Ethernet 1000BASE-X PCS/PMA or SGMII v9.1
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UG155 March 24, 2008
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Appendix B
Core Latency
Core Latency
The standalone core does not meet all the latency requirements specified in
IEEE 802.3
due
to the latency of the Elastic Buffers in both TBI and RocketIO transceiver versions.
However, the core may be used for backplane and other applications where strict
adherence to the IEEE latency specification is not required.
Where strict adherence to the
IEEE 802.3
specification is required, the core may be used
with an Ethernet MAC core that is within the IEEE specified latency for a MAC sublayer.
For example, when the core is connected to the Xilinx 1-Gigabit Ethernet MAC core, the
system as a whole is compliant with the overall
IEEE 802.3
latency specifications.
Latency for 1000BASE-X PCS with TBI
The following measurements are for the core only, and do not include any IOB registers or
the Transmitter Elastic Buffer added in the example design.
Transmit Path Latency
As measured from a data octet input into
gmii_txd[7:0]
of the transmitter side GMII
until that data appears on
tx_code_group[9:0]
on the TBI interface, the latency
through the core in the transmit direction is 5 clock periods of
gtx_clk
.
Receive Path Latency
Measured from a data octet input into the core on
rx_code_group0[9:0]
or
rx_code_group1[9:0]
from the TBI interface (until that data appears on
gmii_rxd[7:0]
of the receiver side GMII), the latency through the core in the receive
direction is equal to 16 clock periods of
gtx_clk,
plus an additional number of clock
cycles equal to the current value of the Receiver Elastic Buffer.
The Receiver Elastic Buffer is 32 words deep. The nominal occupancy will be at half-full,
thereby creating a nominal latency through the receiver side of the core equal to 16 + 16= 32
clock cycles of
gtx_clk
.
