8431AM-21

www.icst.com/products/hiperclocks.html

REV. A APRIL 27, 2005

9

Integrated

Circuit

Systems, Inc.

ICS8431-21

TO

Spread-spectrum clocking is a frequency modulation tech-

nique for EMI reduction. When spread-spectrum is enabled,

a 30kHz triangle waveform is used with 0.5% down-spread

(+0.0% / -0.5%) from the nominal 200MHz clock frequency.

An example of a triangle frequency modulation profile is shown

in

Figure 4A below. The ramp profile can be expressed as:

• Fnom = Nominal Clock Frequency in Spread OFF mode

(200MHz with 16MHz IN)

• Fm = Nominal Modulation Frequency (30kHz)

•

δ = Modulation Factor (0.5% down spread)

(1 -

δ) fnom + 2 fm x δ x fnom x t when 0 < t <

,

(1 -

δ) fnom - 2 fm x δ x fnom x t when

< t <

1

2 fm

1

2 fm

1

fm

The ICS8431-21 triangle modulation frequency deviation will

not exceed 0.6% down-spread from the nominal clock fre-

quency (+0.0% / -0.5%). An example of the amount of down

spread relative to the nominal clock frequency can be seen in

the frequency domain, as shown in

Figure 4B. The ratio of

this width to the fundamental frequency is typically 0.4%, and

will not exceed 0.6%. The resulting spectral reduction will be

greater than 7dB, as shown in

Figure 4B. It is important to

note the ICS8431-21 7dB minimum spectral reduction is the

component-specific EMI reduction, and will not necessarily

be the same as the system EMI reduction.

Δ − 10 dBm

B

A

➤

Fnom

(1 -

δ) Fnom

0.5/fm

1/fm

➤

The ICS8431-21 has been characterized with 18pF parallel resonant

crystals. The capacitor values, C1 and C2, shown in

Figure 3 below

were determined using a 25MHz, 18pF parallel resonant crystal and

were chosen to minimize the ppm error. The optimum C1 and C2

values can be slightly adjusted for different board layouts.

C1

22p

X1

18pF Parallel Crystal

C2

22p

XTAL_OUT

XTAL_IN