si5351 Silicon Laboratories, si5351 Datasheet - Page 10

• Manufacturer Part Number: si5351
• Description: Si5350a Factory-programmable Cmos Clock Generator
• Manufacturer: Silicon Laboratories

Si5351A/B/C
2.4. 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. Up to –15 dB reduction in EMI is
possible. 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. The amount of spread is configurable with the following:


2.5. Control Pins (OEB, SSEN)
The Si5351 offers control pins for enabling/disabling clock outputs and spread spectrum.
2.5.1. 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 asserted (OEB = low). When OEB is released (OEB = high), the clock is allowed to complete its
full clock cycle before going into a disabled state. The operation of the output enable pin is shown in Figure 5.
2.5.2. 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.
10
Frequency
Down spread: –0.5 to –2.5% modulation amplitude
Center spread: ±0.1 to ±1.5% modulation amplitude
Amplitude
Center
No Spread
Spectrum
f
c
OEBx
CLKx
Figure 4. Available Spread Spectrum Profiles
Figure 5. Glitchless Output Enable/Disable
Amplitude
Reduced
and EMI
Clock starts on the
first leading edge
Center Spread
f
c
Rev. 0.1
+/- 0.1% to +/- 1.5%
Spread Amount
Clock continues until
cycle is complete
Amplitude
Reduced
and EMI
Down Spread
f
c
- 0.1% to - 2.5%
Spread Amount