AD9549 datasheet(37/78 Pages) AD

37 / 78 page

Preliminary Technical Data

AD9549

Rev. PrA | Page 37 of 78

Figure 22: Holdover state diagram

Abbreviation Key

OvrdRefPin:

Override REF SEL pin

OvrdHldPin:

Override HOLDOVER pin

AutoRefSel:

Automatic reference select

AutoRcov:

Automatic holdover recovery

AutoHold:

Automatic holdover entry

Logical OR

Logical AND

Logical NOT

RefA:

Reference A selected

RefB:

Reference B selected

HoldOver:

Holdover state

FailA:

Reference A failed

FailB:

Reference B failed

ValidA:

Reference A validated

ValidB:

Reference B validated

Figure 23: Holdover State Diagram Abbreviation Key

States 1 or 2 are in effect when the device is not in the holdover

condition, while states 3 & 4 are in effect when the holdover

condition is active. When REF A is selected as the "active"

reference, then states 1 or 3 are in effect. When REF B is

selected as the "active" reference, then states 2 or 4 are in effect.

A transition between states depends on the reference switchover

and holdover control register settings, the logic state of the

REFSELECT and HOLDOVER pins, and the occurrence of

certain events (e.g., a reference failure).

The state machine and its relationship to control register and

external pin stimuli are shown in the diagram below. The state

machine generates a "derived" reference selection and holdover

state. The actual control signal sent to the reference switchover

logic and the holdover logic, however, depends on the control

signals applied to the MUXes. The "dashed" path leading to the

REFSELECT and HOLDOVER pins is active when the "auto"

mode is selected for reference selection and/or holdover

assertion.

Reference Recovery Timers

Each of the two reference inputs has a dedicated recovery timer.

The status of these timers is used by the holdover state machine

as part of the decision making process for reverting to a

previously faulty reference. For example, suppose that a

reference fails (i.e., an LOR or OOL condition is in effect) and

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