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80 www.xilinx.com Ethernet AVB Endpoint User Guide
UG492 July 23, 2010
Chapter 8: Real Time Clock and Time Stamping
Time Stamp Sampling Position of MAC Frames
A time stamp value should be sampled at the beginning of the first symbol following the
Start of Frame Delimiter (SFD) of the Ethernet MAC frame as seen on the PHY. This is
illustrated in Figure 8-3.
Figure 8-3 also illustrates the actual time stamp sampling position that is used by the core.
Time stamps are taken after the MAC frame SFD is seen not on the GMII, but on the MAC
Client I/F. The time stamping logic is deliberately designed this way for the following
reasons:
1. When the Ethernet AVB Endpoint core is to be connected to the Embedded Tri-Mode
Ethernet MAC, the GMII is not always available to the FPGA fabric logic: specifically
when used with a 1000BASE-X or SGMII physical interface, the GMII exists only as an
internal connection within the embedded block. Therefore, by sampling on the client
interface, we enable the Ethernet AVB Endpoint core to be connected to ANY Xilinx
Tri-Mode MAC used in ANY configuration.
2. Sampling on the MAC Client I/F provides the Ethernet AVB Endpoint core with the
required time stamp exactly when it is needed. Sampling on the GMII would require
the use of sideband Time stamp Value FIFOs (there may be more than a single MAC
frame present in the pipeline stages of the MAC transmitter or receiver). So by
sampling on the MAC Client I/F, we are also able to reduce the need for extra FIFO
logic.
X-Ref Target - Figure 8-3
Figure 8-3: Time Stamping Position
Tri-Mode
Ethernet
MAC
LogiCORE
Ethernet
AVB
Endpoint
LogiCORE
legacy
traffic
AV
traffic
Ethernet
PHY
GMII
MAC
Client I/F
Tx
Tx
Rx Rx
PHY Media
IEEE defined Tx
sample position
IEEE defined Rx
sample position
Xilinx Rx
sample position
Xilinx Tx
sample position
known fixed
Tx latency
known fixed
Rx latency
PHY-specific
Tx latency
PHY-specific
Rx latency