ACS8525T Semtech, ACS8525T Datasheet - Page 30

• Manufacturer Part Number: ACS8525T
• Description: Line Card Protection Switch for SONET/SDH Systems
• Manufacturer: Semtech

Operating Modes (States) of the Device
The ACS8525 has three primary modes of operation, or
operating states: Free-Run, Locked and Digital Holdover.
These are supported by three secondary, temporary
modes (Pre-Locked, Lost-Phase and Pre-Locked2). Refer
to the State Transition Diagram for DPLL1, Figure 8.
The ACS8525 can operate in Forced or Automatic control.
On reset, the ACS8525 reverts to Automatic Control,
where transitions between states are controlled
completely automatically. Forced Control can be invoked
by configuration, allowing transitions to be performed
under external control. This is not the normal mode of
operation, but is provided for special occasions such as
testing, or where a high degree of hands-on control is
required.
Free-run Mode
The Free-run mode is typically used following a
power-on-reset or a device reset before network
synchronization has been achieved. In the Free-run mode,
the timing and synchronization signals generated from
the ACS8525 are based on the 12.800 MHz clock
frequency provided from the external oscillator and are
not synchronized to an input SEC. By default, the
frequency of the output clock is a fixed multiple of the
frequency of the external oscillator, and the accuracy of
the output clock is equal to the accuracy of the oscillator.
However the external oscillator frequency can be
calibrated to improve its accuracy by a software
calibration routine using register
cnfg_nominal_frequency (Reg. 3C and 3D). For example a
500 ppm offset crystal could be made to look like one
accurate to 0.02 ppm.
The transition from Free-run to Pre-locked occurs when
the ACS8525 selects an SEC.
Pre-locked Mode
The ACS8525 will enter the Locked state in a maximum of
100 seconds, as defined by GR-1244-CORE
specification, if the selected SEC is of good quality. If the
device cannot achieve lock within 100 seconds, it reverts
to Free-Run mode and another SEC is selected.
Revision 3.01/August 2005 © Semtech Corp.
ADVANCED COMMUNICATIONS
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FINAL
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Locked Mode
The Locked mode is entered from Pre-locked, Pre-locked2
or Phase-lost mode when an input reference source has
been selected and the DPLL has locked. The DPLL is
considered to be locked when the phase loss/lock
detectors (See“Phase Lock/Loss Detectors” on page 19)
indicate that the DPLL has remained in phase lock
continuously for at least one second. When the ACS8525
is in Locked mode, the output frequency and phase tracks
that of the selected input reference source.
Lost-phase Mode
Lost-phase mode is used whenever the phase loss/lock
detectors (See“Phase Lock/Loss Detectors” on page 19)
indicate that the DPLL has lost phase lock. The DPLL will
still be trying to lock to the input clock reference, if it
exists. If the Leaky Bucket Accumulator calculates that
the anomaly is serious, the device disqualifies the
reference source. If the device spends more than 100
seconds in Lost-phase mode, the reference is disqualified
and a phase alarm is raised on it. If the reference is
disqualified, one of the following transitions takes place:
1. Go to Pre-locked2;
2. Go to Holdover;
Digital Holdover Mode
Digital Holdover mode is the operating condition the
device enters when its currently selected input source
becomes invalid, and no other valid replacement source
is available.
In Digital Holdover mode, the ACS8525 provides the
timing signals to maintain the Line Card but is not phase
locked to an input SEC.
Digital Holdover operates Instantaneously, which means
the DPLL freezes at the frequency it was operating at the
time of entering Digital Holdover mode. This determines
the output frequency accuracy.
- If a known good stand-by source is available.
- If no stand-by sources are available.
ACS8525 LC/P
DATASHEET
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