LMK04832PAPSLASHEM datasheet(33/103 Pages) TI

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8.3.7.1 Calculating Digital Lock Detect Frequency Accuracy

See Digital Lock Detect Frequency Accuracy for more detailed information on programming the registers to

achieve a specified frequency accuracy in ppm with lock detect.

The digital lock detect feature can also be used with holdover to automatically exit holdover mode. See Exiting

Holdover for more information.

8.3.8 Holdover

Holdover mode causes PLL2 to stay locked on frequency with minimal frequency drift when an input clock

reference to PLL1 becomes invalid. While in holdover mode, the PLL1 charge pump is TRI-STATED and a fixed

tuning voltage is set on CPout1 to operate PLL1 in open loop.

8.3.8.1 Enable Holdover

Program HOLDOVER_EN = 1 to enable holdover mode.

Holdover mode can be configured to set the CPout1 voltage upon holdover entry to a fixed user defined voltage

(EN_MAN_DAC = 1) or a tracked voltage (EN_MAN_DAC = 0).

8.3.8.1.1 Fixed (Manual) CPout1 Holdover Mode

By programming MAN_DAC_EN = 1, then the MAN_DAC value will be set on the CPout1 pin during holdover.

The user can optionally enable CPout1 voltage tracking (TRACK_EN = 1), read back the tracked DAC value,

then re-program MAN_DAC value to a user desired value based on information from previous DAC read backs.

This allows the most user control over the holdover CPout1 voltage, but also requires more user intervention.

8.3.8.1.2 Tracked CPout1 Holdover Mode

By programming MAN_DAC_EN = 0 and TRACK_EN = 1, the tracked voltage of CPout1 is set on the CPout1

pin during holdover. When the DAC has acquired the current CPout1 voltage, the DAC_Locked signal is set,

which may be observed on Status_LD1 or Status_LD2 pins by programming PLL1_LD_MUX or PLL2_LD_MUX,

respectively.

Updates to the DAC value for the Tracked CPout1 sub-mode occurs at the rate of the PLL1 phase detector

frequency divided by (DAC_CLK_MULT × DAC_CLK_CNTR).

The DAC update rate should be programmed for ≤ 100 kHz to ensure DAC holdover accuracy.

The ability to program slow DAC update rates, for example one DAC update per 4.08 seconds when using

1024-kHz PLL1 phase detector frequency with DAC_CLK_MULT = 16,384 and DAC_CLK_CNTR = 255, allows

the device to look-back and set CPout1 at a previous good CPout1 tuning voltage values before the event which

caused holdover to occur.

The current voltage of DAC value can be read back using RB_DAC_VALUE, see the RB_DAC_VALUE section.

8.3.8.2 During Holdover

PLL1 is run in open-loop mode.

• PLL1 charge pump is set to TRI-STATE.

• PLL1 DLD is unasserted.

• The HOLDOVER status is asserted

• During holdover, if PLL2 was locked prior to entry of holdover mode, PLL2 DLD continues to be asserted.

• CPout1 voltage is set to:

– a voltage set in the MAN_DAC register (MAN_DAC_EN = 1).

– a voltage determined to be the last valid CPout1 voltage (MAN_DAC_EN = 0).

• PLL1 attempts to lock with the active clock input.

The HOLDOVER status signal can be monitored on the Status_LD1 or Status_LD2 pin by programming the

PLL1_DLD_MUX or PLL2_DLD_MUX register to Holdover Status.

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LMK04832-SEP

SNAS838 – OCTOBER 2022

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