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Ethernet AVB Endpoint User Guide www.xilinx.com 81
UG492 July 23, 2010
IEEE1722 Real Time Clock Format
Because the Xilinx Tri-Mode Ethernet MACs have a known fixed latency, the time stamps
taken can easily be translated into the equivalent GMII position to comply with the
standard. This is performed in the software drivers where the MAC transmitter and
receiver latencies are held in #defines in a header file. The software drivers also contain
placeholder #defines for users to input the PHY-specific latency values for the PHYs used
in the system.
IEEE1722 Real Time Clock Format
The IEEE1722 specification defines the avbtp_timestamp field. This is derived by sampling
the IEEE802.1 AS Real Time Clock and converting the low order time to nanoseconds.
From version 2.1 onwards, this conversion is now performed in the Ethernet AVB
Endpoint core and an alternative RTC, in the 1722 format, is output on the
rtc_nanosec_field_1722[31:0] port.
This port contains a 32-bit word representing nanosecond values. Unlike the IEEE802.1 AS
nanosecond field (which resets back to zero when it reaches 1 second), the IEEE1722
nanosecond field counts fully to 0xFFFFFFFF before wrapping around. The field therefore
wraps around approximately every 4 seconds.
If the system is using the IEEE1722 functionality, this port can be sampled to create the
avbtp_timestamp field. Otherwise this port can be ignored.