ZHCSOZ4 September 2022 LMK5B33414
PRODUCTION DATA
The DPLL domain has a tuning word history monitor block that determines the initial output frequency accuracy upon entry into holdover. When in holdover, the stability of the reference clock (on XO input) determines the long-term stability and accuracy of the output frequency. The tuning word can be updated from one of three sources depending on the DPLL operating mode:
When the history monitor is enabled and the DPLL is locked, the device averages the reference input frequency by accumulating history from the digital loop filter output during a programmable averaging time (TAVG) set by DPLLx_HIST_TIMER. When a valid reference input becomes invalid, the final tuning word value is stored to determine the initial holdover frequency accuracy. Generally, a longer TAVG time will produce a more accurate initial holdover frequency.
If the input reference clock fails and becomes invalid, the history data can be corrupted if the tuning word continues to update before the fail state is indicated by one of the reference input validation monitors. To avoid this scenario, any in progress accumulation is ignored and the recent history data is ignored. The most recent collected average data is discarded such that the actual history used is greater than TAVG but less than 2 × TAVG.
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 TAVG 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 (DPLLx_HIST_EN = 1 → 0 → 1), if needed.
When no tuning word history exists, the free-run tuning word value (DPLLx_FREE_RUN) is used and determines the initial holdover output frequency accuracy.