Altera 40-Gbps Ethernet MAC and PHY MegaCore Function User Manual
Page 70
Figure 3-20: Flow of Frame Through the MAC RX Without Preamble Pass-Through
Illustrates the typical flow of frame through the MAC RX when the preamble pass-through feature is
turned off. In this figure, <p> is payload size (0–1500 bytes), and <s> is the number of pad bytes (0–46
bytes).
Client - MAC Rx Interface
Client Frame
MAC Frame
Destination
Addr[47:0]
Source
Addr[47:0]
Type/
Length[15:0]
Payload
[<p-1>:0]
Destination
Addr[47:0]
SFD[7:0]
Preamble
[47:0]
CRC32
[31:0]
PAD [<s>-1:0]
Source
Addr[47:0]
Start[7:0]
EFD[7:0]
Figure 3-21: Flow of Frame Through the MAC RX With Preamble Pass-Through Turned On
Illustrates the typical flow of frame through the MAC RX when the preamble pass-through feature is
turned on. In this figure, <p> is payload size (0–1500 bytes), and <s> is the number of pad bytes (0–46
bytes).
Client - MAC Rx Interface
Client Frame
MAC Frame
Destination
Addr[47:0]
Source
Addr[47:0]
Type/
Length[15:0]
Payload
[<p-1>:0]
Destination
Addr[47:0]
SFD[7:0]
Preamble
[47:0]
CRC32
[31:0]
PAD [<s>-1:0]
Source
Addr[47:0]
Start[7:0]
SFD[7:0]
Preamble
[47:0]
Start[7:0]
EFD[7:0]
In these figures, normal packet data, highlighted in gray, lasts until the end of the payload section.
However, the IP core might pass along additional padding, marked with PAD, to ensure that the frame
length is at least 64 bytes. The Ethernet standard requires padding insertion when the payload length is
less than 46 bytes. EOP at the RX interface is normally marked after padding, but you can disable CRC
removal to place the EOP at the end of the CRC block.
When enabled, RX automatic pad removal moves the EOP marker to the end of the payload as indicated
in the length field, whether padding has been inserted or not. If the length is greater than 46 bytes, no
padding has been inserted and this feature has no effect. When you enable RX automatic pad removal, the
CRC is excluded from the EOP marker on packets that have stripped padding, and enabling CRC
retention has no effect on padded packets.
Note: Signals ending in
*_fcs_valid
and
*_fcs_error
are not shifted along with the new EOP marker.
Instead, they function as if pad removal were disabled. Do not rely on these signals when operating
in RX automatic pad removal mode.
The
PAD_CONFIG
register controls RX automatic pad removal. By default, pad removal is disabled.
Statistics counting is not affected by RX automatic pad removal; data reports as default, as if the padding
were not removed.
UG-01088
2014.12.15
RX Automatic Pad Removal Control
3-23
Functional Description
Altera Corporation