ACS8515REV2.1 Semtech Corporation, ACS8515REV2.1 Datasheet - Page 29

• Manufacturer Part Number: ACS8515REV2.1
• Description: Line Card Protection Switch for Sonet or SDH Network Elements
• Manufacturer: Semtech Corporation

Selection of Input Reference Clock
Selection of Input Reference Clock
S o u r c e
S o u r c e
Selection of Input Reference Clock
Selection of Input Reference Clock
Selection of Input Reference Clock
S o u r c e
S o u r c e
S o u r c e
Under normal operation, the input reference
sources are selected automatically by an order
of priority, where SEC1 is the highest priority,
SEC2 is the second highest priority and SEC3
is the lowest priority. The priorities can be
re-assigned with external software. The SEC1
reference source has inputs via either a low
speed TTL input port or a high speed PECL/
LVDS input port. Similarly, the SEC2 reference
source has both a low speed TTL or a high
speed PECL/LVDS input port. The SEC3
(standby) reference source only has provision
via a low speed TTL input port. There is provision
for one sync clock input via a TTL port. Whilst
SEC1, SEC2 and SEC3 reference source inputs
can all be active at the same time, only one of
the TTL or PECL/LVDS input ports for the SEC1
and SEC2 reference sources may be used at
any time, the inactive port is ignored, by setting
the priority of that port to zero.
Restoration of repaired reference sources is
handled carefully to avoid inadvertent
disturbance of the output clock. The ACS8510
has two modes of operation; Revertive and
Non-Revertive. In Revertive mode, if a re-
validated (or newly validated) source has a
higher priority than the reference source which
is currently selected, a switch over will take
place. Many applications prefer to minimise 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 switch-over 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
Revision 2.01/December 2005 Semtech Corp.
ADVANCED COMMUNICATIONS
29
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 switch-
over, regardless of whether Revertive or Non-
Revertive mode has been chosen.
Automatic Control Selection
Automatic Control Selection
Automatic Control Selection
Automatic Control Selection
Automatic Control Selection
When automatic selection is required, the
cnfg_ref_selection registers must be set to all-
zero or all-one. The configuration registers,
cnfg_ref_selection_priority, held in the µP port
are organised as 5, 4-bit registers with each
representing an input reference port. Unused
ports should be given the value '0000' 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 4. 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.
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