Xilinx IP Ethernet AVB Endpoint v2.4 UG492 - Manual (Page 60)

60 www.xilinx.com Ethernet AVB Endpoint User Guide
UG492 September 21, 2010
Chapter 6: Ethernet AVB Endpoint Transmission
Figure 6-3 illustrates the timing of a normal frame transfer. When the AV client initiates a
frame transmission, it places the first column of data onto the av_tx_data[7:0] port
and asserts a logic 1 onto av_tx_valid.
After the Ethernet AVB Endpoint core reads the first byte of data, it asserts the av_tx_ack
signal. On the next and subsequent rising clock edges, the client must provide the
remainder of the data for the frame. The end of frame is signalled to the core by taking the
av_tx_valid
to logic 0.
In Figure 6-3 , following the end of frame transmission, the av_tx_done signal is held low,
which indicates to the “Tx Arbiter” that another AV frame is queued. Unless the
configurable bandwidth restrictions have been exceeded, this parks the “Tx Arbiter” onto
the AV traffic queue. Figure 6-3 then illustrates the client asserting the av_tx_valid
signal to request a subsequent frame, and the frame transmission cycle of Figure 6-3
repeats. However, if no further AV traffic frames are queued, the av_tx_done signal
should be set to logic 1 immediately following the end of frame transmission. This than
allows the
“Tx Arbiter” to schedule legacy traffic transmission (if any legacy frames are
queued).
If, following the end of frame reception, the bandwidth allocation for AV traffic has been
exceeded, the
“Tx Arbiter” switches to service the legacy traffic regardless of the state of
the av_tx_done signal.
For this reason, the av_tx_done signal should be considered an aid to the “Tx Arbiter” to
help make best use of the available network bandwidth. Asserting this signal after all AV
traffic has been serviced immediately allows the
“Tx Arbiter” to service the legacy traffic.
This helps achieve in excess of the 25% minimum allocation for the legacy traffic. However,
holding off the assertion of av_tx_done will not act as cheat mode to exceed the
maximum bandwidth allocation for the AV traffic.
X-Ref Target - Figure 6-3
Figure 6-3: Normal Frame Transmission across the AV Traffic Interface
tx_clk
av_tx_data[7:0]
av_tx_data_valid
av_tx_done
av_tx_ack
DA SA DATA L/T
tx_clk_enable
DA