Flow Control Basics - Xilinx LogiCORE IP MAC v8.5 User Manual

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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 6-1 may 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 6-1 may 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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UG144 April 24, 2009