Cypress W42C31-09 Datasheet

W42C31-09

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 3.3V or 5V supply

• Low-power CMOS design

• Available in 8-pin SOIC (Small Outline Integrated
Circuit)

Overview

The W42C31-09 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

VDD

Oscillator or Refer ence
Input

W42C31-09

(EMI suppressed)

Spread Spectrum

Output

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

W42C31-09

0 0 30 to 55 f
0 1 30 to 55 f
1 0 30 to 55 f
1 1 30 to 55 f

Input Frequency

(MHz)

Output Frequency

(MHz)FS1 FS0

±0.625%

IN

±1.25%

IN

±2.5%

IN

–3.75%

IN

Pin Configuration

SOIC

CLKIN

NC

GND

FS1

W42C31-09

1
2
3
4

8
7
6
5

SSON#
CLKOUT
FS0
VDD

Cypress Semiconductor Corporation

• 3901 North First Street • San Jose • CA 95134 • 408-943-2600
January 25, 2000, rev. *B

Pin Definitions

W42C31-09

Pin Name Pin No.

CLKOUT 7 O

T ype Pin Description

Output Modulated Fr equency

clock.

Pin

CLKIN 1 I
NC 2 I
SSON# 8 I

External Reference Frequency Input
No Connect:

This pin must be left unconnected.

Spread Spectrum Control (Active LOW):

internal modulation wa veform on. This pin has an internal pull-down resi stor.

FS0:1 6, 4 I

Frequency Selection Bit 0:

characteristics. Refer to Table 1 . These pins have internal pull-up resistors.
VDD 5 P
GND 3 G

Power Connection:

Ground Connection:

Functional Description

The W42C31-09 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. 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 detec tor al so . The PLL will f or ce the fr equency of
the VCO output sign al to change until the divid ed 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. (Not e: 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 modul ating waveform is s uperi m posed 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 pro­cess has litt le i m pact on system performance.

: Fre quenc y modul ated cop y of the un modul ate d input

Pulling this input signal LOW turns the

These pins select the frequency spreading

Connected to 3.3V or 5V pow er supply.

This should be connect ed to the common ground plane.

Frequency Selection With SSFTG

In Spread Spectrum Frequency Timing Generation, 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
W42C31-09 are shown in Table 1. Other spreading char acter-
istics are available (see separate data sheets) or can be cre­ated 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.

VDD

Clock Input

Reference Input (EMI suppressed)

Freq. Phase

Q

Detector

Feedback

Divider

P

Charge

Pump

GND

Σ

Modulating

Waveform

PLL

VCO

Post

DividersDivider

CLKOUT

Figure 1. System Block Diagra m (Co ncept, not actual implementat ion)

2

W42C31-09

Spread Spectrum Frequency Timing
Generation

The benefits of using Spread Spectrum Frequency Timing
Generation are depicted in Figure 2. An EMI emission profile
of a clock harmonic is shown.

Contrast the typical clock EMI wi th the Cypress Spread Spec­trum Frequency Timing Generation EMI. Notice the spike in
the typical clo c k. This sp ike can mak e system s f ail quas i-peak
EMI testing. The FCC and other regulatory agencies test for
peak emissions. With spr ead spectrum enabled, the peak en­ergy is much lower (at least 8 dB) because the energy is
spread out across a wider ban dwidth.

Modulating Waveform

The shape of the modul ating wa vef orm is critica l to EMI reduc­tion. The modulation scheme used to accomplish the maxi­mum reduction in EMI is shown in Figure 3. The period of the
modulation is shown as a percentage of the period length
along the X axis. The amount that the frequency is varied is
shown along the Y axis, also shown as a percentage of the
total frequency spread.

5dB/div

SSFTG Typical Clock

Cypress frequency selection tables express the modulation
percentage in t wo w ays . The fir st method dis pla ys the sp read ­ing frequency band as a percent of the programmed average
output frequency, symmetric about the programmed average
frequency. This method is always shown usi ng th e expre ssion
f

Center ± XMOD

% in the frequency spr ead selection table.

The second approach is to specify the maximum operating
frequency and the spr eading band as a per centage of this fre­quency. The output signal is swept from the lower edge of the
band to the maximum frequency. The expression for this ap­proach is f
Cypress has taken care to ensure that f

MAX – XMOD

%. Whenever this expression is used,

will never be ex-

MAX

ceeded. This is important in applications where the clock
drives component s with ti ght maxi mum clo c k speed speci fica ­tions.

SSON# Pin

An internal pull-down resistor defaults the chip into a spread
spectrum mode. The SSON# pin enables the spreading fea­ture when set LOW. The SSON# pin disables the spreading
feature when set HIGH (V

EMI Reduction

DD

).

Spread

Spectrum

Enabled

Amplitude (dB)

Frequency Span (MHz)

-SS% +SS%

Non-

Sprea d

Spectrum

Figure 2. Typical Clock and SSFTG Comparison

100%

80%
60%
40%
20%

0%

–20%
–40%

Frequency Shift

–60%
–80%

–100%

10%

20%

30%

40%

50%

60%

70%

80%

90%

Time

100%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Figure 3. Modulation W aveform Prof il e

3