Xilinx IP Ethernet AVB Endpoint v2.4 UG492 - Manual (Page 62)

62 www.xilinx.com Ethernet AVB Endpoint User Guide
UG492 September 21, 2010
Chapter 6: Ethernet AVB Endpoint Transmission
Figure 6-4 illustrates the key features of the credit based algorithm, which are:
•
The Tx Arbiter will schedule queued transmission from the “Tx AV Traffic I/F” if the
algorithm is in credit (greater or equal to 0).
•
If there is less than 0 credit (not shown in Figure 6-4 , but the credit can sink below 0),
then the Tx Arbiter will not allow AV traffic to be transmitted; legacy traffic, if
queued, will be scheduled instead.
• When no AV traffic is queued, any positive credit will be lost and the credit is reset to
0.
• When AV traffic is queued, and until the time at which the Tx Arbiter is able to
schedule it (while waiting for an in-progress legacy frame to complete transmission),
credit can be gained at a rate defined by the idleSlope.
X-Ref Target - Figure 6-4
Figure 6-4: Credit-based Shaper Operation
hiLimit
loLimit
0
increasing
time
credit=0
when no frames
are waiting
idleSlope sendSlope
credits withdrawn
when no frames
are waiting
increasing
credit
number of AV
queued frames
0
transmitting
AV frame
transmitting
Legacy frame
TRUE
TRUE
FALSE
FALSE
1
conflicting legacy traffic present, so queued AV frame is not
transmitted until conflicting legacy frame has been transmitted