Figure 6-4: Flow Control Implementation Triggered From Fifo Occupancy - Xilinx LogiCORE IP MAC v8.5 User Manual

Flow Control Implementation Example
Operation
Figure 6-4
The following describes the sequence of flow control operation.
1.
2.
3.
4.
5.
1-Gigabit Ethernet MAC v8.5 User Guide
UG144 April 24, 2009
-- DISCONTINUED PRODUCT --
illustrates the FIFO occupancy over a period of time.
Full
7/8
3/4
5/8
1/2

Figure 6-4: Flow Control Implementation Triggered from FIFO Occupancy

The average FIFO occupancy of the user system gradually increases over time due to
the clock tolerances. At point A, the occupancy has reached the threshold of 7/8
occupancy. This triggers the maximum duration pause control frame request.
On receiving the pause control frame, the link partner MAC ceases transmission.
After the link partner MAC ceases transmission, the occupancy of the FIFO in the user
system rapidly empties. The occupancy falls to the second threshold of 3/4 occupancy
at point B. This triggers the zero duration pause control frame request (the pause
cancel command).
On receiving this second pause control frame, the link partner MAC resumes
transmission.
Normal operation resumes and the FIFO occupancy again gradually increases over
time. At point C, this Flow Control cycle repeats.
www.xilinx.com
A C
B
R
time
59