LMK05028RGCT.B datasheet(39/94 Pages) TI2

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7.3.8.5 Tuning Word History

Each REF-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, the monitor effectively averages the reference

input frequency by accumulating history from the digital loop filter output during a programmable averaging time

(TAVG). Once the input becomes invalid, the final tuning word value is stored to determine the initial holdover

frequency accuracy. Generally, a longer TAVG time produces a more accurate initial holdover frequency. The

stability of the 0-ppm reference clock (XO or TCXO input) determines the long-term stability and accuracy of the

holdover output frequency.

There is also a separate programmable delay timer (TIGN) that can be set to ignore the history data that is

corrupted just prior to entry into holdover. The history data can be corrupted if a tuning word update occurs

while the input clock is failing and before the input clock is detected by the input monitors. Both TAVG and TIGN

times are programmable through the HISTCNT and HISTDLY register bits, respectively, and are related to the

REF-TDC rate.

The tuning word history is initial cleared after a device hard reset or soft reset. The history monitor begins to

accumulate history once the DPLL locks to a new reference. The previous history is cleared when a switchover

to a new reference occurs assuming the history persistence bit (HIST_HOLD) is not set. The history can be

manually cleared by asserting the history soft reset bit (HIST_SW_RST). If the history persistence bit is set, the

history monitor is not clear the previous history value during reference switchover, holdover exit, or history soft

reset. Whenever the tuning word is cleared, the history monitor waits for the first TAVG timer to expire before

storing the first tuning word value.

Ref Lost

LORÆ 1

History

Reset

History Valid

Holdover

Locked

Ref Valid

LORÆ 0

Free Run

Lock Acq.

Locked

No History

Lock Acq.

Ref Valid

LORÆ 0

Initial holdover

frequency determined

by averaged history.

History Count*

Timer to average history data to

compute initial holdover frequency accuracy.

History Delay*

Delay to ignore

history updates

prior to LOR.

History held*

or cleared on

holdover exit.

Time

History Data Accumulating

LOFL = 0, LOPL = 1

LOFL = 0, LOPLÆ 0

LOFLÆ 0, then LOPLÆ 0

LOFL = 1, LOPL = 1

*Programmable

settings

LOFL = 0, LOPLÆ 1

History Data Accumulating

TAVG(0)

TAVG(1)

TAVG(2..n)

TIGN

Initial start of history

when LOFLÆ 0 only

Figure 7-23. Tuning Word History Windows

If the TAVG period is set very long (minutes or hours) to obtain a more precise historical average frequency, a

switchover or holdover event can 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 TAVG/2K , where K = HIST_INTMD to 0, during the first TAVG period only.

If HIST_INTMD = 0, there is no intermediate update and the first average is stored after the first TAVG period.

However, if HIST_INTMD = 4, then four intermediate averages are taken at TAVG/16, TAVG/8, TAVG/4, and TAVG/2,

as well as at TAVG. After the first TAVG period, all subsequent history updates occur at the TAVG period.

When no tuning word history exists, the free-run tuning word value (TUNING_FREE_RUN) determines the initial

holdover output frequency accuracy.

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LMK05028

SNAS724B – FEBRUARY 2018 – REVISED FEBRUARY 2025

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