AD9548 datasheet(31/111 Pages) AD

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Data Sheet

AD9548

Rev. G | Page 31 of 111

The promoted priority parameter allows the user to assign a

higher priority to a reference after it becomes the active

reference. For example, suppose four references have a selection

priority of 3 and a promoted priority of 1, and the remaining

references have a selection priority or 2 and a promoted priority

of 2. Now, assume that one of the Priority 3 references becomes

active because all of the Priority 2 references have failed. Some-

time later, however, a Priority 2 reference becomes valid. The

switchover logic normally attempts to automatically switch over

to the Priority 2 reference because it has higher priority than the

presently active Priority 3 reference. However, because the

Priority 3 reference is active, its promoted priority of 1 is in

effect. This is a higher priority than the newly validated

reference’s priority of 2, so the switchover does not occur. This

mechanism enables the user to give references preferential

treatment while they are selected as the active reference. An

example of promoted vs. nonpromoted priority switching

appears in state diagram form in Figure 37. Figure 38 shows a

block diagram of the interrelationship between the reference

inputs, monitors, validation logic, profile selection, and priority

selection functionality.

ACTIVE

A FAULTED

B FAULTED

ALL VALID

INITIAL

STATE

A VALID

B VALID

A VALID

B VALID

INPUT

PRIORITY

PROMOTED

PRIORITY TABLE

COMMON

WITHOUT PROMOTION

WITH PROMOTION

Figure 37. Example of Priority Promotion

PROFILE

SELECTION

VALIDATION

LOGIC

PRIORITY

SELECTION

÷R

MONITORS

A/AA

B/BB

C/CC

D/DD

TDC

LOOP

CONTROLLER

Figure 38. Reference Clock Block Diagram

AD9548 Datasheet(PDF) 31 Page - Analog Devices

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