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Chapter 6: Using Flow Control

The user MAC on the left side has a reference clock slightly slower than the nominal

125 MHz. The link partner MAC on the right side has a reference clock slightly faster than the nominal 125 MHz. As a result, the user MAC receives data at a faster line rate than that at which it can transmit. The MAC on the left is shown performing a loopback implementation which results in the FIFO filling up over time. Without Flow Control, this FIFO will eventually fill and overflow, resulting in the corruption or loss of Ethernet frames. Enabling Flow Control in the MAC provides a mechanism to solve this data rate matching problem.

Flow Control Basics

A MAC may transmit a pause control frame to request that its link partner cease transmission for a defined period of time. For example, the user MAC on the left side of Figure 6-1may initiate a pause request when its client FIFO (illustrated) reaches a nearly full state.

A MAC should respond to received pause control frames by ceasing transmission of frames for the period of time defined in the received pause control frame. For example, the link partner MAC in Figure 6-1may cease transmission after receiving the pause control frame transmitted by the user MAC. In a well designed system, the link partner MAC would cease transmission before the client FIFO experienced an overflow condition. This provides time for the FIFO to be emptied to a safe level before normal operation resumes, thus safeguarding the system against FIFO overflow conditions and frame loss.

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1-Gigabit Ethernet MAC v8.5 User Guide

 

 

UG144 April 24, 2009

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Xilinx UG144 manual Flow Control Basics, Using Flow Control

UG144 specifications

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