PM5347 datasheet(59/302 Pages) PMC

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PM5347 S/UNI-PLUS

DATA SHEET

PMC-941033

ISSUE 6

SATURN USER NETWORK INTERFACE PLUS

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE

FUNCTIONAL DESCRIPTION

9.1

Clock Recovery

The clock recovery unit recovers the clock from the incoming bit serial data

stream. The clock recovery unit is fully compliant with SONET and SDH jitter

tolerance requirements. The clock recovery unit utilizes a low frequency

reference clock to train and monitor its clock recovery PLL. Under loss of signal

conditions, the clock recovery unit continues to output a line rate clock that is

locked to this reference for keep alive purposes. The clock recovery unit can be

configured to utilize reference clocks at 6.48 or 19.44 MHz. The clock recovery

unit provides status bits that indicate whether it is locked to data or the

reference. The clock recovery unit also supports diagnostic loopback and a loss

of signal input that squelches normal input data.

Initially, the PLL locks to the reference clock, RRCLK+/-. When the frequency of

the recovered clock is within 488 ppm of the reference clock, the PLL attempts

to lock to the data. Once in data lock, the PLL reverts to the reference clock if no

data transitions occur in 80 bit periods or if the recovered clock drifts beyond 488

ppm of the reference clock.

When the transmit clock is derived from the recovered clock (loop timing), the

accuracy of the transmit clock is directly related to the RRCLK+/- reference

accuracy in the case of a loss of signal condition. To meet the Bellcore GR-253-

CORE SONET Network Element free-run accuracy specification, the reference

must be within +/-20ppm. When not loop timed, the RRCLK+/- accuracy may be

relaxed to +/-50ppm.

The loop filter transfer function is optimized to enable the PLL to track the jitter,

yet tolerate the minimum transition density expected in a received SONET data

signal. The total loop dynamics of the clock recovery PLL yield a jitter tolerance

which exceeds the minimum tolerance proposed for SONET equipment by GR-

253-CORE (Figure 3). The jitter tolerance illustrated is associated with the

external loop filter components recommended in the Operation section.

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