Silicon Laboratories SI5351A/B/C - Manual (Page 13)

Si5351A/B/C
Preliminary Rev. 0.95 13
3.4. Spread Spectrum
Spread spectrum can be enabled on any of the clock outputs that use PLLA as its reference. Spread spectrum is
useful for reducing electromagnetic interference (EMI). Enabling spread spectrum on an output clock modulates its
frequency, which effectively reduces the overall amplitude of its radiated energy. See “AN554: Si5350/51 PCB
Layout Guide” for details. Note that spread spectrum is not available on clocks synchronized to PLLB or to the
VCXO.
The Si5351 supports several levels of spread spectrum allowing the designer to chose an ideal compromise
between system performance and EMI compliance.
Figure 6. Available Spread Spectrum Profiles
3.5. Control Pins (OEB, SSEN)
The Si5351 offers control pins for enabling/disabling clock outputs and spread spectrum.
3.5.1. Output Enable (OEB)
The output enable pin allows enabling or disabling outputs clocks. Output clocks are enabled when the OEB pin is
held low, and disabled when pulled high. When disabled, the output state is configurable as disabled high, disabled
low, or disabled in high-impedance.
The output enable control circuitry ensures glitchless operation by starting the output clock cycle on the first leading
edge after OEB is pulled low. When OEB is pulled high, the clock is allowed to complete its full clock cycle before
going into a disabled state.
3.5.2. Spread Spectrum Enable (SSEN)—Si5351A and Si5351B only
This control pin allows disabling the spread spectrum feature for all outputs that were configured with spread
spectrum enabled. Hold SSEN low to disable spread spectrum. The SSEN pin provides a convenient method of
evaluating the effect of using spread spectrum clocks during EMI compliance testing.
f
c
Reduced
Amplitude
and EMI
Down Spread
f
c
Reduced
Amplitude
and EMI
Center Spread
f
c
No Spread
Spectrum
Center
Frequency
Amplitude