Altera 40-Gbps Ethernet MAC and PHY MegaCore Function - Manual (Page 69)

By default, the MAC RX removes all Start, SFD, preamble, and IPG bytes from accepted frames. However,
if you turn on the RX preamble pass-through feature, by setting bit 0 of the
Preamble Pass-Through
Configuration
register at offset 0x125, the MAC RX does not remove the eight-byte preamble sequence.
Related Information
MAC Feature Configuration Registers on page 3-105
Information about the
Preamble Pass-Through Configuration
register.
40-100GbE IP Core FCS (CRC-32) Removal
Independent user configuration register bits control FCS CRC removal at runtime. CRC removal supports
both narrow and wide bus options. Bit 1 of the
CRC_CONFIG
register enables and disables CRC removal; by
default, CRC removal is enabled.
In the user interface, the EOP signal (
l<n>_rx_endofpacket
or
dout_last_data
) indicates the end of
CRC data if CRC is not removed. When CRC is removed, the EOP signal indicates the final byte of
payload.
By default, the IP core asserts the FCS error signal (
l<n>_rx_fcs_error
or
dout_fcs_error)
and the
EOP signal on the same clock cycle if the current frame has an FCS error. However, if the IP core is in RX
automatic pad removal mode, the signals might not be asserted in the same clock cycle.
Related Information
RX Automatic Pad Removal Control on page 3-22
40-100GbE IP Core CRC Checking
The 32-bit CRC field is received in the order: X32, X30, . . . X1, and X0 , where X32 is the most significant
bit of the FCS field and occupies the least significant bit position in the first FCS byte.
If a CRC32 error is detected, the RX MAC marks the frame invalid by asserting the
dout_fcs_error
and
dout_fcs_valid
signals.
When operating in the cut-through or store and forward mode, with Avalon–ST or the custom streaming
client interface, the FCS result is always preserved.
RX CRC Forwarding
The CRC-32 field is forwarded to the client interface after the final byte of data, if the CRC removal
option is not enabled.
Related Information
40-100GbE IP Core FCS (CRC-32) Removal on page 3-22
RX Automatic Pad Removal Control
In the 40GbE and 100GbE MAC configurations, you can enable and disable RX automatic pad removal
with a configuration register bit in run-time.
The following figures illustrate the normal format of received data at the MAC RX interface.
3-22 40-100GbE IP Core FCS (CRC-32) Removal
UG-01088
2014.12.15
Altera Corporation Functional Description
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