ACS8520A datasheet(19/150 Pages) SEMTECH

Part #	ACS8520A

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

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Manufacturer	SEMTECH [Semtech Corporation]

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ACS8520A SETS

ADVANCED COMMS & SENSING

FINAL

DATASHEET

Revision 1.00/September 2007© Semtech Corp.

Page 19

www.semtech.com

It is also possible to combine the internal averaging filters

with some additional software filtering. For example the

internal fast filter could be used as an anti-aliasing filter

and the software could further filter this before

determining the actual Holdover frequency. To support

this feature, a facility to read out the internally averaged

frequency has been provided. By setting Reg. 40, Bit 5,

cnfg_holdover_modes, read_average, the value read

back from the cnfg_holdover_frequency register will be

the filtered value. The filtered value is available

regardless of what actual Holdover mode is selected.

Clearly this results in the register not reading back the

data that was written to it.

Example: Software averaging to eliminate temperature drift.

Select Manual Holdover mode by setting Reg. 34 Bit 4,

cnfg_input_mode, man_holdover High.

Select Fast Holdover Averaging mode by setting Reg. 40

Bit 6, cnfg_holdover_modes, auto_averaging High and

Reg. 40 Bit 7 High.

Select to be able to read back filtered output by setting

Reg. 40 Bit 5, cnfg_holdover_modes, read_average High.

Software periodically reads averaged value from the

cnfg_holdover_frequency register and the temperature

(not supplied from ACS8520A). Software processes

frequency and temperature and places data in software

look-up table or other algorithm. Software writes back

appropriate averaged value into the

cnfg_holdover_frequency register.

Once Holdover mode is entered, software periodically

updates the cnfg_holdover_frequency register using the

temperature information (not supplied from ACS8520A).

Mini-holdover Mode

Holdover mode so far described refers to a state to which

the internal state machine switches as a result of activity

or frequency alarms, and this state is reported in Reg. 09.

To avoid the DPLL’s frequency being pulled off as a result

of a failed input, then the DPLL has a fast mechanism to

freeze its current frequency within one or two cycles of the

input clock source stopping. Under these circumstances

the DPLL enters Mini-holdover mode; the Mini-holdover

frequency used being determined by Reg. 40, Bits [4:3],

cnfg_holdover_modes, mini_holdover_mode.

Mini-holdover mode only lasts until one of the following

happens:

A new source has been selected, or

The state machine enters Holdover mode, or

The original fault on the input recovers.

External Factors Affecting Holdover Mode

If the external TCXO/OCXO 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

fluctuations are). It is advantageous to shield the

TCXO/OCXO 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 Holdover is

directly related to the stability of the external oscillator.

Pre-locked2 Mode

This state is very similar to the Pre-Locked 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 ACS8520A will enter the Locked state in a

maximum of 100 seconds, as defined by GR-1244-

CORE[19] 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.

DPLL Architecture and Configuration

A Digital PLL gives a stable and consistent level of

performance that can be easily programmed for different

dynamic behavior or operating range. It is not affected by

operating conditions or silicon process variations. Digital

synthesis is used to generate all required SONET/SDH

output frequencies. The digital logic operates at

204.8 MHz that is multiplied up from the external

12.800 MHz oscillator module. Hence the best resolution

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