ACS8520T Semtech, ACS8520T Datasheet - Page 14

• Manufacturer Part Number: ACS8520T
• Description: Synchronous Equipment Timing Source (SETS) for Stratum 3/4E/4 and SMC Systems
• Manufacturer: Semtech

Interrupts for Activity Monitors
The loss of the currently selected reference source will
eventually cause the input to be considered invalid,
triggering an interrupt. The time taken to raise this
interrupt is dependant 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.
Leaky Bucket Timing
The time taken (in seconds) to raise an inactivity alarm on
a reference source that has previously been fully active
(Leaky Bucket empty) will be:
where n is the number of the Leaky Bucket Configuration.
If an input is intermittently inactive then this time can be
longer. The default setting of cnfg_upper_threshold_n is
6, therefore the default time is 0.75 s.
The time taken (in seconds) to cancel the activity alarm on
a previously completely inactive reference source is
calculated, for a particular Leaky Bucket, as:
where:
The default setting is shown in the following:
Frequency Monitoring
The ACS8520 performs frequency monitoring to identify
reference sources which have drifted outside the
Revision 3.02/October 2005 © Semtech Corp.
ADVANCED COMMUNICATIONS
(cnfg_upper_threshold_n) / 8
[2
a = cnfg_decay_rate_n
b = cnfg_bucket_size_n
c = cnfg_lower_threshold_n
(where n = the number of the relevant Leaky
Bucket Configuration in each case).
[2
1
(a)
x (8 - 4)] /8 = 1.0 secs
x (b - c)]/ 8
FINAL
Page 14
acceptable frequency range measured with respect either
to the output clock or to the XO clock.
The sts_reference_sources out-of-band alarm for a
particular reference source is raised when the reference
source is outside the acceptable frequency range. With
the default register settings a soft alarm is raised if the
drift is outside ±11.43 ppm and a hard alarm is raised if
the drift is outside ±15.24 ppm. Both of these limits are
programmable from 3.8 ppm up to 61 ppm.
The ACS8520 DPLL has a programmable lock and
capture range frequency limit up to ±80 ppm (default is
±9.2 ppm).
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 defined by the default
configuration and can be changed via the microprocessor
interface by the Network Manager. 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. For this, the ACS8520 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 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 take place under
software control or if the currently selected source fails.
To enable software control, the software should briefly
enable Revertive mode to effect a switch-over to the
ACS8520 SETS
DATASHEET
www.semtech.com