Low Latency 100-Gbps Ethernet IP Core User Guide

ID 683160
Date 4/15/2021
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3.2.2.5.1. Order of Transmission

The IP core transmits bytes on the Ethernet link starting with the preamble and ending with the FCS in accordance with the IEEE 802.3 standard. Transmit frames the IP core receives on the client interface are big-endian. Frames the MAC sends to the PHY on the XGMII/CGMII between the MAC and the PHY are little-endian; the MAC TX transmits frames on this interface beginning with the least significant byte.

Figure 15. Byte Order on the Client Interface Lanes Without Preamble Pass-ThroughDescribes the byte order on the Avalon-ST interface when the preamble pass-through feature is turned off. Destination Address[40] is the broadcast/multicast bit (a type bit), and Destination Address[41] is a locally administered address bit.


For example, the destination MAC address includes the following six octets AC-DE-48-00-00-80. The first octet transmitted (octet 0 of the MAC address described in the 802.3 standard) is AC and the last octet transmitted (octet 7 of the MAC address) is 80. The first bit transmitted is the low-order bit of AC, a zero. The last bit transmitted is the high order bit of 80, a one.

The preceding table and the following figure show that in this example, 0xAC is driven on DA5 (DA[47:40]) and 0x80 is driven on DA0 (DA[7:0]).

Figure 16. Octet Transmission on the Avalon-ST Signals Without Preamble Pass-Through Illustrates how the octets of the client frame are transferred over the TX datapath when preamble pass-through is turned off.
Figure 17. Byte Order on the Avalon-ST Interface Lanes With Preamble Pass-ThroughDescribes the byte order on the Avalon-ST interface when the preamble pass-through feature is turned on.

Destination Address[40] is the broadcast/multicast bit (a type bit), and Destination Address[41] is a locally administered address bit.



Figure 18. Octet Transmission on the Avalon-ST Signals With Preamble Pass-ThroughIllustrates how the octets of the client frame are transferred over the TX datapath when preamble pass-through is turned on. The eight preamble bytes precede the destination address bytes. The preamble bytes are reversed: the application must drive the Start byte on l8_tx_data[455:448] and the SFD byte on l8_tx_data[511:504] .

The destination address and source address bytes follow the preamble pass-through in the same order as in the case without preamble pass-through.

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