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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

been raised, then the DPLL will continue to lock to the Each source is monitored over a 128 ms period. If, within

input, with little disturbance. In this scenario, with the

a 128 ms period, an irregularity occurs that is not deemed

DPLL in the “locked” state, the DPLL uses “nearest edge

locking” mode (±180° capture) avoiding cycle slips or

glitches caused by trying to lock to an edge 360° away, as

would happen with traditional PLLs.

to be due to allowable jitter/wander, then the

Accumulator is incremented.

The Accumulator will continue to increment up to the

point that it reaches the programmed Bucket size. The “fill

Activity Monitoring

rate” of the Leaky Bucket is, therefore, 8 units/second.

The “leak rate” of the Leaky Bucket is programmable to

The ACS8522 has a combined inactivity and irregularity be in multiples of the fill rate (x 1, x 0.5, x 0.25 and

monitor. The ACS8522 uses a Leaky Bucket Accumulator,

which is a digital circuit which mimics the operation of an

analog integrator, in which input pulses increase the

output amplitude but die away over time. Such integrators

are used when alarms have to be triggered either by fairly

x 0.125) to give a programmable leak rate from

8 units/sec down to 1 unit/sec. A conflict between trying

to “leak” at the same time as a “fill” is avoided by

preventing a leak when a fill event occurs.

regular defect events, which occur sufficiently close

Disqualification of a non-selected reference source is

together, or by defect events which occur in bursts. Events based on inactivity, or on an out-of-band result from the

which are sufficiently spread out should not trigger the frequency monitors. The currently selected reference

alarm. By adjusting the alarm setting threshold, the point

at which the alarm is triggered can be controlled. The

point at which the alarm is cleared depends upon the

decay rate and the alarm clearing threshold.

source can be disqualified for phase, frequency, inactivity

or if the source is outside the DPLL lock range. If the

currently selected reference source is disqualified, the

next highest priority, qualified reference source is

selected.

On the alarm setting side, if several events occur close

together, each event adds to the amplitude and the alarm

Interrupts for Activity Monitors

will be triggered quickly; if events occur further apart, but

still sufficiently close together to overcome the decay, the

alarm will be triggered eventually. If events occur at a rate

which is not sufficient to overcome the decay, the alarm

will not be triggered. On the alarm clearing side, if no

defect events occur for a sufficient time, the amplitude

will decay gradually and the alarm will be cleared when

the amplitude falls below the alarm clearing threshold.

The ability to decay the amplitude over time allows the

importance of defect events to be reduced as time passes

by. This means that, in the case of isolated events, the

alarm will not be set, whereas, once the alarm becomes

set, it will be held on until normal operation has persisted

for a suitable time (but if the operation is still erratic, the

alarm will remain set). See Figure 3.

The loss of the currently selected reference source will

eventually cause the input to be considered invalid,

triggering an interrupt, if not masked. The time taken to

raise this interrupt is dependent on the Leaky Bucket

Configuration of the activity monitors. The fastest Leaky

Bucket setting will still take up to 128 ms to trigger the

interrupt. The interrupt caused by the brief loss of the

currently selected reference source is provided to

facilitate very fast source failure detection if desired. It is

triggered after missing just a couple of cycles of the

reference source. Some applications require the facility to

switch downstream devices based on the status of the

reference sources. In order to provide extra flexibility, it is

possible to flag the main_ref_failed interrupt (Reg. 06 Bit

There is one Leaky Bucket Accumulator per input channel.

Each Leaky Bucket can select from four Configurations

(Leaky Bucket Configuration 0 to 3). Each Leaky Bucket

6) on the pin TDO. This is simply a copy of the status bit in

the interrupt register and is independent of the mask

register settings. The bit is reset by writing to the interrupt

Configuration is programmable for size, alarm set and status register in the normal way. This feature can be

reset thresholds, and decay rate.

enabled and disabled by writing to Reg. 48 Bit 6.

Revision 5/November 2006 © Semtech Corp.

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