Xilinx Virtex-4 User Manual page 62

Chapter 3: Client, Host, and MDIO Interfaces
Flow Control Basics
An Ethernet MAC transmits a pause control frame for the link partner to cease
transmission for a defined period of time. For example, the left Ethernet MAC of
Figure 3-28
state.
An Ethernet MAC responds 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
right Ethernet MAC of
frame transmitted by the left Ethernet MAC. In a well-designed system, the right Ethernet
MAC ceases transmission before the client FIFO of the left Ethernet MAC is overflowed.
This provides time to empty the FIFO to a safe level before normal operation resumes. It
also safe guards the system against FIFO overflow conditions and frame loss.
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Client Logic MAC
Tx
Rx
FIFO
Figure 3-28: Requirement for Flow Control
initiates a pause request when the client FIFO (illustrated) reaches a nearly full
Figure 3-28
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125 MHz –100 ppm
125 MHz +100 ppm
ceases transmission after receiving the pause control
Embedded Tri-Mode Ethernet MAC User Guide
MAC
Rx
Tx
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