+/+…when timing is critical
C9825B
Low EMI Clock Generator for Intel 133MHz/3DIMM Chipset Systems
Preliminary
Dial-a-Frequency™ Feature (Cont.)
P is a large value PLL constant that depends on the last frequency selection achieved through the hardware selectors
(S3, S2, S1, S0) or through the software selectors (byte0, bits7,6,5,4). P value may be determined from the following
table:
S(3:0)
P
0000
0001, 1100
32005333
48008000
64010666
96016000
0010, 0011, 0100, 1000, 1111
0110, 0111, 0101, 1001, 1010, 1011, 1101, 1110
Therefore, if a 145MHz (use 145x106 )value is desired, then we should apply 145 into equation 1, and start by
choosing R to be 47 (assume the last frequency selection has the value P = 96016000):
145X106 = 96016000 X N
47
=> N = 70.97775371
Since this N number must be entered in Binary, it can only be an integer, so it must be rounded up or down. Here we
can rounded it up to 71, which will give us an exact CPU frequency of:
Fcpu = 96016000 X N = 145.045 MHz (accuracy + 310 ppm)
47
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 = 145.0038 MHz (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.).
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST.,
MILPITAS, CA 95035, USA. TEL: 408-263-6300, FAX 408-263-6571
http://www.imicorp.com
Rev 1.2
4/12/2000
Page 9 of 18
CYPRESS [ CYPRESS ]
