Preemption Header Insertion/Extraction - 1.3 English

Versal Devices Integrated 100G Multirate Ethernet MAC Subsystem Product Guide (PG314)

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1.3 English

Preemption leverages the MRMAC custom preamble features to insert and extract the necessary Preamble, SMD, and FRAG_COUNT fields. Insertion and extraction of custom preamble is enabled on a per-port basis by setting the ctl_tx_custom_preamble_enable_<N> field of the CONFIGURATION_TX_REG register to 1. This feature must be enabled for preemption to work properly.

When custom preamble is enabled, the user logic communicates the desired preamble field for outbound frames on a port-by-port, packet-by-packet basis by means of the signals tx_preamblein_N[55:0]. In the receive direction, the signals rx_preambleout_N[55:0] are used to communicate the extracted preamble from the received packets.

Note that the tx_preamblein/rx_preambleout signals are 7B wide. The first byte of preamble must always be set to 0x55, as required by IEEE 802.3. Consequently, it is not necessary to include this first byte in the in/out signals, as it is always the same. Byte 0 ([7:0]) of the in/out signals map to byte 1 ([15:8]) of the preamble field.