VITESSE

SEMICONDUCTOR CORPORATION

Multi-Rate SONET/SDH

Clock and Data Recovery IC

Data Sheet

VSC8122

G52228-0, Rev 4.1

Page 3

01/05/01

© VITESSE SEMICONDUCTOR CORPORATION • 741 Calle Plano • Camarillo, CA 93012

Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com

Internet: www.vitesse.com

Outputs can also be AC terminated as shown in Figure 3. The output differential voltage and common-mode

voltage range are specified in Table 4, High-Speed Inputs and Outputs.

Figure 3: High-Speed Clock and Data Output AC Termination

Clock Recovery

The VSC8122 has a selectable input data rate. Two pins (FSEL0 and FSEL1) select the data rate to be pro-

vided to the VSC8122.

Table 1: Input Data Rate Select

The incoming data is presented both to the clock recovery circuit and the data retiming circuit. When there

is a phase error between the incoming data and the on-chip Voltage-Controlled Oscillator (VCO), the loop filter

raises or lowers the control voltage of the VCO to null the phase difference.

The lock detector monitors the frequency difference between the REFCK (optionally divided by a pres-

caler) and the recovered clock divided by 128. In the event of the loss of an input signal, or if the input is switch-

ing randomly, the VCO will move in one direction. At the time the VCO differs by more than 1MHz from the

REFCK based 2.48832GHz rate, the lock detector will assert the LOL output. LOL is designed to be asserted

from between 2.3µs and 100µs after the interruption of data. The VCO will continue to be frequency-locked at

approximately 1MHz off of the REFCK based 2.48832GHz rate.

When NRZ data is again presented at the data input, the phase detector will permit the VCO to lock to the

incoming data. Hysteresis is provided which delays the deassertion of LOL until approximately 160µs following

the restoration of valid data.

The NOREF output will go high to indicate that there is no signal on the REFCK input, or that the REFCK

is more than approximately 25% above or below the expected value.

Input Data Rate

FSEL0

FSEL1

2.48832Gb/s or 2.5Gb/s

00

1.24416Gb/s or 1.25Gb/s

10

622.08Mb/s or 625Mb/s

01

155.52Mb/s or 156.25Mb/s

11

VSC8122

50

Ω

50

Ω

100

Ω

100

Ω

CO+ / DO+

CO- / DO-

0.1

µF

0.1

µF