G52154-0, Rev 4.2

© VITESSE SEMICONDUCTOR CORPORATION

Page 7

3/19/99

741 Calle Plano, Camarillo, CA 93012 • 805/388-3700 • FAX: 805/987-5896

VITESSE

SEMICONDUCTOR CORPORATION

Data Sheet

VSC8113

ATM/SONET/SDH 622 Mb/s Transceiver Mux/Demux

with Integrated Clock Generation and Clock Recovery

Good analog design practices should be applied to the board design for these external components. Tightly

controlled analog ground and power planes should be provided for the PLL portion of the circuitry. The dedi-

cated PLL power (VDDANA) and ground (VSSANA) pins should have quiet supply planes to minimize jitter

generation within the clock synthesis unit. This is accomplished by either using a ferrite bead or a C-L-C choke

(

π filter) on the (VDDANA) power pins. Note: Vitesse recommends a (π filter) C-L-C choke over using a ferrite

bead. All ground planes should be tied together using multiple vias.

The VSC8113 features a lock detect function for the CMU, called “CMULOCKDET”. It generates low

going pulses when the CMU is locked to the incoming REFCLK. This is accomplished by comparing the phase

of the synthesized clock to the reference clock. If the “CMULOCKDET” output remains high for > 10

µs, the

CMU is locked.

Reference Clocks

To improve jitter performance and to provide flexibility, an additional differential PECL reference clock

input is provided. This reference clock is internally XNOR’d with a TTL reference clock input to generate the

reference for the CMU. Vitesse recommends using the differential PECL input and tieing the unused TTL refer-

ence clock low. If the TTL reference clock is used the positive side of the differential PECL reference clock

“REFCLKP+” should be tied to ground. “REFCLKP+/-” are internally biased with on-chip resistors to 1.65

volts, see figure 14 for schematic of internal biasing of differential I/O’s.

The CRU has the option of either using the CMU’s reference clock or its own independent reference clock

“CRUREFCLK”. If the CMU reference clock is used, it must be 78MHz. This is accomplished with the control

signal “CRUREFSEL”. The “CRUREFCLK” should be used if the system is being operated in either a regener-

ation or looptiming mode. In either of these modes the quality of the “CRUREFCLK” is not a concern, thus it

can be driven by a simple 77.76MHz crystal, the key is its’ independent of the CMU’s reference clock.

Table 1: Recommended External Capacitor Values

Reference

Frequency

[MHz]

Divide Ratio

CP

CN

Type

Size

Tol.

19.44

32

0.1

0.1

X7R

0603/0803

+/-10%

38.88

16

0.1

0.1

X7R

0603/0803

+/-10%

51.84

12

0.1

0.1

X7R

0603/0803

+/-10%

77.76

8

0.1

0.1

X7R

0603/0803

+/-10%