CYPRESS CY25901 User Manual

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CY25901

Spread Spectrum Clock Generator

Features

• Supports clock requirements for Spread Spectrum

• 40-MHz Spread Spectrum clock output

• Reference clock output

• Two spread bandwidths: 1% , 3%

• External clock or Cera-Lock input

• 3.3V operation

• 8-pin SOIC package

Ta ble 1. Function Table

SSSEL SSON REFOUT

00XIN= XIN ± 0.35% (0.7% center) = XIN ± 0.5% (1.0% center)
1 0 XIN = XIN ± 1.20% (2.4% center) = XIN ± 1.5% (3.0% center)

X (don’t care) 1 XIN = XIN (No Spread) = XIN (No Spread)

Block Diagram

XIN

Oscillator

XOUT

REFOUT

Description

The CY25901 clock generator provi des a low-elec tromagnetic
interference (EMI) clock output. It features Spread Spectrum
technology, a modulation technique designed specifically for
reducing EMI at the fund amental frequency and it s harmonics.

CLKOUT (Spread Spectrum)

CY25901SC CY25901SC-1

Pin Configuration

CLKOUT

VDD

VSS

XIN

1
2
3
4

8
7
6
5

SSON#
REFOUT
SSSEL
XOUT

SSON

SSSEL

Cypress Semiconductor Corporation • 3901 North First Street • San Jose, CA 95134 • 408-943-2600
Document #: 38-07521 Rev. ** Revised February 18, 2003

PLL

CLKOUT

SSCG

CY25901

Pin Description

Pin No. Name I/O Description

1CLKOUTOSpread Spectrum Clock Output. See Table 1 for frequency selections.
2VDD PWRPower Supply.
3 VSS PWR Common Ground.
4XIN IOscillator Buffer Input. Connect to an external parallel resona nt crystal or externally ge nerated

5XOUT OOscill ator Buffer Output. Connect to an external p arallel resonant cryst al. Do not connect when

6 SSSEL I Spread Spectrum Select Input. See Table 1. internally pulled up.
7REFOUTOBuffered Output of XIN.
8 SSON# I Spread Spectrum Enable Input. When asserted low, Spread Spectrum is enabled.

reference clock.

an externally generated reference clock is applied at XIN.

Internally pulled down.

Spread Spectrum Clock Generator

Spread Spectrum Clock Generator (SSCG) is a frequency
modulation techniqu e used to reduce EMI radiatio n generated
by repetitive digital signals, mainly cl ocks. A clock radiates EM
energy at its fundamental frequency as well as its harmonics.
Spread Spectrum distributes this energy over a small
frequency bandwidth, and decreasing the peak value of

Figure 1. Modulation Frequency Profile

0

-10

-20

-30

-40

dBm

-50

-60

-70

-80
38 38 39 39 40 41 41 42 43

Frequ ency( MHz)

radiated energy over the spectrum . This technique is achieved
by modulating the clock around or below the center of its
reference frequency by a certain percentage (which also
determine s the energy distribution bandwidth).

The SSCG function is enabled when SSON pin is set to low.
Resulting in a spread ban dwidth tha t is center spread, amount
as selected by SSSEL (see Table 1).

Spread S pec t rum

SS-ON
SS-OFF

Figure 2. Spread Spectrum

Document #: 38-07521 Rev. ** Page 2 of 5