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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
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Figure 8-3: Time Stamping Position
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.
80 | www.xilinx.com | Ethernet AVB Endpoint User Guide |
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| UG492 July 23, 2010 |