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Cypress Semiconductor Electronic Components Datasheet



W166

Spread Spectrum Frequency Timing Generator



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W166 pdf
W166
Spread Spectrum Frequency Timing Generator
Features
• Maximized EMI suppression using Cypress’s Spread
Spectrum technology
• Generates a spread spectrum copy of the provided
input
• Selectable spreading characteristics
• Integrated loop filter components
• Operates with a 3.3V or 5V supply
• SSON# pin enables frequency spreading
• Low power CMOS design
• Available in 8-pin SOIC (Small Outline Integrated
Circuit)
Overview
The W166 incorporates the latest advances in PLL spread
spectrum frequency synthesizer techniques. By frequency
modulating the output with a low-frequency carrier, peak EMI
is greatly reduced. Use of this technology allows systems to
pass increasingly difficult EMI testing without resorting to cost-
ly shielding or redesign.
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
W166
FS1 FS0
Input
Frequency
(MHz)
Output
Frequency (MHz)
00
01
10
11
50 to 65
50 to 65
50 to 65
50 to 65
fIN ±0.625%
fIN ±1.25%
fIN ±2.5%
fIN –3.75%
Simplified Block Diagram
3.3V or 5V
Oscillator or Reference
Input
W166
Spread Spectrum
Output
(EMI suppressed)
Pin Configuration
CLKIN
NC
GND
FS1
1
2
3
4
8 SSON#
7 CLKOUT
6 FS0
5 VDD
Cypress Semiconductor Corporation • 3901 North First Street • San Jose • CA 95134 • 408-943-2600
December 20, 1999, rev. **



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W166 pdf
W166
Pin Definitions
Pin Name
CLKOUT
Pin No.
7
CLKIN
NC
SSON#
1
2
8
FS0:1
6, 4
VDD
GND
5
3
Pin
Type
O
I
NC
I
I
P
G
Pin Description
Output Modulated Frequency: Frequency modulated copy of the reference input
(SSON# asserted).
External Reference Frequency Input: Clock input.
No Connect: This pin must be left unconnected.
Spread Spectrum Control (Active LOW): Asserting this signal (active LOW) turns the
internal modulation waveform on. This pin has an internal pull-down resistor.
Frequency Selection Bits 0,1: These pins select the frequency spreading characteris-
tics. 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 W166 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 W166 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 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.
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, nar-
row and wide modulation selections are provided.
Clock Input
Reference Input
Freq.
Divider
Q
VDD
Phase
Detector
Charge
Pump
Σ
VCO
Post
Dividers
CLKOUT
(EMI suppressed)
Feedback
Divider
P
Modulating
Waveform
PLL
GND
Figure 1. System Block Diagram
2



Part Number W166
Description Spread Spectrum Frequency Timing Generator
Maker Cypress Semiconductor - Cypress Semiconductor
Total Page 7 Pages
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