Output Modulated Frequency: Frequency modulated copy of the unmodulated input
Crystal Connection or External Reference Frequency Input: This pin has dual
functions. It may either be connected to an external crystal, or to an external reference
Crystal Connection: If using an external reference, this pin must be left unconnected.
Output Enable (Active LOW): This pin three-states the output when HIGH. It has an
internal pull-down resistor.
Frequency Selection Bit 0: This pin selects the frequency spreading characteristics.
Refer to Table 1. This pin has a pull-up resistor.
Frequency Selection Bit 1: This pin selects the frequency range. Refer to Table 1.
This pin has a pull-up resistor.
Power Connection: Connected to 5V power supply.
Ground Connection: This should be connected to the common ground plane.
The W42C31-03 uses a phase-locked loop (PLL) to frequency
modulate an input clock. The result is an output clock whose
frequency is slowly swept over a narrow band near the input
signal. The basic circuit topology is shown in Figure 1. An
on-chip crystal driver causes the crystal to oscillate at its fun-
damental. The resulting reference signal is divided by Q and
fed to the phase detector. A signal from the VCO is divided by
P and fed back to the phase detector also. The PLL will force
the frequency of the VCO output signal to change until the
divided output signal and the divided reference signal match
at the phase detector input. The output frequency is then equal
to the ratio of P/Q times the reference frequency. The unique
feature of the Spread Spectrum Clock Generator is that a mod-
ulating waveform is superimposed at the input to the VCO.
This causes the VCO output to be slowly swept across a pre-
determined frequency band.
Because the modulating frequency is typically 1000 times
slower than the fundamental clock, the spread spectrum pro-
cess has little impact on system performance.
Frequency Selection With SSFTG
In Spread Spectrum Frequency Timing Generation, EMI re-
duction depends on the shape, modulation percentage, and
frequency of the modulating waveform. While the shape and
frequency of the modulating waveform are fixed, the modula-
tion percentage may be varied.
Using frequency select bits (FS1:0 pins), various spreading
percentages can be chosen (see Table 1).
A larger spreading percentage improves EMI reduction. How-
ever, large spread percentages may either exceed system
maximum frequency ratings or lower the average frequency to
a point where performance is affected. For these reasons,
spreading percentages between ±0.5% and ±2.5% are most
The W42C31 features the ability to select from various spread
spectrum characteristics. Selections specific to the
W42C31-03 are shown in Table 1. Other spreading character-
istics are available (see separate data sheets) or can be cre-
ated with a custom mask.
Figure 1. System Block Diagram