Micrel, Inc.

SY69753AL

August 2007

6

M9999-082107-E

Functional Block

Functional Description

Clock Recovery

Clock Recovery, as shown in the block diagram,

generates a clock that is at the same frequency as the

incoming data bit rate at the Serial Data input. The

clock is phase aligned by a PLL so that it samples the

data in the center of the data eye pattern.

The phase relationship between the edge transitions

of the data and those of the generated clock are

compared by a phase/frequency detector. Output

pulses from the detector indicate the required

direction of phase correction. These pulses are

smoothed by an integral loop filter. The output of the

loop filter controls the frequency of the Voltage

Controlled Oscillator (VCO), which generates the

recovered clock.

Frequency

stability,

without

incoming

data,

is

guaranteed by an alternate reference input (REFCLK)

that the PLL locks onto when data is lost. If the

Frequency of the incoming signal varies by greater

than approximately 1000ppm with respect to the

synthesizer frequency, the PLL will be declared out of

lock, and the PLL will lock to the reference clock.

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. This transfer function yields a 30µs data

stream of continuous 1's or 0's for random incoming

NRZ data.

Lock Detect

The SY69753AL contains a link fault indication circuit,

which monitors the integrity of the serial data inputs. If

the received serial data fails the frequency test, then

the PLL will be forced to lock to the local reference

clock. This will maintain the correct frequency of the

recovered clock output under loss of signal or loss of

lock conditions. If the recovered clock frequency

deviates from the local reference clock frequency by

more than approximately 1000ppm, the PLL will be

declared out of lock. The lock detect circuit will poll the

input data stream in an attempt to reacquire lock to

data. If the recovered clock frequency is determined to

be within approximately 1000ppm, the PLL will be

declared in lock and the lock detect output will go

active.

During the interval when the CDR is not locked onto

the RDIN input, the LFIN output will not be a static

LOW, but will be changing.

.