ACS8510 datasheet(39/69 Pages) SEMTECH

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Revision 2.07/Jan 2001 ã2001 Semtech Corp

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ACS8510 SETS

ADVANCED COMMUNCIATIONS

FINAL

Holdover mode

The Holdover mode is used when the ACS8510

was in Locked mode for long enough to acquire

stable frequency data, but the final selected

reference source has become unavailable and

a replacement has not yet been qualified for

selection.

In Holdover mode, the ACS8510 provides the

timing and synchronisation signals to maintain

the Network Element (NE), but they are not

phase locked to any input reference source.

The timing is based on a stored value of the

frequency ratio obtained during the last Locked

mode period.

The Holdover performance is mainly limited by

what is happening to the TCXO. The ACS8510

has 3 ways of determining Holdover, either;

1. By external frequency setting (cnfg_holdover_offset

2. By an internal frequency measuring and averaging

system which averages the last 20 minutes

3. By just using the last frequency (as reported by the

sts_curr_inc_offset register). This value can be read out

of the device and used to build up a longer term average

using an external averaging circuit. This value can then to

readback into the device and used as the Holdover offset

(via cnfg_holdover_offset register).

By default it uses the internal averager. This

means that if the TCXO frequency is varying

due to temperature fluctuations in the room,

then the instantaneous value can be different

from the average value, and then it may be

possible to exceed the 0.05 ppm limit

(depending on how extreme the temperature

flucuations are). It is advantageous to shield

the TCXO to slow down frequency changes due

to drift and external temperature fluctuations.

The frequency accuracy of Holdover mode has

to meet the ITU-T, ETSI and Telcordia

performance requirements. The performance

of the external oscillator clock is critical in this

mode, although only the frequency stability is

important - the stability of the output clock in

Hold-Over is directly related to the stability of

the external oscillator.

Pre-Locked(2) mode

This state is very similar to the Pre-Locked(1)

state. It is entered from the Holdover state

when a reference source has been selected

and applied to the phase locked loop. It is also

entered if the device is operating in revertive

mode and a higher-priority reference source is

restored.

Upon applying a reference source to the phase

locked loop, the ACS8510 will enter the Locked

state in a maximum of 100 seconds, as defined

by GR-1244-CORE specification, if the selected

reference source is of good quality.

If the device cannot achieve lock within 100

seconds, it reverts to Holdover mode and

another reference source is selected.

Protection Facility

The ACS8510 supports redundancy protection.

The primary functions of this include:

- Alignment of the priority tables of both Master

and Slave ACS8510 devices so as to align the

selection of reference sources of both Master

and Slave ACS8510 devices.

- Alignment of the phases of the 8 kHz and 2

kHz clocks in both Master and Slave ACS8510

devices to within one cycle of the 77.76 MHz

internal clock.

When two ACS8510 devices are to be used in

a redundancy-protection scheme within an NE,

one will be designated as the Master and the

other as the Slave. It is expected that an NE

will use the T

OUT0 output for its internal

operations because the T

OUT4 output is intended

to feed an SSU/BITS system. An SSU/BITS will

not be bothered by phase differences between

ACS8510 Datasheet(PDF) 39 Page - Semtech Corporation

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