W185

3

Overview

The W185 products are one series of devices in the Cypress

PREMIS family. The PREMIS family incorporates the latest

advances in PLL spread spectrum frequency synthesizer tech-

niques. By frequency modulating the output with a low-fre-

quency carrier, peak EMI is greatly reduced. Use of this tech-

nology allows systems to pass increasingly difficult EMI testing

without resorting to costly 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.

Functional Description

The W185 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 W184 the output

frequency is nominally 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 predetermined 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 for a given

frequency, the modulation percentage may be varied.

Using frequency select bits (FS1:2 pins), the frequency range

can be set. Spreading percentage may be selected as either

1.25% or 3.75% (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 percentage options are provided.

Freq.

Phase

Modulating

VCO

Post

CLKOUT

Detector

Charge

Pump

Waveform

Dividers

Divider

Feedback

Divider

PLL

GND

Σ

Q

P

Clock Input

Reference Input

(EMI suppressed)

Figure 1. Functional Block Diagram