The DPLL domain has a tuning word history monitor block that determines the initial output frequency accuracy upon entry into holdover. The tuning word can be updated from one of three sources depending on the DPLL operating mode:

1. Locked Mode: From the output of the digital loop filter when locked
2. Holdover Mode: From the final output of the history monitor
3. Free Run Mode: From the free-run tuning word register (user defined)

When the history monitor is enabled and the DPLL is locked, it effectively averages the reference input frequency by accumulating history from the digital loop filter output during a programmable averaging time (T_AVG). Once the input becomes invalid, the final tuning word value is stored to determine the initial holdover frequency accuracy. Generally, a longer T_AVG time will produce a more accurate initial holdover frequency. The stability of the 0-ppm reference clock (XO input) determines the long-term stability and accuracy of the holdover output frequency.

There is also a separate programmable delay timer (T_IGN) that can be set to ignore the history data that is corrupted just prior to entry into holdover. The history data could be corrupted if a tuning word update occurs while the input clock is failing and before it is detected by the input monitors. Both T_AVG and T_IGN times are programmable through the HISTCNT and HISTDLY register bits, respectively, and are related to the TDC rate.

The tuning word history is initially cleared after a device hard-reset or soft-reset. After the DPLL locks to a new reference, the history monitor waits for the first T_AVG timer to expire before storing the first tuning word value and begins to accumulate history. The history monitor will not clear the previous history value during reference switchover or holdover exit. The history can be manually cleared or reset by toggling the history enable bit (HIST_EN = 1 → 0 → 1), if needed.

(1) History count and delay windows are programmable.

Figure 9-20 Tuning Word History Windows

If the T_AVG period is set very long (minutes or hours) to obtain a more precise historical average frequency, it is possible for a switchover or holdover event to occur before the first tuning word is stored and available for use. To overcome this, there is an intermediate history update option (HIST_INTMD). If the history is reset, then the intermediate average can be updated at intervals of T_AVG/2^K , where K = HIST_INTMD to 0, during the first T_AVG period only. If HIST_INTMD = 0, there is no intermediate update and the first average is stored after the first T_AVG period. However, if HIST_INTMD = 4, then four intermediate averages are taken at T_AVG/16, T_AVG/8, T_AVG/4, and T_AVG/2, as well as at T_AVG. After the first T_AVG period, all subsequent history updates occur at the T_AVG period.

When no tuning word history exists, the free-run tuning word value (TUNING_FREE_RUN) determines the initial holdover output frequency accuracy.