W42C31-09 Datasheet, PDF (2/7 Pages) Cypress Semiconductor – Spread Spectrum Frequency Timing Generator

W42C31-09
Pin Definitions
Pin Name
CLKOUT
CLKIN
NC
SSON#
FS0:1
VDD
GND
Pin No.
7
1
2
8
6, 4
5
3
Pin
Type
O
I
I
I
I
P
G
Pin Description
Output Modulated Frequency: Frequency modulated copy of the unmodulated input
clock.
External Reference Frequency Input
No Connect: This pin must be left unconnected.
Spread Spectrum Control (Active LOW): Pulling this input signal LOW turns the
internal modulation waveform on. This pin has an internal pull-down resistor.
Frequency Selection Bit 0: These pins select the frequency spreading
characteristics. Refer to Table 1. These pins have internal pull-up resistors.
Power Connection: Connected to 3.3V or 5V power supply.
Ground Connection: This should be connected to the common ground plane.
Functional Description
The W42C31-09 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. The
input 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. (Note: For the W42C31-09 the
output frequency is equal to the input frequency.) The unique
feature of the Spread Spectrum Frequency Timing Generator
is that a modulating waveform is superimposed at the input to
the VCO. This causes the VCO output to be slowly swept
across a predetermined 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
common.
The W42C31 features the ability to select from various spread
spectrum characteristics. Selections specific to the
W42C31-09 are shown in Table 1. Other spreading character-
istics are available (see separate data sheets) or can be cre-
ated with a custom mask. Also, other devices in the W42C31
family offer frequency multiplication in addition to the spread
spectrum function. This will allow the use of less expensive
fundamental mode crystals.
Clock Input
Reference Input
Freq.
Divider
Q
VDD
Phase
Detector
Charge
Pump
Σ
VCO
Post
Dividers
Feedback
Divider
P
Modulating
Waveform
PLL
CLKOUT
(EMI suppressed)
GND
Figure 1. System Block Diagram (Concept, not actual implementation)
2