+/+…when timing is critical
C9866
Low EMI Clock Generator for Intel 815 Chipset Systems
Advanced Information
IMI Confidential
If the above frequency is not accurate enough, then you must choose another R value and start from the beginning.
For example choose R = 49 and this will yield an N = 73.99808365, which is rounded to 74. If the 74 is applied in the
formula 1, then Fcpu = 145.0038MHz(accuracy +26 ppm).
Other R values within the above limits may also be evaluated.
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.8A) or around the center (Fig.8B) of its resting frequency by a certain percentage (which also determines the energy
distribution bandwidth). In this device, Spread Spectrum is enabled by setting I2C Byte0, Bit3 = 1. The default of the
device at power up keeps the Spread Spectrum disabled, it is therefore, important to have I2C accessibility to turn-on the
Spread Spectrum function. Once the Spread Spectrum is enabled, the spread bandwidth option is selected by I2C Byte
5, Bits 5, 6 & 7 following tables 4A, and 4B below.
In Down Spread mode the center frequency is shifted down from its rested (non-spread) value by ½ of the total spread
%. (eg.: assuming the center frequency is 100MHz in non-spread mode; when down spread of –0.5% is enabled, the
center frequency shifts to 99.75MHz.).
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Rev 0.02
4/13/2000
Page 11 of 19
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