Ethernet Design Example Components User Guide

ID 683044
Date 9/27/2021
Public

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1.4.2. Adjusting Offset, Jitter, and Wander

The TOD clock supports several types of adjustments:
  • Offset—use the OffsetNS and OffsetFNS registers to adjust large offsets in assisting faster system convergence. The offset can be positive or negative. The maximum correction is (109 - 50) ns.
  • Jitter—use the JitterTimer and JitterAdjust registers to achieve small time scales (milliseconds or microseconds) frequency correction.

    The jitter adjustment can either be a positive or negative adjustment per unit time. This helps achieve better frequency corrections. For very low values of the jitter, such as 1 ns correction for every second, the timer must be larger and the adjustment value must be smaller.

    For example, to achieve 1 ns correction every second in a clock domain of 3.2 ns period, configure the registers as follow:
    • JitterTimer = 0x12A05F20, which is the hexadecimal value of (1000000000/3.2).
    • JitterAdjust = 0x10000, which sets the nanosecond field to 0x1 and the fractional nanosecond field to 0x0.
  • Wander—use the WanderTimeLSB, WanderTimeMSB, and WanderAdjust registers to achieve large time scale correction.

    The wander adjustment can either be a positive or negative adjustment per unit time. Wander adjustments are typically on larger time scales such as per hour. For very low values of the wander such as 1 ns per 24 hours, the timer must be larger and the adjustment value must be smaller.

    For example, to achieve 1 ns correction every 24 hours in a clock domain of 3.2 ns period, configure the registers as follow:
    • WanderTimerLSB[29:0] = 0x2D68_B000
    • WanderTimerMSB[15:0] = 0x6239
Note: The adjustments are available only when PERIOD_CLOCK_FREQUENCY is set to 0.