F-Tile Ethernet Intel® FPGA Hard IP User Guide

ID 683023
Date 3/28/2022
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7.2.3. Using MAC Avalon ST skip_crc Signal to Control Source Address, PAD, and CRC Insertion

The i_tx_skip_crc port allows the CRC to turn on and off per packet basis.
Figure 30. Using tx_skip_crc

Some systems have multiple streams of data feeding the same Ethernet link. If some of those streams provide packets with CRC already calculated, you can use the i_tx_skip_crc signal to not allow their recalculation. Some systems use frames smaller than the Ethernet minimum size. In this case, you must disable the frame padding.

Some protocols modify the CRC to indicate special conditions. You can use the i_tx_skip_crc signal to pass these special CRCs to the line without turning CRC off for all packets. Use the i_tx_skip_crc signal when bridging packets and respacing them without changing the received CRC.

Table 35.  Consequences of Asserting the tx_skip_crc Signal
MAC Field i_tx_skip_crc = 0 i_tx_skip_crc = 1
Source Address When you enable Use Source Address Insertion, i_txmac_saddr replaces the content of the source address bytes. Regardless of Use Source Address Insertion status, the source address bytes are not replaced.
Padding Frames with size of 64 bytes of less are padded to 64 bytes. No padding is added.
CRC (Frame Check Sequence) Calculates the packet's CRC and append it to the end. No CRC calculation. Instead, uses last four bytes of the i_tx_data as a CRC value.