AD9546SLASHPCBZ datasheet(84/205 Pages) AD

84 / 205 page

AD9546

Data Sheet

Rev. 0 | Page 84 of 205

Validation Time (tVALID)

In some applications, a reference signal must maintain its

validity (that is, not faulted) for some minimum duration

(tVALID) before the system considers the reference valid. The

reference monitor has a built in timer to handle such timed

validation requirements. Each reference monitor has an

associated unsigned 20-bit integer for assigning tVALID per the

Table 56. Validation Timer Address Ranges

Reference Input

REFA

0x0410 to 0x0412

REFAA

0x0430 to 0x0432

REFB

0x0450 to 0x0452

REFBB

0x0470 to 0x0472

Auxiliary REF0

0x0490 to 0x0492

Auxiliary REF1

0x04B0 to 0x04B2

Auxiliary REF2

0x04D0 to 0x04D2

Auxiliary REF3

0x04F0 to 0x04F2

The programmed value of tVALID carries units of milliseconds,

up to a maximum of approximately 1048 sec (about 17.5 min).

For example, a validation time of 10 min requires

tVALID = (10 min) × (60 sec/min) × (1000 ms/sec)

= 600,000 ms (0x927C0 hexadecimal)

The validation timer starts when the reference goes from a

faulted condition to an unfaulted condition. When the

validation timer expires (that is, in Figure 60, the done output

of the validation timer = 1), then the reference exhibits valid

status (valid = 1).

After the validation timer starts, if a faulted condition occurs,

the validation timer restarts. This behavior ensures that

reference validation occurs only after a reference maintains an

unfaulted condition for the full duration of the validation

timer.

Note that programming a new value for tVALID causes the

validation timer to reset. Resetting the validation timer

invalidates the reference even if the reference was valid prior to

changing tVALID. However, even though the reference exhibits an

invalid status when the validation timer is reset, the reference

monitor continues to indicate the appropriate fault or

unfaulted condition based on the quality of the reference input

signal.

The user can force a reference to invalid status without

affecting the normal operation of the reference monitor (see the

Invalidate section).

Timeout

The user has the option to force the validation timer to

immediately time out via Register 0x2002, Bits[7:0], where each

bit in the register corresponds to a particular reference.

Specifically, starting with D7 and ending with D0, the

corresponding references are auxiliary REF3, auxiliary REF2,

auxiliary REF1, auxiliary REF0, REFBB, REFB, REFAA, and

REFA.

Programming timeout = 1 causes the validation timer to time

out immediately, giving the reference valid status (valid = 1).

Bypass

In certain circumstances, a user may find it necessary to bypass

the reference monitor control of the validation timer. To this

end, each reference input has a dedicated control bit (bypass)

via Register 0x2004, Bits[7:0], where each bit in the register

corresponds to a particular reference. Specifically, starting with

D7 and ending with D0, the corresponding references are

auxiliary REF3, auxiliary REF2, auxiliary REF1, auxiliary REF0,

REFBB, REFB, REFAA, and REFA.

The bypass function does not actually bypass the reference

monitor itself, but only the behavior of valid status.

When a reference has bypass = 0 (default), the normal fault and

unfault conditions prescribed by the reference monitor control

the start of the validation timer, which controls the valid state.

Per Figure 60, when bypass = 0, the user has the option to force

an unfaulted reference to invalid status via the invalidate

control mechanism (see the Invalidate section). However, the

user cannot force a faulted reference to be valid via the

invalidate control mechanism when bypass = 0.

When a reference has bypass = 1, starting the validation timer

falls under user control via the invalidate control mechanism

(see the Invalidate section). That is, programming invalidate =

0 is equivalent to the reference monitor exhibiting an unfaulted

condition. Conversely, programming invalidate = 1 is

equivalent to the reference monitor exhibiting a faulted

condition. The validation timer responds accordingly to the

imposed fault and unfault conditions.

Note that regardless of the state of the bypass or invalidate

control bit, the fast, slow, LOS, and excess jitter status

indicators of the reference monitor function normally and

remain accessible to the user via their respective status registers

(see Figure 60).

Invalidate

In certain circumstances, a user may find it necessary to force a

reference from a valid state to an invalid state. To this end, each

reference input has a dedicated control bit (invalidate) via

corresponds to a particular reference. Specifically, starting with

D7 and ending with D0, the corresponding references are

auxiliary REF3, auxiliary REF2, auxiliary REF1, auxiliary REF0,

REFBB, REFB, REFAA, and REFA.

A scenario for using invalidate = 1 when bypass = 0 is when the

reference signal must be traceable to a primary timing source,

but the user has external knowledge that the source is no longer

traceable. Under these circumstances, the reference signal may

still satisfy the parameters for being in tolerance. Therefore, the

reference monitor indicates the reference as unfaulted.

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