STC5455 datasheet(21/56 Pages) CONNOR-WINFIELD

21 / 56 page

Page 21 of 56 TM121 Rev: P1.3

Date: October 24, 2011

STC5455

Synchronous Clock for SETS

Functional Specification

Data sheet

Preliminary

value. The alarm assert threshold value must be

greater than the alarm de-assert threshold value and

less than or equal to the bucket size value. Attempted

writes of invalid values will be ignored. Therefore,

user must carefully plan an appropriate sequence of

writes when re-configure the activity monitor. See

Assert Threshold for details.

Alarms appear in the Refs Activity register. A “1”

indicates activity, and a “0” indicates an alarm, no

activity. Note that if a reference is detected as a differ-

ent frequency, the leaky bucket accumulator is set to

the bucket size value and the reference will become

inactive immediately.

Input Qualification

A selected reference is “qualified” if it passes the

activity evaluation and its frequency offset is within

the programmed qualification range for over a pre-

programmed soaking time.

A reference qualification range may be programmed

up to 102.3 ppm by writing to register Qualification

Range, and a disqualification range set up to 102.3

ppm, by writing to register Disqualification Range.

The qualification range must be set less than the dis-

qualification range. Additionally, qualification soaking

time may be programmed from 0 to 63 seconds by

writing to register Qualification Soaking Time. The

pull-in range is the same as the disqualification range.

Figure 3: Reference Qualification Scheme

Activity

Not Good

Activity

Good

Qualified

Activity Alarm

Asserted

Activity Alarm

Asserted

Activity Alarm

De-Asserted

Out of Disqualification Range

Within Offset Qualification

Range for more than

Qualification soaking Time

Continuously

The frequency offset of each reference is relative to

the internal freerun clock may be read by selecting

the reference in the Ref Info Selector register and

then reading the offset value from register Ref Info.

Figure 3 shows the reference qualification scheme. A

reference is qualified if it has no activity alarm and is

continuous within the qualification range for more

than the qualification soaking time. An activity alarm

or frequency offset beyond the disqualification range

will disqualify the reference. It may then be re-quali-

fied if the activity alarm is off and the reference is

within the qualification range for more than the qualifi-

cation time.

The reference qualification status of each reference

may be read from register Ref Qual.

Automatic Reference Election Mechanism

The STC5455 has an auto reference elector always

elect the best candidate from the reference inputs

according to the revertivity status, each reference’s

priority and qualification. This mechanism operates

independent of reference selection mode. In other

word, regardless what the current reference selection

mode is, the auto reference elector always work in

this mechanism. The detail description of the refer-

ence selection mode is in following sections.

The reference priority is indicated in the reference pri-

ority table which is shown in register Ref Priority

Table individually for each timing generator. Each ref-

erence has one entry in the table, which may be set to

value from 0 to 15. ‘0’ revokes the reference from the

election, while 1 to 15 set the priority, where ‘1’ has

the highest, and ‘15’ has the lowest priority. The high-

est priority pre-qualified reference then is a candidate

selected by the automatic reference elector. If multi-

ple references share the same priority, the one that

has been qualified for the longest time will be recom-

mended to be the candidate. If the current highest pri-

ority reference input fails, the next-highest priority

reference is selected as the candidate.

In order to avoid disturbance of the clock output, the

candidate reference selected by automatic reference

elector should be handled in two different mode.

Revertive mode and non-revertive mode. The mode

is determined by either enabling or disabling the

“revertive” bit of the Control Mode to “1” for revertive

or to “0” for non-revertive operation.

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