Figure 1

VCLK

XTALOUT

N/C

MCLK

MS1

EXTFREQ

FS0

FS1

STROBE

FS2

FS3

MS0

10

C3

C2

.1

22

ICS2495

5.0V

NOTES:

FS3-FS0, MS1-MS0, EXTFREQ, and STROBE inputs are all equipped with pull-ups and need not be tied high.

Mount decoupling capacitors as close as possible to the device and connect device ground to the ground plane where available.

Mount crystal and its circuit traces away from switching digital lines and the VCLK, MCLK and XTALOUT lines.

Reference Oscillator & Crystal Selection

In cases where the on-chip crystal oscillator is used to generate

the reference frequency, the accuracy of the crystal oscillation

frequency will have a very small effect on output accuracy.

The external crystal and the on-chip circuit implement a Pierce

oscillator. In a Pierce oscillator, the crystal is operated in its

parallel-resonant (also called anti-resonant mode). This means

that its actual frequency of oscillation depends on the effective

capacitance that appears across the terminals of the quartz

crystal. Use of a crystal that is characterized for use in a

series-resonant circuit is fine, although the actual oscillation

frequency will be slightly higher than the value stamped on the

crystal can (typically 0.025%-0.05% or so). Normally, this

error is not significant in video graphics applications, which is

why the ICS2495 will typically derive its frequency reference

from a series resonant crystal connected between pins 1 and

16.

As the entire operation of the phase-locked loop depends on

having a stable reference frequency, the crystal should be

mounted as close as possible to the package. Avoid routing

digital signals or the ICS2495 outputs underneath or near these

traces. It is also desirable to ground the crystal can to the

ground plane, if possible.

Power Supply Conditioning

The ICS2495 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 most applications it is judicious to decouple the power

supply as shown in Figure 1.

Layout Considerations

Utilizing the ICS2495 in video graphics adapter cards or on

PS2 motherboards is simple, but does require precautions in

board layout if satisfactory jitter-free performance is to be

realized. Care should be exercised to ensure that components

not related to the ICS2495 do not share its ground. In applica-

connected to the ground plane.

Frequency Reference

The internal reference oscillator contains all of the passive

components required. An appropriate crystal should be con-

nected between XTAL1 (16) and XTAL2 (1). In IBM compat-

ible applications this will typically be a 14.31818 MHz crystal,

but fundamental mode crystals between 10 MHz and 25 MHz

have been tested. Maintain short lead lengths between the

crystal and the ICS2495. In some applications, it may be

desirable to utilize the bus clock. If the signal amplitude is

equal to or greater than 3.5 volts, it may be connected directly

to XTAL1 (16). If the signal amplitude is less than 3.5 volts,

connect the clock through a .047 microfarad capacitor to

XTAL1 (16), and keep the lead length of the capacitor to

XTAL1 (16) to a minimum to reduce noise susceptibility . This

coupling the input restores noise immunity. The ICS2495 is

not sensitive to the duty cycle of the bus clock; however, the

quality of this signal varies considerably with different moth-

erboard designs. As the quality of this signal is typically outside

of the control of the graphics adapter card manufacturer, it is

suggested that this signal be buffered on the graphics adapter

board. XTAL2 (1) must be left open in this configuration.

ICS2495

2