30 Pin Descriptions

Symbol

Pin

IO

Description

No

16

28

pins

DGND

15

Digital Ground

Internal circuit power supply return

EXTGND

1

External Ground

Output buffer power supply return

AGND

14

Analog Ground

Substrate ground used to ensure proper device biasing and isolation

18

XTL IN

8

I

External Crystal Oscillator Input

XTL IN can also be used as a CMOS compatible reference frequency

input for the PLL This input is selected when REF SEL is at a logical LOW level The component

connections required for oscillator operation are shown in the application diagrams

XTL OUT

6

External Crystal Oscillator Output

XTL OUT is not intended for use as a logic drive output pin

REF IN

5

I

Reference Input

TTL compatible input for use as the PLL’s phase comparator reference frequency

input when the REF SEL is at a logic HI level This input is for use in concentrator configurations where

there are multiple CDD devices at a given site requiring synchronization

FEEDBK IN

4

I

Feedback Input

TTL compatible input for use as the PLL’s phase comparator feedback input to close

the loop This input is intended to be driven from one of the LBCs (Local Byte Clocks) This input is

designed to provide the same frequency and within 2 ns of the same phase as REF IN when REF IN is in

active operation

REF SEL

9

I

Reference Select

TTL compatible input which selects either the crystal oscillator inputs XTL IN and

XTL OUT or the REF IN inputs as the reference frequency inputs for the PLL The crystal oscillator inputs

are selected when REF SEL is at a logic LOW level and the REF IN input is selected as the reference

frequency when REF SEL is at a logic HI level

FILTER

10

O

Filter

Low pass PLL loop filter pin A three element filter consisting of one capacitor in parallel with a

resistor and another capacitor should be connected between this pin and ground

VCO SEL

17

I

VCO Select

TTL compatible input used to select either the internal VCO or an external VCO through the

XVCO IN and XVCO INB pins The internal VCO is selected when the VCO SEL pin is at a logic HIGH

level and the external VCO is selected when at a logic LOW level

XVCO IN

13

I

External VCO Inputs

Differential inputs for use with an external VCO These inputs are DC biased to

XVCO INB

12

VCO To use a single-ended VCO couple the signal into one of the inputs through a series low value

capacitor and bypass the other input to GND through a 001 mF capacitor When not in use ground one

input and let the other float

5