C9806C
I
2C Frequency Clock Generator for Mobile Pentium II Applications.
Preliminary
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST.
Rev.1.0
2/9/2000
MILPITAS, CA 95035. TEL: 408-263-6300. FAX 408-263-6571
Page 8 of 15
http://www.imicorp.com
Reducing Clock Noise with Power Bypassing
Figure 4
The C9806C produces many different frequency clocks simultaneously. If these clocks are allowed to couple back into
the Internal Analog circuitry that may cause a destablizing noise injection condition. To eliminate this potential problem
and produce the cleanest design and clock outputs, IMI recommends bypassing the 3.3V VDD supply to the Analog
Power Pin (pin 25) with the circuitry shown in Figure 4. This will effectively isolate the device’s Analog Power Supply from
any noise that is present on it that will cause increased jitter and phase noise within it internal analog circuitry.
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
or around the center 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, bit1 = 1, and bit0 = 0. 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 SST (0:2)
in I2C byte 0, bits 4, 5 & 6 following table 5 below.
Down Spread
Center Spread
25
R
X
3.3 Volt VDD
.I nF