Xilinx LOGICORE UG144 - Manual (Page 43)

1-Gigabit Ethernet MAC v8.5 User Guide www.xilinx.com 43
UG144 April 24, 2009
Receiving Inbound Frames R
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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 core receives a frame which exceeds the maximum
legal length, rx_bad_frame is asserted. When jumbo frame handling is enabled, frames
which are longer than the legal maximum are received in the same way as shorter frames.
For more information about enabling and disabling jumbo frame handling, see
“Configuration Registers,” on page 78 .
Length/Type Field Error Checks
Enabled
Default operation is with the length/type error checking enabled (see “Receiver
Configuration,” on page 79 ). In this mode, the following checks are made on all frames
received. If either of these checks fail, the frame is marked as bad.
• A value in the length/type field that is greater than or equal to decimal 46, but less
than decimal 1536 (a length interpretation), is checked against the actual data length
received.
• A value in the length/type field that is less than decimal 46 is checked to see that the
data field is padded to exactly 46 bytes (so that the resultant frame is a minimum
frame size of 64 bytes total in length).
Furthermore, if padding is indicated (the length/type field is less than decimal 46) and
client-supplied FCS passing is disabled, the length value in the length/type field will be
used to deassert rx_data_valid after the indicated number of data bytes so that the
padding bytes are removed from the frame. See
“Client-Supplied FCS Passing.”
Disabled
When the length/type error checking is disabled and the length/type field has a length
interpretation, the MAC does not check the length value against the actual data length
received. See
“Receiver Configuration” in Chapter 8 . A frame containing only this error is
marked as good.
However, if the length/type field is less than decimal 46 then the MAC will mark a frame
as bad if it is not the minimum frame size of 64 bytes.
If padding is indicated and client-supplied FCS passing is disabled, then a length value in
the length/type field will not be used to deassert rx_data_valid. Instead,
rx_data_valid
is deasserted before the start of the FCS field, and any padding is not
removed from the frame.