AD9548 datasheet(30/111 Pages) AD

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AD9548

Data Sheet

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 AD9548 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

(Address 0x0A01). The user selection mode bits (Register 0x0A01,

Bits[4:3]) allow the user to select one of the reference switchover

state machine’s four operating modes, as follows:

•

Automatic mode (Address A01, Bits[4:3] = 00)

•

Fallback mode (Address 0A01, Bits[4:3] = 01)

•

Holdover mode (Address 0A01, Bits[4:3] = 10)

•

Manual mode (Address 0A01, Bits[4:3] = 11)

In automatic mode, a fully automatic priority-based algorithm

selects which reference is the active reference. When programmed

for automatic mode, the device ignores the user selection reference

bits (Register 0x0A01, Bits[2:0]). However, when programmed for

any of the other three modes, the device makes use of the user

reference bits. These bits specify a particular input reference

(000 = REF A, 001 = REF AA ..., 111 = REF DD).

In fallback mode, the user reference is the active reference

whenever 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

whenever 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

particular reference declared as the user reference. The reason is

that if the user reference fails and its redetect timer expires, then

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 AD9548 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, Bit 6 and Bit 5, respectively]). 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 AD9548 has a two-tiered, automatic, priority-based

algorithm that is in effect for both automatic and fallback

reference switchover. 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 AD9548 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, then the device

chooses the reference having the lowest index value. For

example, if the A, B, and CC references (Index 0, Index 2, and

Index 5, respectively) share the same profile, then 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.

When using multiple differential reference inputs, physically

connect the reference input signal with the highest priority to

the reference input with the lowest index value. For example, a

differential signal on Reference Input B should not have a

priority that exceeds a differential signal on Reference Input A.

A differential reference on Reference Input C should not have a

priority that exceeds a differential signal on Reference Input B,

and a differential signal on Reference Input D should not have a

priority that exceeds a differential signal on Reference Input C.

A differential reference on Reference Input A has no priority

restrictions. Table 23 shows four valid priority settings for four

differential reference inputs.

Table 23. Valid Differential Reference Priority Examples1

Reference

Input A

Reference

Input B

Reference

Input C

Reference

Input D

Any reference input configured for a CMOS input is exempt from these

considerations.

Rev. G | Page 30 of 111

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