ACS8509 Semtech Corporation, ACS8509 Datasheet - Page 36

• Manufacturer Part Number: ACS8509
• Description: Synchronous Equipment Timing Source for Sonet or SDH Network Elements
• Manufacturer: Semtech Corporation

Selection of Input Reference Clock Source
Under normal operation, the input reference sources are
selected automatically by an order of priority. But, for
special circumstances, such as chip or board testing, the
selection may be forced by configuration.
Automatic operation selects a reference source based on
its pre-defined priority and its current availability. A table
is maintained which lists all reference sources in the order
of priority. This is initially downloaded into the ACS8509
via the microprocessor interface by the Network Manager,
and is subsequently modified by the results of the ongoing
quality monitoring. In this way, when all the defined
sources are active and valid, the source with the highest
programmed priority is selected but, if this source fails,
the next-highest source is selected, and so on.
Restoration of repaired reference sources is handled
carefully to avoid inadvertent disturbance of the output
clock. The ACS8509 has two modes of operation;
Revertive and Non-Revertive. In Revertive mode, if a
revalidated (or newly validated) source has a higher
priority than the reference source which is currently
selected, a switchover will take place. Many applications
prefer to minimize the clock switching events and choose
Non-Revertive mode. In Non-Revertive mode, when a
re-validated (or newly validated) source has a higher
priority then the selected source will be maintained. The
re-validation of the reference source will be flagged in the
sts_sources_valid register and, if not masked, will
generate an interrupt.
Selection of the re-validated source can only take place
under software control - the software should briefly
enable Revertive mode to affect a switchover to the higher
priority source. If the selected source fails under these
conditions the device will indicate that it is still locked to
the failed reference. It will not select the higher priority
source until instructed to do so by the software; by briefly
setting the Revertive mode bit. When there is a reference
available with higher priority than the selected reference,
there will be NO change of reference source as long as the
Non-Revertive mode remains on AND the device will
remain indicating a locked state on the failed reference.
This is the case even if there are lower priority references
available or the currently selected reference fails. When
the ONLY valid reference sources that are available have
a lower priority than the selected reference, a failure of
the selected reference will always trigger a switchover,
regardless of whether Revertive or Non-Revertive mode
has been chosen.
Revision 2.00/January 2006 © Semtech Corp.
ADVANCED COMMUNICATIONS
FINAL
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Also, in a Master/Slave redundancy-protection scheme,
the Slave device(s) must follow the Master device. The
alignment of the Master and Slave devices is part of the
protection mechanism. The availability of each source is
determined by a combination of local and remote
monitoring of each source. Each input reference source
supplied to each ACS8509 device is monitored locally and
the results are made available to other devices.
Forced Control Selection
A configuration register, cnfg_ref_selection, controls both
the choice of automatic or forced selection and the
selection itself (when forced selection is required). The
forced selection of an input reference source occurs when
the cnfg_ref_selection variable contains a non-zero value,
the value then representing the input port required to be
selected. This is not the normal mode of operation, and
the cnfg_ref_selection variable is defaulted to the all-one
value on reset, thereby adopting the automatic selection
of the reference source.
Automatic Control Selection
When an automatic selection is required, the
cnfg_ref_selection register must be set to all zero or all
one. The configuration registers,
cnfg_ref_selection_priority, held in the µP port block,
consists of 3, 8-bit registers organised as one 4-bit
register per input reference port. Each register holds a 4-
bit value which represents the desired priority of that
particular port. Unused ports should be given the value,
“0000” or “1111”, in the relevant register to indicate they
are not to be included in the priority table. On power-up, or
following a reset, the whole of the configuration file will be
defaulted to the values defined by Table 6. The selection
priority values are all relative to each other, with lower-
valued numbers taking higher priorities. Each reference
source should be given a unique number, the valid values
are 1 to 15 (dec). A value of 0 disables the reference
source. However if two or more inputs are given the same
priority number those inputs will be selected on a first in,
first out basis.
If the first of two same priority number sources goes
invalid the second will be switched in. If the first then
becomes valid again, it becomes the second source on
the first in, first out basis, and there will not be a switch. If
a third source with the same priority number as the other
two becomes valid, it joins the priority list on the same
first in, first out basis. There is no implied priority based on
the channel numbers.
ACS8509 SETS
DATASHEET
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