Datasheet

Data Sheet AD9548
Rev. E | Page 31 of 112
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 36. Figure 37 shows a
block diagram of the interrelationship between the reference
inputs, monitors, validation logic, profile selection, and priority
selection functionality.
A
ACTIVE
B
ACTIVE
C
ACTIVE
A FAULTED
B FAULTED
ALL VALID
INITIAL
STATE
A VALID
B VALID
A VALID
B VALID
INPUT PRIORITY PROMOTED
A0 0
B1 0
C2 1
D3 2
PRIORITY TABLE
COMMON
WITHOUT PROMOTION
WITH PROMOTION
08022-011
Figure 36. Example of Priority Promotion
PROFILE
SELECTION
VALIDATION
LOGIC
PRIORITY
SELECTION
÷RMONITORS
A/AA
B/BB
C/CC
D/DD
TDC
LOOP
CONTROLLER
……
…
………
08022-012
Figure 37. Reference Clock Block Diagram