ACS8514 Semtech Corporation, ACS8514 Datasheet - Page 11

• Manufacturer Part Number: ACS8514
• Description: Synchronous Equipment Timing Source Partner IC for 2nd T4 Dpll, Accurate Monitoring & Input Extender
• Manufacturer: Semtech Corporation

occur sufficiently close together, or by defect events which
occur in bursts. Events which are sufficiently spread out
should not trigger the 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.
On the alarm setting side, if several events occur close
together, each event adds to the amplitude and the alarm
will be triggered quickly; if events occur a little more
spread out, 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 .
There is one Leaky Bucket Accumulator per input channel.
Each Leaky Bucket can select from one of four Configurations
(Leaky Bucket Configuration 0 to 3). Each Leaky Bucket
Configuration is programmable for size, alarm set and
reset thresholds, and decay rate.
Figure 3 Inactivity and Irregularity Monitoring
Revision 3.00 April 2007 © Semtech Corp.
ADVANCED COMMS & SENSING
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FINAL
Each source is monitored over a 128 ms period. If, within
a 128 ms period, an irregularity occurs that is not deemed
to be due to allowable jitter/wander, then the Accumulator
is incremented. Irregularity is defined as too much or too
little activity (corresponding to +/- 1000ppm on a
frequency basis).
The Accumulator will continue to increment up to the point
that it reaches the programmed Bucket size. The "fill rate"
of the Leaky Bucket is, therefore, 8 units/second. The
"leak rate" of the Leaky Bucket is programmable to be in
multiples of the fill rate (x 1, x 0.5, x 0.25 and 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.
Disqualification of a non-selected reference source is
based on inactivity, or on an out-of-band result from the
frequency monitors. The currently selected reference
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.
To avoid the DPLL being pulled off by clock inactivity on a
shorter timescale than 128ms, the DPLL contains a fast
activity detector such that within approximately two
missing input clock cycles, a no-activity flag is raised and
the DPLL is frozen in holdover mode, holding the last
output frequency value. With the DPLL in holdover mode it
is isolated from further disturbances. If the input
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