The clocking architecture for the Low Latency 40/50G MAC with PCS/PMA clocking is illustrated in the following figure. Low latency is achieved by omitting the RX FIFO, which results in different clocking arrangement. There are two clock domains in the datapath, as illustrated by the dashed lines in the following figure.
refclk_p0, refclk_n0, tx_serdes_refclk
The refclk
differential pair is required to be an
input to the FPGA. The example design includes a buffer to convert this clock to a
single-ended signal refclk
, which is used as the
reference clock for the GT block. The tx_serdes_refclk
is directly derived from refclk
. Note that refclk
must be
chosen so that the tx_serdes_refclk
meets the
requirements of 802.3, which is within 100 ppm of 312.5 MHz for 40G, and 390.625 MHz
for 50G.
tx_clk_out
This clock is used for clocking data into the TX AXI4-Stream
Interface and it is also the reference clock for the TX control and status signals.
It is the same frequency as tx_serdes_refclk
.
Because there is no TX FIFO, you must respond immediately to the tx_axis_tready
signal.
rx_clk_out
The rx_clk_out
output signal is presented as a
reference for the RX control and status signals processed by the RX core. It is the
same frequency as the rx_serdes_clk
. Because there
is no RX FIFO, this is also the clock which drives the RX AXI4-Stream Interface. In this arrangement, rx_clk_out
and tx_clk_out
are
different frequencies and have no defined phase relationship to each other.
dclk
The dclk
signal must be a convenient stable clock.
It is used as a reference frequency for the GT helper blocks which initiate the GT
itself. In the example design, a typical value is 75 MHz, which is readily derived
from the 300 MHz clock available on the VCU107 evaluation board. Note that the
actual frequency must be known to the GT helper blocks for proper operation.