Xilinx UG144 manual Flow Control Implementation Example

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Flow Control Implementation Example

R

Operation

Figure 6-4illustrates the FIFO occupancy over a period of time.

	Full
	A	C
FIFO occupancy	7/8
	3/4
		B
	5/8
	1/2
		time

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

The following describes the sequence of flow control operation.

1.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.

2.On receiving the pause control frame, the link partner MAC ceases transmission.

3.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).

4.On receiving this second pause control frame, the link partner MAC resumes transmission.

5.Normal operation resumes and the FIFO occupancy again gradually increases over time. At point C, this Flow Control cycle repeats.

1-Gigabit Ethernet MAC v8.5 User Guide www.xilinx.com 59

UG144 April 24, 2009