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Preliminary Technical Data

AD9549

Rev. PrA | Page 33 of 78

detection process in the previous section. However, a different

control register is used: the Frequency Lock Watchdog Timer

REFERENCE MONITORS

Loss of Reference

The AD9549 can set an alert when one or both of the reference

signals are not present. Each of the two reference inputs (REFA,

REFB) has a dedicated LOR (Loss of Reference) circuit enabled

via the I/O Register Map. Detection of an LOR condition sets

the appropriate LOR bit in both a status register and an IRQ

internally available to the multi-purpose "status" pins (S1:4) of

the AD9549. By setting the appropriate bit in the I/O Register

Map, the user can assign a status pin to each of the LOR flags.

This provides a means to control external hardware based on

the state of the LOR flags directly.

The LOR circuits are internal ‘watchdog’ timers with a

programmable period. The period of the timer is set via the I/O

monitored reference signal. The rising edge of the reference

signal continuously resets the watchdog timer. If the timer

reaches a full count, this indicates that the reference was either

lost or its period was longer than the timer period. LOR does

not differentiate between these.

The period for each of the LOR timers is controlled by a 16-bit

word in the I/O Register Map. The period of the timer clock

(TCLK) is 2/fS. Therefore, the period of the watchdog timer

(TWD) is:

TWD = (2/fS)N

Where N is the value of the 16-bit word stored in the I/O

of N so that the watchdog period is greater than the input

reference period, expressed mathematically as:

( )

floor

where fR is the frequency of the input reference. The value of N

results in establishing two frequencies. One for which the LOR

signal will never be triggered (fPRESENT), and one for which the

LOR signal will always be active (fLOST). Between these

frequencies the LOR signal will intermittently toggle between

states.

The values of the two frequency bounds are:

()

LOST

PRESENT

Note that when N is chosen to be

( ) 1

floor

, the LOR

circuit is capable of indicating an LOR condition in little more

than a single input reference period. For example, if fS=1GHz

and fR=2.048MHz, then the smallest useable N value is:

()

(

) 245

048

floor

MIN

Which yields values for fPRESENT and fLOST as:

520

032

816

048

LOST

PRESENT

and

NOTE: N should be chosen sufficiently large to account for any

acceptable deviation in the period of the input reference signal.

Notice that the value of N is inversely proportional to the

reference frequency, meaning that as the reference frequency

goes up, the precision for adjusting the threshold goes down.

Proper operation of the LOR circuit requires that N be no less

than 3. Therefore, the highest reference frequency for which

the LOR circuit will function properly is given by: fLOR_MAX =

fS/6.

Reference Frequency Monitor

The AD9549 can set an alert whenever one or both of the

reference inputs drift in frequency beyond user-specified limits.

Each of the two references has a dedicated Out of Limits (OOL)

circuit enabled/disabled via the I/O Register Map. Detection of

an OOL condition sets the appropriate OOL bit in both a status

can also assign a status pin (S1-S4) to each of the OOL flags by

setting the appropriate bit in the I/O Register Map. This

provides a means to control external hardware based on the

state of the OOL flags directly.

Each reference monitor contains three main building blocks: a

programmable reference divider, a 32-bit counter, and a 32-bit

digital comparator.

Figure 18: Reference Monitor

Four values are needed to calculate the correct values of the

reference monitor: The system clock frequency, fS (usually

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