ACS8525AP datasheet(11/112 Pages) SEMTECH

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ACS8525A LC/P

ADVANCED COMMS & SENSING

FINAL

DATASHEET

Revision 1.00/September 2007 © Semtech Corp.

Page 11

www.semtech.com

Disqualification of a non-selected SEC is based on

inactivity noted by the Activity Monitors. The currently

selected SEC can be disqualified for being out-of phase,

inactive, or if the source is outside the DPLL lock range.

If the currently selected SEC is disqualified, the next

highest priority qualified SEC is selected.

Interrupts for Activity Monitors

The loss of the currently selected SEC 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 SEC is

provided to facilitate very fast source failure detection if

desired. It is triggered after missing just a couple of cycles

of the SEC. Some applications require the facility to switch

downstream devices based on the status of the SECs. 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

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

an SEC that has previously been fully active (Leaky Bucket

empty) will be:

(cnfg_upper_threshold_n) / 8

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 SEC is calculated, for a

particular Leaky Bucket, as:

[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

Fast Activity Monitor

Anomalies on the selected clock have to be detected as

they occur and the PLL must be temporarily isolated until

the clock is once again pure. The SEC activity monitoring

process cannot be used for this because the high degree

of accuracy required dictates that the process be slow. To

achieve the immediacy required, the PLL requires an

alternative mechanism. The phase locked loop itself

contains a fast activity detector such that within

approximately two missing input clock cycles, a no-activity

flag is raised and the DPLL is frozen in Digital Holdover

mode. This flag can also be read as the DPLL1

main_ref_failed bit (from Reg. 06 sts_interrupts, Bit 6)

and can be set to indicate a phase lost state by enabling

Reg. 73, Bit 6. With the DPLL in Digital Holdover mode it

is isolated from further disturbances. If the input becomes

available again before the activity monitor rejection alarm

has been raised, then the DPLL will continue to lock to the

input, with little disturbance. In this scenario, with the

DPLL in the “locked” state, the DPLL uses “nearest edge

locking” mode (±180° capture) avoiding cycle slips or

glitches caused by trying to lock to an edge 360° away, as

would happen with traditional PLLs.

Selector

This block has two main functions:

Selection of the Input reference clock source via

Reg. 33 force_select_reference_source

Forcing of the Operating mode of the device, via

Reg. 32 cnfg_operating_mode

Selection of Input SECs

Under normal operation, the input SECs are selected

automatically by an order of priority given in the Priority

Table. For special circumstances however, such as chip or

board testing, the selection may be forced by

configuration.

Automatic operation selects an SEC based on its

predefined priority and its current validity. A table is

maintained which lists all valid SECs in the order of

priority. This is initially downloaded into the ACS8525A via

the Serial interface by the Network Manager, and is

subsequently modified by the results of the ongoing

quality monitoring. In this way, when all the defined

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