W254B
Document #: 38-07233 Rev. *A
Page 7 of 17
Spread Spectrum Frequency Timing
Generation
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 am-
plitudes of the radiated electromagnetic emissions are re-
duced. This effect is depicted in Figure 7.
As shown in Figure 7, 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 8. 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 +0.5% or –0.5% of the se-
lected frequency. Figure 8 details the Cypress spreading pat-
tern. Cypress does offer options with more spread and greater
EMI reduction. Contact your local Sales representative for de-
tails on these devices.
Spread Spectrum clocking is activated or deactivated by se-
lecting the appropriate value for bit 3 in data byte 0 of the
SMBus data stream. Refer to page 9 for more details.
SSFTG
Typical Clock
Frequency Span (MHz)
Spread
Spectrum
Enabled
EMI Reduction
Spread
Spectrum
Non-
Frequency Span (MHz)
Down Spread
Center Spread
Figure 7. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation
MAX.
MIN.
Figure 8. Typical Modulation Profile
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