W150

Rev 1.0, November 24, 2006

Page 4 of 14

Spread Spectrum Generator

The device generates a clock that is frequency modulated in

order to increase the bandwidth that it occupies. By increasing

the bandwidth of the fundamental and its harmonics, the ampli-

tudes of the radiated electromagnetic emissions are reduced.

This effect is depicted in Figure 3.

As shown in Figure 3, a harmonic of a modulated clock has a

much lower amplitude than that of an unmodulated signal. The

reduction in amplitude is dependent on the harmonic number

and the frequency deviation or spread. The equation for the

reduction is

dB = 6.5 + 9*log10(P) + 9*log10(F)

Where P is the percentage of deviation and F is the frequency

in MHz where the reduction is measured.

The output clock is modulated with a waveform depicted in

Figure 4. This waveform, as discussed in “Spread Spectrum

Clock Generation for the Reduction of Radiated Emissions” by

Bush, Fessler, and Hardin produces the maximum reduction

in the amplitude of radiated electromagnetic emissions. The

deviation selected for this chip is specified in Table 6. Figure 4

details the Cypress spreading pattern. Cypress does offer

options with more spread and greater EMI reduction. Contact

your local Sales representative for details on these devices.

Spread Spectrum clocking is activated or deactivated by

selecting the appropriate values for bits 1–0 in data byte 0 of

the SMBus data stream. Refer to Table 7 for more details.

Power-on

Reset

Timer

Output Three-state

Data

Latch

Hold

QD

W150

Clock Load

R

10 k

Output

Buffer

Output

Low

Output Strapping Resistor

Series Termination Resistor

Jumper Options

Resistor Value R

Figure 2. Input Logic Selection Through Jumper Option

Figure 3. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation

SSFTG

Typical Clock

Frequency Span (MHz)

–1.0

+1.0

0

–0.5%

–SS%

+SS%

5 dB/div

+0.5%