AD9547SLASHPCBZ datasheet(30/104 Pages) AD

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AD9547

Rev. 0 | Page 30 of 104

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

allow the user to select one of the reference switchover state

machine’s four operating modes, as follows:

•

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)

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 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.

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