AD9548SLASHPCBZ datasheet(29/112 Pages) AD

29 / 112 page

AD9548

Rev. 0 | Page 29 of 112

the B reference is differential, then only the REFB nibble

matters (the device ignores the REFBB nibble).

The MSB of each nibble is the manual profile bit, whereas the

three LSBs of each nibble identify one of the eight profiles

(0 to 7). A Logic 1 for the manual profile bit assigns the

associated reference to the profile identified by the three LSBs of

the nibble. A Logic 0 for the manual profile bit configures the

associated reference for automatic reference-to-profile

assignment (the three LSBs are ignored in this case). Note that

references configured for automatic reference-to-profile

assignment require activation (see the Reference-to-Profile

Assignment State Machine section).

Reference-to-Profile Assignment State Machine

The functional flexibility of the AD9548 resides in the way that

it assigns a particular input reference to one of the eight

reference profiles. The reference-to-profile assignment state

machine effectively builds a reference-to-profile table that maps

the index of each input reference to a profile (see Table 22).

Each entry in the profile column consists of a profile number

(0 to 7) or a null value. A null value appears when a reference-

to-profile assignment does not exist for a particular reference

input (following a reset, for example). The information in

Table 22 appears in the register map (Register 0D0C to Register

0D13) so that the user has access to the reference-to-profile

assignments on a real-time basis. Register 0D0C contains the

information for REF A, Register 0D0D contains the inform-

ation for REF AA, and so on to Register 0D13 for REF DD. Bit 7

of each register is the null indicator for that particular reference.

If Bit 7 = 0, then the profile assignment for that particular

reference is null. If Bit 7 = 1, then that particular reference is

assigned to the profile (0 to 7) identified by Bits[6:4]. Note that

Bits[6:4] are meaningless unless Bit 7 = 1.

Table 22. Reference-to-Profile Table

Reference

Input

Reference

Index

Profile

Profile number (or null value)

Following a reset, the reference-to-profile assignment state

machine is inactive to avoid improperly assigning a reference to

a profile before the system clock stabilizes. The reason is that

the state machine relies on accurate information from the

reference monitors, which, in turn, rely on a stable system clock.

Because the reference-to-profile assignment state machine is

inactive at power-up, the user must initiate it manually by

writing to the reference profile detect register (Address 0A0D).

The state machine activates immediately, unless the system

clock is not stabilized, in which case, activation occurs upon

expiration of the system clock stability timer. Note that

initialization of the state machine is on a per-reference basis.

That is, each reference input is associated with an independent

initialization control bit.

Once initialized for processing a reference, the state machine

continuously monitors that reference until the occurrence of a

device reset. This is true even when the user programs a

reference for manual profile selection, in which case, the state

machine associated with that particular reference operates with

its activity masked. The masked background activity allows for

seamless operation if the user subsequently reprograms the

reference for automatic profile selection.

Reference-to-Profile Assignment

When a reference is programmed for manual profile assignment

(see Register 0503 to Register 0506), the reference-to-profile

assignment state machine simply puts the programmed manual

profile number into the profile column of the reference-to-

profile table (see Table 22) in the row associated with the appro-

priate reference. However, when the user programs a reference

for automatic profile assignment, the state machine must figure

out which profile to assign to the reference.

As long as a null entry appears in the reference-to-profile table

for a particular input reference, the validation logic for that

reference enters a period estimation mode. Note that a null

entry is the default state following a reset, but it also occurs

when a reference redetect timer expires. The period estimation

mode enables the validation logic to make a blind estimate of

the period of the input reference with a tolerance of 0.1%. The

validation logic remains in the period estimation mode until it

successfully estimates the reference period.

Upon a successful reference period measurement by the

validation logic, the state machine compares the measured

period to the nominal reference period programmed into each

of the eight profiles. The state machine assigns the reference to

the profile with the closest match to the measured period. If

more than one profile exactly matches the reference period,

then the state machine chooses the profile with the lowest

numeric index. For example, if the reference period in both

Profile 3 and Profile 5 matches the measured period, then

Profile 3 is given the assignment.

To safeguard against making a poor reference-to-profile

assignment, the state machine ensures that the measured

reference period is within 6.25% of the nominal reference

period that appears in the closest match profile. Otherwise, the

state machine does not make a profile assignment and leaves

the null entry in the reference-to-profile table.

As long as there are input references programmed for automatic

profile assignment, and for which the profile assignment is null,

the state machine continues to cycle through those references

searching for a profile match. Furthermore, unless an input

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