Cypress W42C31-06 Datasheet

W42C31-06

Spread Spectrum Frequency Timing Generator

Features

• Maximized EMI suppression usi ng Cypress’s Spread
Spec trum te chnol ogy

• Generates a spread spectrum 3X copy of the 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 W42C31-06 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

Simplified Block Diagram

5.0V

X1

XTAL
Input

X2

W42C31-06

Spread Spectrum
Output

(EMI suppressed)

to pass increasingly difficult EMI testing without resorting to
costly shiel ding or redesign.

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

W42C31-06 Oscillator

Input

Frequency

(MHz)

0 14 to 20 14 to 20 3f
1 14 to 20 14 to 20 3f

XTAL Input

Frequency

(MHz)

Output

Frequency

(MHz)FS0

±0.75%

IN

±1.25%

IN

Pin Configuration

SOIC

X1
X2

GND

FS0

W42C31-06

1
2
3
4

8
7
6
5

SSON#
NC
VDD
CLKOUT

5.0V

Oscillator or Refer ence
Input

W42C31-06

Cypress Semiconductor Corporation

Spread Spectr um

(EMI suppressed)

Output

• 3901 North First Street • San Jose • CA 95134 • 408-943-2600
September 29, 1999, r ev. **

Pin Definitions

W42C31-06

Pin Name Pin No.

CLKOUT 5 O

T ype Pin Description

Output Modulated Fr equency

clock (multiplier of 3).

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
SSON# 8 I

Crystal Connection:

Spread Spectrum (Active LOW):

modulating waveform off. This pin has an inte rnal pul l-down resistor.
FS0 4 I

Frequency Selection Bit 0:

characteristics. Refer to Table 1 . This pin has an internal pull -up resistor.
NC 7 NC
VDD 6 P
GND 3 G

No Connect:

This pin must be left unconnected.

Power Connection:

Ground Connection:

Functional Description

The W42C31-06 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 .

Pulling this inp ut si gnal HIGH turns the internal

This pin selects the fre quency spreadi ng

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 the frequency select bit (FS0), the spreading percent­age 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
W42C31-06 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

V

DD

Phase

Detector

Feedback

Divider

P

Charge

Pump

GND

Σ

Modulating

Waveform

PLL

Figure 1. System Block Diagram

2

VCO

Post

DividersDivider

CLKOUT