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Cypress Semiconductor Electronic Components Datasheet

W42C31-03 Datasheet

Spread Spectrum Frequency Timing Generator

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W42C31-03
Spread Spectrum Frequency Timing Generator
Features
• Maximized EMI suppression using Cypress’s Spread
Spectrum technology
• Generates a spread spectrum copy of the provided
input
• Integrated loop filter components
• Operates with a 5V supply
• Low power CMOS design
• Available in 8-pin SOIC (Small Outline Integrated
Circuit)
Overview
The W42C31-03 incorporates the latest advances in PLL
spread spectrum frequency synthesizer techniques. By fre-
quency modulating the output with a low-frequency carrier,
EMI is greatly reduced. Use of this technology allows systems
to pass increasingly difficult EMI testing without resorting to
costly shielding or redesign.
Simplified Block Diagram
5.0V
In a system, not only is EMI reduced in the various clock lines,
but also in all signals which are synchronized to the clock.
Therefore, the benefits of using this technology increase with
the number of address and data lines in the system. The Sim-
plified Block Diagram shows a simple implementation.
Table 1. Frequency Spread Selection
W42C31-03
FS1 FS0
00
01
10
11
Oscillator
Input
Frequency
(MHz)
10 to 20
10 to 20
20 to 33
20 to 33
XTAL Input
Frequency
(MHz)
Output
Frequency
(MHz)
10 to 20
10 to 20
20 to 25
20 to 25
fIN ±1.875%
fIN ±1.0%
fIN ±1.875%
fIN –2.0%
Pin Configuration
XTAL
Input
X1
X2 W42C31-03
Spread Spectrum
Output
(EMI suppressed)
X1
X2
GND
FS0
SOIC
1 8 OE#
2 7 FS1
3 6 VDD
4 5 CLKOUT
5.0V
Oscillator or Reference
Input
W42C31-03
Spread Spectrum
Output
(EMI suppressed)
Cypress Semiconductor Corporation • 3901 North First Street • San Jose • CA 95134 • 408-943-2600
September 28, 1999, rev. **


Cypress Semiconductor Electronic Components Datasheet

W42C31-03 Datasheet

Spread Spectrum Frequency Timing Generator

No Preview Available !

W42C31-03
Pin Definitions
Pin Name
CLKOUT
X1
X2
OE#
FS0
FS1
VDD
GND
Pin No.
5
1
2
8
4
7
6
3
Pin
Type
O
I
I
I
I
I
P
G
Pin Description
Output Modulated Frequency: Frequency modulated copy of the unmodulated input
clock
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
clock.
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.
Functional Description
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
common.
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.
VDD
XTAL
X1
X2
Freq.
Divider
Q
Crystal load
capacitors
as needed
Phase
Detector
Charge
Pump
Σ
VCO
Post
Dividers
Feedback
Divider
P
Modulating
Waveform
PLL
GND
Figure 1. System Block Diagram
CLKOUT
2


Part Number W42C31-03
Description Spread Spectrum Frequency Timing Generator
Maker Cypress Semiconductor
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