Xilinx LOGICORE UG144 - Manual (Page 59)

1-Gigabit Ethernet MAC v8.5 User Guide www.xilinx.com 59
UG144 April 24, 2009
Flow Control Implementation Example R
-- DISCONTINUED PRODUCT --
Operation
Figure 6-4 illustrates the FIFO occupancy over a period of time.
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.
Figure 6-4: Flow Control Implementation Triggered from FIFO Occupancy
time
FIFO occupanc
y
3/4
7/8
5/8
1/2
Full
A
B
C