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ACS8522

Synchronous Equipment Timing Source for Stratum 3/4E/4 and SMC Systems

Semtech Corporation 

ACS8522 SETS LITE

ADVANCED COMMUNICATIONS

FINAL

DATASHEET

An input reference monitor is assigned to each of the four z Greater programmable bandwidth from 0.1 Hz to

inputs. The monitors operate continuously such that at all

times the status of all of the inputs to the device are

70 Hz in 10 steps (T4 path programmable bandwidth

in 3 steps, 18, 35 and 70 Hz)

known. Each input can be monitored for both frequency

and activity, activity alone, or the monitors can be

disabled.

z Noise rejection on low frequency input

z Manual Holdover frequency control

The frequency monitors have a “hard” (rejection) alarm

limit and a “soft” (flag only) alarm limit for monitoring

frequency, whilst the reference is still within its allowed

frequency band. Each input reference can be

programmed with a priority number allowing references to

be chosen according to the highest priority valid input. The

two paths (T0 and T4) have independent priorities to allow

completely independent operation of the two paths. Both

paths operate either automatic or external source

selection.

For automatic input reference selection, the T0 path has

a more complex state machine than the T4 path.

The T0 and T4 PLL paths support the following common

features:

z Automatic source selection according to input

priorities and quality level

z Different quality levels (activity alarm thresholds) for

each input

z Variable bandwidth, lock range and damping factor

z Direct PLL locking to common SONET/SDH input

frequencies or any integer multiple of 8 kHz up to

100 MHz

z Automatic mode switching between Free-run, Locked

and Holdover states

z Fast detection on input failure and entry into Holdover

mode (holds at the last good frequency value)

z Frequency translation between input and output rates

via direct digital synthesis

z Controllable automatic Holdover frequency filtering

z Frame Sync pulse alignment.

Either the software or an internal state machine controls

the operation of the DPLL in the T0 path. The state

machine for the T4 path is very simple and cannot be

manually/externally controlled, however the overall

operation can be controlled by manual reference source

selection. One additional feature of the T4 path is the

ability to measure a phase difference between two inputs.

The T0 path DPLL always produces an output at

77.76 MHz to feed the APLL, regardless of the frequency

selected at the output pins. The T4 path can be operated

at a number of frequencies. This is to enable the

generation of extra output frequencies, which cannot be

easily related to 77.76 MHz. When the T4 path is selected

to lock to the T0 path, the T4 DPLL locks to the 8 kHz from

the T0 DPLL. This is because all of the frequencies of

operation of the T4 path can be divided to 8 kHz and this

will ensure synchronization of all the frequencies within

the two paths.

Both of the DPLLs’ outputs are connected to multiplying

and filtering APLLs. The outputs of these APLLs are

divided making a number of frequencies simultaneously

available for selection at the output clock ports. The

various combinations of DPLL, APLL and divider

configurations allow for generation of a comprehensive

set of frequencies as listed in Table 12).

z High accuracy digital architecture for stable PLL

dynamics combined with an APLL for low jitter final

output clocks.

There are a number of features supported by the T0 path

that are not supported by the T4 path, although these can

also all be externally controlled by software.

The additional T0 features supported are:

To synchronize the lower output frequencies when the T0

PLL is locked to a high frequency reference input, an

additional input is provided. The SYNC2K pin (pin 28) is

used to reset the dividers that generate the 2 kHz and

8 kHz outputs such that the output 2/8 kHz clocks are

lined up with the input 2 kHz. This synchronization

method could allow for example, a master and a slave

device to be in precise alignment.

z Non-revertive mode

z Phase Build-out on source switch (hit-less source

switching)

z I/O phase offset control

The ACS8522 also supports Sync pulse references of

4 kHz or 8 kHz although in these cases frequencies lower

than the Sync pulse reference may not necessarily be in

phase.

Revision 5/November 2006 © Semtech Corp.

Page 8

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