AD9547/PCBZ Analog Devices Inc, AD9547/PCBZ Datasheet - Page 30

• Manufacturer Part Number: AD9547/PCBZ
• Description: Clock Generator/Synchronizer Evaluation Board
• Manufacturer: Analog Devices Inc

Specifications of AD9547/PCBZ

• Silicon Manufacturer: Analog Devices
• Application Sub Type: Network Clock Generator/Synchronizer
• Kit Application Type: Clock & Timing
• Silicon Core Number: AD9547
• Main Purpose: Timing, Clock Generator
• Embedded: No
• Utilized Ic / Part: AD9547
• Primary Attributes: 2 Differential or 4 Single Ended Inputs
• Secondary Attributes: CMOS, LVPECL & LVDS Compatible
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD9547
As long as there are input references programmed for automatic
profile assignment, and for which the profile assignment is null,
the state machine continues to cycle through those references
searching for a profile match. Furthermore, unless an input
reference is assigned to a profile, it is considered invalid and
excluded as a candidate for a reference switchover.
REFERENCE SWITCHOVER
An attractive feature of the AD9547 is its versatile reference
switchover capability. The flexibility of the reference switchover
functionality resides in a sophisticated prioritization algorithm
coupled with register-based controls. This scheme provides the
user with maximum control over the state machine that handles
reference switchover.
The main reference switchover control resides in the loop mode
register (Address 0x0A01). The user selection mode bits (Bits[4:3])
allow the user to select one of the reference switchover state
machine’s four operating modes, as follows:
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•
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In automatic mode, a fully automatic, priority-based algorithm
selects the active reference. When programmed for automatic
mode, the device ignores the user reference selection bits (Register
0x0A01, Bits[1:0]). However, when programmed for any of the
other three modes, the device uses the user reference bits. They
specify a particular input reference (00 = REF A, 01 = REF AA ,
10 = REF B, 11 = REF BB).
In fallback mode, the user reference is the active reference when
it is valid. Otherwise, the device switches to a new reference using
the automatic priority-based algorithm.
In holdover mode, the user reference is the active reference when
it is valid. Otherwise, the device switches to holdover mode.
In manual mode, the user reference is the active reference whether
it is valid or not. Note that, when using this mode, the user must
program the reference-to-profile assignment (see Register 0x0503
and Register 0x0504) as manual for the particular reference that
is declared as the user reference. The reason is that if the user refer-
ence fails and its redetect timer expires, its profile assignment
(shown in Table 22) becomes null. This means that the active
reference (user reference) does not have an assigned profile, which
places the AD9547 into an undefined state.
The user also has the option to force the device directly into
holdover or free-run operation via the user holdover and user
free-run bits (Register 0x0A01, Bits[6:5]). In free-run mode, the
free-running frequency tuning word register (Address 0x0300
to Address 0x0305) defines the DDS output frequency.
In holdover mode, the DDS output frequency depends on the
holdover control settings (see the Holdover section).
Automatic mode (Address 0x0A01, Bits[4:3] = 00)
Fallback mode (Address 0x0A01, Bits[4:3] = 01)
Holdover mode (Address 0x0A01, Bits[4:3] = 10)
Manual mode (Address 0x0A01, Bits[4:3] = 11)
Rev. B | Page 30 of 104
Automatic Priority-Based Reference Switchover
The AD9547 has a two-tiered, automatic, priority-based algorithm
that is in effect for both automatic and fallback reference switch-
over. The algorithm relies on the fact that each reference profile
contains both a selection priority and a promoted priority. The
selection and promoted priority values range from 0 (highest
priority) to 7 (lowest priority). The selection priority determines
the order in which references are chosen as the active reference.
The promoted priority is a separate priority value given to a
reference only after it becomes the active reference.
An automatic reference switchover occurs on failure of the active
reference or when a previously failed reference becomes valid
and its selection priority is higher than the promoted priority
of the currently active reference (assuming that the automatic or
fallback reference switchover is in effect). When performing an
automatic reference switchover, the AD9547 chooses a reference
based on the priority settings within the profiles. That is, the device
switches to the reference with the highest selection priority (lowest
numeric priority value). It does so by using the reference-to-profile
table (see Table 22) to determine the reference associated with
the profile exhibiting the highest priority.
If multiple references share the same profile, the device chooses
the reference having the lowest index value. For example, if the
A, B, and BB references (Index 0, Index 2, and Index 3, respectively)
share the same profile, a switchover to Reference A occurs because
Reference A has the lowest index value. Note, however, that only
valid references are included in switchover of the selection process.
The switchover control logic ignores any reference with a status
indication of invalid.
The promoted priority parameter allows the user to assign a higher
priority to a reference after it becomes the active reference. For
example, suppose that two references have a selection priority of 3
and a promoted priority of 1, and the remaining references have
a selection priority of 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. Sometime 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 refer-
ence. An example of promoted vs. nonpromoted priority switching
appears in state diagram form in Figure 33. Figure 34 shows a
block diagram of the interrelationship between the reference
inputs, monitors, validation logic, profile selection, and priority
selection functionality.