Overwrite Interface - 2.3 English

Versal Devices Integrated 100G Multirate Ethernet MAC Subsystem Product Guide (PG314)

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2.3 English

The straightforward available adjustment method is to perform a direct update of the system_timer using the Overwrite interface. In this procedure, the user logic provides its current timer value at regular intervals. The per-port inputs ctl_tx/rx_ptp_systemtimer_N are used to provide the master clock value. A transition on ctl_tx/rx_ptp_st_sync_N triggers the overwrite. Both of these signals are latched on the master time-of-day clock, ts_tx/rx_clk_N.

The overwrite mechanism is very effective. An overwrite interval of 64 ns, using the default settings for the timer_increment, allows the MRMAC to maintain the target accuracy even with the clock at ±200 PPM.

During the overwrite process, a fixed offset can be applied to the ctl_tx/rx_ptp_systemtimer_N value using the ctl_tx/rx_st_offset_N input. This allows the system to account for any system latency (for example, latency due to re-sampling delays). It also allows you to move the reference plane, the point in the datapath that the timestamp represents.
Note: By default, the offset should be set to 4.75x the clock period.

When the master time-of-day clock domain ts_tx/rx_clk_N and the ctl_tx/rx_ptp_st_sync_N signals are asynchronous to the local clock, it is possible that the new system timer value is retimed and sampled in the wrong clock cycle, leading to unwanted inaccuracy. To counter this, an embedded DDR phase detector automatically adjusts the provided system_timer value to account for whether the new timer value was sampled on a rising or falling clock edge. This is the phase adjust function depicted in the diagram.

If additional accuracy is required, the user logic can overwrite the system timer at start-up and then continually monitor the stat_tx/rx_ptp_systemtimer_N making frequency and phase adjustments to the system timer as necessary to precisely match the master timer. Using this method, accuracy can be improved significantly.