AD9549ABCPZ datasheet(30/76 Pages) AD

30 / 76 page

AD9549

Rev. D | Page 30 of 76

RESET

FAILA & VALIDB & AUTOREFSEL & OVRDREFPIN

FAILB & VALIDA & AUTOREFSEL & OVRDREFPIN

REFB

HOLDOVER

REFA

HOLDOVER

REFB

HOLDOVER

REFA

HOLDOVER

FAI

IDB

AUT

RCO

V &

VRDRE

VRDHL

FAI

IDA

AUT

RCO

V &

VRDRE

VRDHL

REFA:

REFB:

HOLDOVER:

FAILA:

FAILB:

VALIDA:

VALIDB:

REFERENCE A SELECTED

REFERENCE B SELECTED

HOLDOVER STATE

REFERENCE A FAILED

REFERENCE B FAILED

REFERENCE A VALIDATED

REFERENCE B VALIDATED

OVRDREFPIN:

OVRDHLDPIN:

AUTOREFSEL:

AUTORCOV:

AUTOHOLD:

||:

OVERRIDE REF SEL PIN

OVERRIDE HOLDOVER PIN

AUTOMATIC REFERENCE SELECT

AUTOMATIC HOLDOVER RECOVERY

AUTOMATIC HOLDOVER ENTRY

LOGICAL OR

LOGICAL AND

LOGICAL NOT

ABBREVIATION KEY

Figure 37. Holdover State Diagram

Holdover and Reference Switchover State Machine

Figure 37 shows the interplay between the input reference

signals and holdover, as well as the various control signals

and the four states.

State 1 or State 2 is in effect when the device is not in the holdover

condition, and State 3 or State 4 is in effect when the holdover

condition is active. When REFA is selected as the active reference,

State 1 or State 3 is in effect. When REFB is selected as the active

reference, State 2 or State 4 is 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 (for example, a reference

failure).

The state machine and its relationship to control register and

external pin stimuli are shown in Figure 37. 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 automatic mode is

selected for reference selection and/or holdover assertion.

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