W181

Document #: 38-07152 Rev. *D

Page 2 of 9

Key Specifications

Frequency Range: ............................ 28 MHz

Crystal Reference Range.................. 28 MHz

Cycle to Cycle Jitter: ....................................... 300 ps (max.)

Selectable Spread Percentage: ................... 1.25% or 3.75%

Output Duty Cycle: ............................... 40/60% (worst case)

Output Rise and Fall Time: .................................. 5 ns (max.)

Overview

The W181 products are one series of devices in the Cypress

PREMIS family. The PREMIS family 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 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

Simplified Block Diagram on page 1 shows a simple imple-

mentation.

Functional Description

The W181 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 W181 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

predetermined frequency band.

Pin Definitions

Pin Name

Pin No.

(SOIC)

Pin No.

(TSSOP)(-01)

Pin

Type

Pin Description

CLKOUT

5

8

O

Output Modulated Frequency: Frequency modulated copy of the

unmodulated input clock (SSON# asserted).

CLKIN or X1

1

2

I

Crystal Connection or External Reference Frequency Input: This pin

has dual functions. It may either be connected to an external crystal, or

to an external reference clock.

NC or X2

2

3

I

Crystal Connection: If using an external reference, this pin must be left

unconnected.

SSON#

8(02/03/52/53)

--

I

Spread Spectrum Control (Active LOW): Asserting this signal (active

LOW) turns the internal modulation waveform on. This pin has an internal

pull-down resistor.

FS1:2

7, 8 (01/51)

12, 1

I

Frequency Selection Bit(s) 1 and 2: These pins select the frequency

range of operation. Refer to Table 2. These pins have internal pull-up

resistors.

SS%

4

6

I

Modulation Width Selection: When Spread Spectrum feature is turned

on, this pin is used to select the amount of variation and peak EMI

reduction that is desired on the output signal. This pin has an internal

pull-up resistor.

VDD

6

10

P

Power Connection: Connected to 3.3V or 5V power supply.

GND

3

4

G

Ground Connection: Connect all ground pins to the common system

ground plane.

NC

5, 7, 9, 11, 13,

14

NC No Connection

Table 1. Modulation Width Selection

SS%

W181-01, 02, 03 Output W181-51, 52, 53 Output

0

–1.25%

(Down Spread)

±0.625

(Center Spread)

1

–3.75%

(Down Spread)

±1.875%

(Center Spread)

Table 2. Frequency Range Selection

W181 Option#

FS2

FS1

-01, 51

(MHz)

-02, 52

(MHz)

-03, 53

(MHz)

00

28

N/A

01

38

N/A

10

46

N/A

46

11

58

N/A

58