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ACS8520

Synchronous Equipment Timing Source for Stratum 3/4E/4 and SMC Systems

Semtech Corporation 

ACS8520 SETS

ADVANCED COMMUNICATIONS

FINAL

DATASHEET

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

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).

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:

(cnfg_upper_threshold_n) / 8

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

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:

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.

[2 (a) x (b - c)]/ 8

where:

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).

The default setting is shown in the following:

[21 x (8 - 4)] /8 = 1.0 secs

Frequency Monitoring

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.

The ACS8520 performs frequency monitoring to identify To enable software control, the software should briefly

reference sources which have drifted outside the

enable Revertive mode to effect a switch-over to the

Revision 3.02/October 2005 © Semtech Corp.

Page 14

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