+/+…when timing is critical
C9853
High Performance Pentium III Clock Generator
Preliminary
Spectrum Spread Clocking Description
Spread Spectrum is a modulation technique for distributing clock period over a certain bandwidth (called Spread
Bandwidth). This technique allows the distribution of the energy (EMI) over a range of frequencies therefore reducing
the radiation generated from clocks. As the spread is a percentage of the rested (non-spread) frequency, it is effective
at the fundamental and all its harmonics.
In this device Spread Spectrum is enabled through pin 42 (Spread#). As the name suggests, spread spectrum is
enabled when Spread# is low. This pin has a 250KΩ internal pull up, therefore, defaults to a high (Spread Spectrum
disabled) unless externally forced to a low.
When Spread# is forced low, the device will be down spread (fig.5B) mode at –0.5%, and 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.), see fig.4 below.
Down Spread
With Spectrum Spread
Without Spectrum Spread
Amplitude
(dB)
Frequency (MHz)
Spectrum Analysis
Fig. 4
Spectrum Spreading Selection Table
Unspread
Frequency in MHz
Spread Spectrum Parameter
Down Spreading
F Min
(MHz)
99.5
F Center
F Max
Spread
(%)
- 0.5
- 0.5
(MHz)
99.75
132.6
(MHz)
100.0
133.3
100.0
133.3
132.3
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST.
MILPITAS, CA TEL 408-263-6300, FAX 408-263-6571
http://www.imicorp.com
1.0
4/13/2000
Page 10 of 16
CYPRESS [ CYPRESS ]
