Frequency Reference

The internal reference oscillator contains all of the passive

components required. An appropriate series-resonant crystal

should be connected between XTAL1 (1) and XTAL2 (2). In

IBM-compatible applications, this will typically be a

14.31818 MHz crystal, but fundamental mode crystals be-

tween 10 MHz and 25 MHz have been tested. Maintain short

lead lengths between the crystal and the ICS2694. In order to

optimize the quality of the quartz crystal oscillator, the input

switching threshold of XTAL1 is VDD/2 rather than the con-

ventional 1.4V of TTL. Therefore, XTAL1 may not respond

properly to a legal TTL signal since TTL is not required to

exceed VDD/2. Therefore, another clock input CLKIN (pin 13)

has been added to the chip which is sized to have an input

switching point of 1.4V. Inside the chip, these two inputs are

ANDED. Therefore, when using the XTAL1 and XTAL2,

CLKIN should be held high. (It has a pull-up.) When using

CLKIN, XTAL1 should be held high. (It does not have a

pull-up because a pull-up would interfere with the oscillator

bias.)

It is anticipated that some applications will use both clock

inputs, properly gated, for either board test or unique system

functions. By generating all the system clocks from one refer-

ence input, the phase and delay relationships between the

various outputs will remain relatively fixed, thereby eliminat-

ing problems arising from totally unsynchronized clocks inter-

acting in a system.

Power Supply Conditioning

The ICS2694 is a member of the second generation of dot clock

products. By incorporating the loop filter on chip and upgrad-

ing the VCO, the ease of application has been substantially

improved over earlier products. If a stable and noise-free power

supply is available, no external components are required. How-

ever, in some applications it may be judicious to decouple the

power supply as shown in Figures 1 or 2. Figure 1 is the normal

configuration for 5 volt only applications. Which of the two

provides superior performance depends on the noise content of

the power supplies. In general, the configuration of Figure 1 is

satisfactory. Figure 2 is the more conventional if a 12 volt

analog supply is available, although the improved performance

comes at a cost of an extra component; however, the cost of the

discretes used in Figure 1’s are less than the cost of Figure 1’s

discrete components.

Since the ICS2694 outputs a large number of high-frequency

clocks, conservative design practices are recommended. Care

should be exercised in the board layout of supply and ground

traces, and adequate power supply decoupling capacitors con-

sistent with the application should be used.

R1

33

+5

+5

DVDD

VSS, AVSS

AVDD

C2

22µV

C1

C3

.µ1F

.µ1F

Figure 1

D1

R1

470

+120

+50

DVDD

VSS, AVSS

AVDD

4.7V

C1

C2

.µ1F

.µ1F

Figure 2

ICS2694

3