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ACS8522 Datasheet(PDF) 11 Page - Semtech Corporation |
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ACS8522 Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 118 page  ADVANCED COMMUNICATIONS FINAL DATASHEET Revision 5/November 2006 © Semtech Corp. Page 11 www.semtech.com ACS8522 SETS LITE been raised, then the DPLL will continue to lock to the input, with little disturbance. In this scenario, with the 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. Activity Monitoring The ACS8522 has a combined inactivity and irregularity 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 regular defect events, which 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 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. 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 Configuration is programmable for size, alarm set and reset thresholds, and decay rate. 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. 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. Interrupts for Activity Monitors 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 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 status register in the normal way. This feature can be enabled and disabled by writing to Reg. 48 Bit 6. |
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