Low Latency 40-Gbps Ethernet IP Core User Guide

ID 683745
Date 3/08/2021
Document Table of Contents

3.2.1. LL 40GbE IP Core TX Datapath

The TX MAC module receives the client payload data with the destination and source addresses and then adds, appends, or updates various header fields in accordance with the configuration specified. The MAC does not modify the destination address, the source address, or the payload received from the client. However, the TX MAC module adds a preamble (if the IP core is not configured to receive the preamble from user logic), pads the payload of frames greater than eight bytes to satisfy the minimum Ethernet frame payload of 46 bytes, and if you set Enable TX CRC insertion or turn on flow control, calculates the CRC over the entire MAC frame. (If padding is added, it is also included in the CRC calculation. If you turn off Enable TX CRC insertion, the client must provide the CRC bytes and must provide frames that have a minimum size of 64 bytes and therefore do not require padding). If you set Average interpacket gap to 8 or 12, the TX MAC module inserts IDLE bytes to maintain an average IPG. In addition, the TX MAC inserts an error in the Ethernet frame if the client requests to insert an error.

The LL 40GbE IP core does not process incoming frames of less than nine bytes correctly. You must ensure such frames do not reach the TX client interface.

Figure 12. Typical Client Frame at the Transmit InterfaceIllustrates the changes that the TX MAC makes to the client frame. This figure uses the following notational conventions:
  • <p> = payload size, which is arbitrarily large.
  • <s> = padding bytes = 0–46 bytes.
  • <g> = number of IPG bytes

The following sections describe the functions that the TX module performs: