W42C31-06 Datasheet, PDF (3/6 Pages) Cypress Semiconductor – Spread Spectrum Frequency Timing Generator

W42C31-06
Spread Spectrum Frequency Timing
Generation
The benefits of using Spread Spectrum Frequency Timing
Generation are depicted in Figure 2. An EMI emission profile
of a clock harmonic is shown.
Contrast the typical clock EMI with the Cypress Spread Spec-
trum Frequency Timing Generation EMI. Notice the spike in
the typical clock. This spike can make systems fail quasi-peak
EMI testing. The FCC and other regulatory agencies test for
peak emissions. With spread spectrum enabled, the peak en-
ergy is much lower (at least 8 dB) because the energy is
spread out across a wider bandwidth.
Modulating Waveform
The shape of the modulating waveform is critical to EMI reduc-
tion. The modulation scheme used to accomplish the maxi-
mum reduction in EMI is shown in Figure 3. The period of the
modulation is shown as a percentage of the period length
along the X axis. The amount that the frequency is varied is
shown along the Y axis, also shown as a percentage of the
total frequency spread.
Cypress frequency selection tables express the modulation
percentage in two ways. The first method displays the spread-
ing frequency band as a percent of the programmed average
output frequency, symmetric about the programmed average
frequency. This method is always shown using the expression
f_Center ± X_MOD% in the frequency spread selection table.
The second approach is to specify the maximum operating
frequency and the spreading band as a percentage of this fre-
quency. The output signal is swept from the lower edge of the
band to the maximum frequency. The expression for this ap-
proach is f_MAX – X_MOD%. Whenever this expression is used,
Cypress has taken care to ensure that f_MAX will never be ex-
ceeded. This is important in applications where the clock
drives components with tight maximum clock speed specifica-
tions.
5dB/div
SSFTG
Typical Clock
Figure 2. Typical Clock and SSFTG Comparison
SSON# Pin
An internal pull-down resistor defaults the chip into spread
spectrum mode. The SSON# pin enables the spreading fea-
ture when set LOW. When disabled (SSON# HIGH), the
W42C31-06 simply passes through the input clock. Upon go-
ing LOW, the device takes 2 ms to re-track the spreading algo-
rithm.
100%
80%
60%
40%
20%
0%
–20%
–40%
–60%
–80%
–100%
Time
Figure 3. Modulation Waveform Profile
3