Cypress W42C31-03 Datasheet
W42C31-03
Spread Spectrum Frequency Timing Generator
Features
• Maximized EMI suppression usi ng Cypress’s Spread
Spec trum te chnol ogy
• Generates a spread spectrum copy of the provided
input
• Integrated loop filter components
• Operates with a 5V supply
• Low po w er 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 frequency 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
X1
XTAL
Input
X2
W42C31-03
Spread Spectrum
Output
(EMI suppressed)
In a system, not on ly is EMI reduced i n the v ari ous cl oc k lin es,
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 Simplified Block Diagram shows a simple implementation.
T able 1. Frequency Spread Selection
W42C31-03 Oscillator
Input
Frequency
(MHz)
0 0 10 to 20 10 to 20 f
0 1 10 to 20 10 to 20 f
1 0 20 to 33 20 to 25 f
1 1 20 to 33 20 to 25 f
XTAL Input
Frequency
(MHz)
Output
Frequency
(MHz)FS1 FS0
±1.875%
IN
±1.0%
IN
±1.875%
IN
–2.0%
IN
Pin Configuration
SOIC
X1
X2
GND
FS0
W42C31-03
1
2
3
4
8
7
6
5
OE#
FS1
VDD
CLKOUT
5.0V
Oscillator or Refer ence
Input
W42C31-03
Cypress Semiconductor Corporation
Spread Spectr um
(EMI suppressed)
Output
• 3901 North First Street • San Jose • CA 95134 • 408-943-2600
September 28, 1999, r ev. **
Pin Definitions
W42C31-03
Pin Name Pin No.
CLKOUT 5 O
T ype Pin Description
Output Modulated Fr equency
clock
Pin
X1 1 I
Crystal Connection or External Reference Frequency Input:
functions. I t ma y e ither b e connec ted to an e xternal crystal, or to an ex ternal ref er ence
clock.
X2 2 I
OE# 8 I
Crystal Connection:
Output Enable (Active LOW):
internal pull-down resistor.
FS0 4 I
Frequency Selection Bi t 0:
Refer to Table 1 . This pin has a pull-up resistor.
FS1 7 I
Frequency Selection Bit 1:
This pin has a pull-up resistor.
VDD 6 P
GND 3 G
Power Connection:
Ground Connection:
Functional Description
The W42C31-03 uses a p hase- loc k ed loop ( PLL) t o freq uency
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 fundamental. The resulting reference signal is divided by Q and
fed to the phase det ector. 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 phas e detector input. The output frequenc y is th en equal
to the ratio of P/Q times the reference frequency. The unique
featur e of the Spread Spect rum Clock Generat or is t hat a modulating waveform is superimposed at the input to the VCO.
This causes the VCO output to be slowly swept across a predetermined frequen cy band.
Because the modulating frequency is typically 1000 times
slower than the fundamental clock, the spread spectrum process has litt le i m pact on system performance.
: Fre quenc y modul ated cop y of the un modul ate d input
This pin has dual
If usi ng an external reference , thi s pin mus t be l eft unc onnected .
This pin three-states the output when HIGH. It has an
This pin selec ts the fr equenc y spreadi ng charac terist ics .
This pin selects the fre quency range. Refer to Table 1.
Connected to 5V power supply.
This should be connect ed to the common ground plane.
Frequency Selection With SSFTG
In Spread Spectrum Frequency Timing Generati on, EMI reduction depends on the shape, modulation percentage, and
frequency of the modulating waveform. While the shape and
frequency of the modulating waveform are fixed, the modulation percentage may be varied.
Using frequency select bits (FS1:0 pins), various spreading
percentages can be chosen (see Table 1).
A larger spreading per centage improves EMI reduction . However, large spread percentages may either exceed system
maximum frequ ency ra tings or lo wer the a v erag e fr equency t o
a point where performance is affected. For these reasons,
spreading percentages between ±0.5% and ±2.5% are most
common.
The W42C31 feat ures t he abil ity to sel ect from v arious 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 created w i th a custom mask.
XTAL
Crystal load
capacitors
as needed
X1
X2
Freq.
Q
VDD
Phase
Detector
Feedback
Divider
P
Charge
Pump
GND
Σ
Modulating
Waveform
PLL
Figure 1. System Block Diagram
2
VCO
Post
DividersDivider
CLKOUT
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