AD9558 datasheet(34/104 Pages) AD

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AD9558

Data Sheet

Rev. A | Page 34 of 104

LOOP CONTROL STATE MACHINE

Switchover

Switchover occurs when the loop controller switches directly

from one input reference to another. The AD9558 handles a

reference switchover by briefly entering holdover mode, loading

the new DPLL parameters, and then immediately recovering.

During the switchover event, however, the AD9558 preserves

the status of the lock detectors to avoid phantom unlock

indications.

Holdover

The holdover state of the DPLL is typically used when none of

the input references are present, although the user can also

manually engage holdover mode. In holdover mode, the output

frequency remains constant. The accuracy of the AD9558 in

holdover mode is dependent on the device programming and

availability of tuning word history.

Recovery from Holdover

When in holdover mode and a valid reference becomes

available, the device exits holdover operation. The loop state

machine restores the DPLL to closed-loop operation, locks to

the selected reference, and sequences the recovery of all the

loop parameters based on the profile settings for the active

reference.

Note that, if the user holdover bit is set, the device does not

automatically exit holdover when a valid reference is available.

However, automatic recovery can occur after clearing the user

holdover bit (Bit 6 in Register 0x0A01).

AD9558 Datasheet(PDF) 34 Page - Analog Devices