
81 00 V1V2