Datasheet

Data Sheet AD9559
Rev. C | Page 33 of 120
The AD9559 evaluation software includes a frequency planning
wizard that configures the profile parameters, based on the
input and output frequencies.
REFERENCE SWITCHOVER
An attractive feature of the AD9559 is its versatile reference
switchover capability. The flexibility of the reference switchover
functionality resides in a sophisticated prioritization algorithm
that is 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 user mode
registers in the PLL_0/PLL_1 operational controls registers. The
reference switching mode bits (Bits[4:2] in Register 0x0A22 for
DPLL_0 and Register 0x0A42 for DPLL_1) allow the user to
select one of the five operating modes of the reference
switchover state machine, as follows:
• Automatic revertive mode
• Automatic nonrevertive mode
• Manual with automatic fallback mode
• Manual with automatic holdover mode
• Full manual mode without holdover
In the automatic modes, a fully automatic priority-based algorithm
selects the active reference. When programmed for an automatic
mode, the device chooses the highest priority valid reference.
When two or more references have the same priority, REFA has
preference over REFB, and so on in alphabetical order. However,
the reference position is used only as a tiebreaker and does not
initiate a reference switch.
The following list gives an overview of the five operating modes:
• Automatic revertive mode. The device selects the highest
priority valid reference and switches to a higher priority
reference if it becomes available, even if the reference in use
is still valid. In this mode, the user reference is ignored.
• Automatic nonrevertive mode. The device stays with the
currently selected reference as long as it is valid, even if
a higher priority reference becomes available. The user
reference is ignored in this mode.
• Manual with automatic fallback mode. The device uses the
user reference for as long as it is valid. If it becomes invalid,
the reference input with the highest priority is chosen in
accordance with the priority-based algorithm.
• Manual with automatic holdover mode. The user reference
is the active reference until it becomes invalid. At that
point, the device automatically goes into holdover.
• Full manual mode without holdover. The user reference is
the active reference, regardless of whether or not it is valid.
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. In free run mode, the free run frequency tuning
word register defines the free run output frequency. In holdover
mode, the output frequency depends on the holdover control
settings (see the Holdover section).
Phase Build-Out Reference Switching
The AD9559 supports phase build-out reference switching,
which is the term given to a reference switchover that
completely masks any phase difference between the previous
reference and the new reference. That is, there is virtually no
phase change detectable at the output when a phase build-out
switchover occurs.