PRELIMINARY
C9851
Clock Generator for Pentium III Server and Workstation Applications
Lumped Test Load Configurations (Cont.)
3.3V signals
tDC
-
-
3.3V
Output under Test
Probe
2.4V
1.5V
Load Cap
0.4V
0V
Tr
Tf
Fig. 1B
Spread Spectrum Clock Generation (SSCG)
Spread Spectrum is a modulation technique applied here for maximum efficiency in minimizing Electro-Magnetic
Interference radiation generated from repetitive digital signals mainly clocks. A clock accumulates EM energy at the
center frequency it is generating. Spread Spectrum distributes this energy over a small frequency bandwidth therefore
spreading the same amount of energy over a spectrum. This technique is achieved by modulating the clock down from
(Fig.2) its resting frequency by a certain percentage (which also determines the energy distribution bandwidth). The
default of the device at power up keeps the Spread Spectrum disabled, therefore, in order to enable this function pin23,
Spread#, must be connect to ground (a low state.). See table 3 for Spread bandwidth description.
In Down Spread mode the center frequency is shifted down from its rested (non-spread) value by -0.25%. (ex.:
assuming the center frequency is 100MHz in non-spread mode; when down spread is enabled, the center frequency
shifts to 99.75MHz.).
In Center Spread mode, the Center frequency remains the same as in the non-spread mode.
Cypress Semiconductor Corporation
525 Los Coches St.
Document#: 38-07068 Rev. **
05/04/2001
Milpitas, CA 95035. Tel: 408-263-6300, Fax: 408-263-6571
http://www.cypress.com
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