October 2003

ASM3P2821B

rev 1.0

Alliance Semiconductor

2575, Augustine Drive

• Santa Clara, CA • Tel: 408.855.4900 • Fax: 408.855.4999 • www.alsc.com

Notice: The information in this document is subject to change without notice.

Features

FCC approved method of EMI attenuation.

Generates a 1X low EMI spread spectrum clock of

the input frequency.

Input frequency range: 15MHz – 27MHz.

Internal loop filter minimizes external components

and board space.

Frequency deviation: -2.5%

Low inherent cycle-to-cycle jitter.

3.3V operating voltage.

TTL or CMOS compatible inputs and outputs.

Ultra-low power CMOS design.

o

TBD mA @ 3.3V, 24MHz

Pinout

compatible

with

Cypress

CY25811.

Products are available for industrial temperature

range.

Available in 8-pin SOIC and TSSOP.

Product Description

The ASM3P2821B is a versatile spread spectrum

frequency modulator designed specifically for input clock

frequencies in the range of 15MHz - 27MHz. The

ASM3P2821B can generate an EMI reduced clock from a

ceramic resonator or system clock. The ASM3P2821B

offers a percentage deviation of –2.5%.

The ASM3P2821B reduces electromagnetic interference

(EMI) at the clock source, allowing system wide reduction

of EMI of

down stream clock and data dependent

signals. The ASM3P2821B allows significant system cost

savings by reducing the number of circuit board layers

ferrite beads, shielding and other passive components

that are traditionally required to pass EMI regulations.

The ASM3P2821B uses the most efficient and optimized

modulation profile approved by the FCC and is

implemented in a proprietary all digital method.

The ASM3P2821B modulates the output of a single PLL

in order to “spread” the bandwidth of a synthesized clock,

and more importantly, decreases the peak amplitudes of

its harmonics. This results in significantly lower system

EMI compared to the typical narrow band signal produced

by oscillators and most frequency generators. Lowering

EMI by increasing a signal’s bandwidth is called ‘spread

spectrum clock generation’.

Applications

The ASM3P2821B is targeted towards EMI management

for high speed digital applications such as PC peripheral

devices, consumer electronics and embedded controller

systems.

Block Diagram

VSS

XIN

Crystal

Oscillator

Frequency

Divider

Feedback

Divider

Modulation

Phase

Detector

Loop

Filter

VCO

Output

Divider

MODOUT

PLL

VDD