AD9549APCBZ datasheet(27/76 Pages) AD

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AD9549

Rev. D | Page 27 of 76

The following four values are needed to calculate the correct

values of the reference monitor:

•

System clock frequency, fS (usually 1 GHz)

•

Reference input frequency, fR (in Hz)

•

Error bound, E (1% = 0.01)

•

Monitor window size (W)

The monitor window size is the difference between the maximum

and minimum number of counts accumulated between adjacent

edges of the reference input. If this window is too small, random

variations cause the OOL detector to indicate incorrectly that

a reference is out of limits. However, the time required to

determine if the reference frequency is valid increases with window

size. A window size of at least 20 is a good starting point.

The four input values mentioned previously are used to calculate

the OOL divider (D) and OOL nominal value (N), which, in

turn, are used to calculate the OOL upper limit (U) and OOL

lower limit (L), according to the following formulas:





































ceil

535

min

max

)

floor(

)

floor(

−

)

floor(

)

ceil(

The timing accuracy is dependent on two factors. The first is

the inherent accuracy of fS because it serves as the time base for

the reference monitor. As such, the accuracy of the reference

monitor can be no better than the accuracy of fS. The second

factor is the value of W, which must be sufficiently large (≥20)

so that the timer resolves the deviation between a nominal value

of fR and a value that is out of limits.

As an example, let fR = 10 MHz, Ε = 0.05%, fS = 1 GHz, and

W = 20. The limits are then

D = 79

Lower Limit = 1980

Upper Limit = 2020

Next, let Ε = 0.0005%. Then the limits are

D = 7999

Lower Limit = 199980

Upper Limit = 200020

Note that the number of counts (and time) required to make

this measurement has increased by 100×. In addition, it is

recommended that D be an odd number.

REFERENCE SWITCHOVER

The AD9549 supports dual input reference clocks. Reference

switchover can be accomplished either automatically or manually

by appropriately programming the automatic selector bit in the

I/O register map (Register 0x01C0, Bit 2).

Transition to a newly selected reference depends on a number

of factors:

•

State of the REFSELECT pin

•

State of the REF_AB bit (Register 0x01C1[2])

•

State of the enable ref input override bit (Register 0x01C1[3])

•

Holdover status

A functional diagram of the reference switchover and holdover

logic is shown in Figure 33.

AUTOREFSEL

OVERRIDE REFPIN

AUTOHOLD

OVERRIDE HLDPIN

REF_AB

HLDOVR

DERIVED

REFSEL STATE

DERIVED

HOLDOVER

STATE

REFSELECT

HOLDOVER

STATE

MACHINE

REFERENCE

SWITCHING

CONTROL

LOGIC

HOLDOVER

CONTROL

LOGIC

ACTIVE REFSEL STATE

ACTIVE HOLDOVER STATE

Figure 33. Reference Switchover and Holdover Logic

In manual mode, the active reference is determined by an exter-

nally applied logic level to the REFSELECT pin. In automatic

mode, an internal state machine determines which reference is

active, and the REFSELECT pin becomes an output indicating

which reference the state machine is using.

The user can override the active reference chosen by the internal

state machine via the enable ref input override bit. The REF_AB

bit is then used to select the desired reference. When in override,

it is important to note that the REFSELECT pin does not

indicate the physical reference selected by the REF_AB bit.

Instead, it indicates the reference that the internal state machine

would select if the device were not in the override mode. This

allows the user to force a reference switchover by means of the

programming registers while monitoring the response of the

state machine via the REFSELECT pin.

The same type of operation (manual/automatic and override)

also applies to the holdover function, as shown in the reference

switchover logic diagram (see Figure 33). The dashed arrows in

the diagram indicate that the state machine output is available

to the REFSELECT and HOLDOVER pins when in override mode.

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