AD9546SLASHPCBZ datasheet(60/205 Pages) AD

60 / 205 page

AD9546

Data Sheet

Rev. 0 | Page 60 of 205

Although the local time scale is precise (frequency stable), it is

likely not accurate. That is, the time indicated by the local time

scale is with respect to an arbitrary starting time. Stated another

way, the epoch of the local time scale is arbitrary. The purpose

of the CCS is to provide a means to produce an internal

common time scale with a precise epoch determined by the

user. In practice, the user provides a physical trigger event (the

synchronization arrow in Figure 51) along with a numeric

entry of the time associated with the trigger event (the time

arrow in Figure 51), which effectively assigns an epoch. The

CCS uses the trigger event and numeric entry to produce a

precise time shifted version of the local time scale (the common

time scale) synchronized to the desired epoch. More

importantly, in the context of multiple AD9546 devices in a

digitized clocking system, the user can use the CCS of each

device to obtain precise alignment of the common time scale

across devices.

The diagram in Figure 51 is a functional representation of the

CCS. The key points of interest are the time and synchronization

inputs, synchronization offset refinement, the slew limiter, and

the summing node with the common time scale (a time shifted

version of the local time scale) as its output.

SYNCHRONIZATION SOURCE

The synchronization source constitutes the time stamp source

associated with the synchronization input of Figure 51. The

arrival of each time stamp from the selected synchronization

source constitutes a synchronization event. In operation, when

a synchronization event occurs, the user updates the synchroniza-

tion time (see the Synchronization Time section) associated

with that synchronization event, but before the occurrence of

the next synchronization event.

Because the common clock DPLL has access to a primary and

secondary (optional) reference source for producing the local

time scale, the CCS also has access to a primary and secondary

(optional) synchronization source. Whenever the common

clock DPLL switches between primary and secondary

references, the CCS automatically switches to its primary or

secondary synchronization source, commensurate with the

common clock DPLL (a one to one correspondence).

To assign the primary synchronization source, use Bits[4:0] of

source, use Bits[4:0] of Register 0x0D21. In legacy auxiliary

DPLL operation, the CCS uses the source specified in Bits[4:0]

of Register 0x0D11.

If possible, the recommendation is to use REFB and/or REFBB

as the synchronization source to the CCS because REFB and

REFBB provide access to the analog loopback function (see the

Analog Clock Loopback section).

The local time scale appears as one of the synchronization

sources associated with the synchronization input in Figure 51.

The user can employ the local time scale for immediate

synchronization rather than using a time stamp source. Immediate

synchronization is useful for forcing a synchronization trigger

in applications where the user is not transporting the common

time scale across multiple devices or does not require the

common time scale to have a specific epoch, but wants to use

one (or more) of the digitized clocking resources (a UTS, for

example). In such cases, the immediate trigger is necessary to

make the CCS ready for use by the digitized clocking resources.

To select immediate synchronization, use 0x1E or 0x1F for

Bits[4:0] associated with the primary or secondary

synchronization source selection.

When the common clock DPLL experiences a reference

switchover, there is a direct impact on the CCS. First, the

synchronization source changes to primary or secondary in

keeping with the new reference source (primary or secondary).

Likewise, the synchronization time (see the Synchronization

Time section) and skew (see the Adding Time Skew to the

Synchronization Time section) change in keeping with the new

reference source (primary or secondary). In addition, the CCS

flushes any synchronization data previously captured.

However, the synchronization offset presently applied by the

CCS to the common time scale remains in effect. Likewise, the

filtering state of the synchronization offset refinement block

(see the Synchronization Offset Refinement section) and the

condition of the slew limiter (see the Synchronization Slew

Limiter section) also remain in effect.

Because the CCS flushes old synchronization data on a

reference switchover, the CCS has no synchronization event to

which it can compare the first synchronization time update.

This condition causes a synchronization error and prevents the

CCS from performing the intended synchronization. However,

the next complete synchronization event clears the

synchronization error with synchronization proceeding

normally from that point onward.

TAGGED SYNCHRONIZATION

Tagged synchronization only applies when the synchronization

source originates from a TDC generating tagged time stamps.

Thus, tagged synchronization does not apply to immediate

synchronization (that is, using the common time scale as the

synchronization source).

For a source with tagged time stamps (see the Tagged Time

Stamps section), the user has the option of synchronizing to

only the tagged time stamps from that source. To enable the

tagged synchronization feature, program a Logic 1 in Bit 7 of

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