AD9549 datasheet(51/78 Pages) AD

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Preliminary Technical Data

AD9549

Rev. PrA

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POWER-ON RESET

On initial power-up, the AD9549 internally generates a 75ns

RESET pulse. The pulse is initiated when both of the following

two conditions are met:

The 3.3v supply is greater than 2.35± 0.1V

The 1.8v supply is greater than 1.4± 0.05V

Less than 1 ns after RESET goes high, the S1-S4 configuration

pins go high impedance, and remain high impedance until

RESET is deactivated. This allows strapping and configuration

during RESET.

Because of this reset sequence, power supply sequencing isn’t

critical.

AD9549 POWER UP AND PROGRAMMING

SEQUENCE

The following sequence should be followed when initializing

the AD9549:

Apply Power. The AD9549 will perform an internal

reset.

IMPORTANT: Make sure the desired configuration

registers have Single Tone mode(R0100[5]) set, and

Lock Loop (R0100[0]) cleared. If the Lock Loop bit is

set on initial loading, the AD9549 will attempt to lock

the loop before it has been configured.

Once the registered are loaded, the OOL (Out of

limits) and LOR (loss of reference) can be monitored

to insure that a valid reference signal is present on

RefA or RefB.

If a valid reference is present, Register 0100 can be

reprogrammed to clear Single Tone Mode and lock the

loop.

Automatic Holdover mode can now be used to make

the AD9549 immune to any disturbance on the

reference inputs.

The following sequence should be followed when changing

frequencies the AD9549:

Open the loop and enter single tone mode via Register

0100.

Enter the new register settings.

Write 1E to Register 0012.

Once the registered are loaded, the OOL (Out of

limits) and LOR (loss of reference) can be monitored

to insure that a valid reference signal is present on

RefA or RefB.

If a valid reference is present, Register 0100 can be

reprogrammed to clear Single Tone Mode and lock the

loop.

Automatic Holdover mode can now be used to make

the AD9549 immune to any disturbance on the

reference inputs.

Notes:

Attempting to lock the loop without a valid reference can put

the AD9549 into a state that requires a reset.

Automatic Holdover mode is not available unless the loop has

been successfully closed.

If the user desires to open and close the loop manually, we

recommend writing 1E to Register 0012 prior to reclosing the

loop.

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