Ethernet Design Example Components User Guide

ID 683044
Date 9/27/2021

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1.4.1. Adjusting TOD Drift

You can use the DriftAdjust and DriftAdjustRate registers to correct drifts in the TOD clock due to insufficient binary representation of the 16-bit fractional nanosecond field in the Period register.

For example, the Period register is set to 6.4 ns for a 10G Ethernet application. The hexadecimal representation of this value is 0x6 ns and 0x6666.4 fns.
  • Fractional nanosecond field is 16 bits wide: 0.4 fns = 0.4 * 216 = 26214.4 in decimal.
  • Converting to hexadecimal: 26214 + 0.4 = 0x6666 + 0x0000.4 = 0x6666.4 fns.
The fractional nanosecond value, 0x0000.4, cannot be represented in 16 bits thus causing the time of day to drift from the actual time by 0x0002 fns every 5 clock cycles. In other words, the time of day drifts 953.6 ns every 1 second. To correct this situation, configure the registers as follow:
  • DriftAdjust = 0x02, which sets the nanosecond field to 0x0 and the fractional nanosecond field to 0x2.
  • DriftAdjustRate = 0x5.