C9531
Document #: 38-07034 Rev. *D
Page 6 of 10
Spread Spectrum Clocking
Down Spread 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
undesirable electromagnetic energy (EMI) over a wide range
of frequencies therefore reducing the average radiated energy
present at any frequency over a given time period. As the
spread is specified as a percentage of the resting (non-spread)
frequency value, it is effective at the fundamental and, to a
greater extent, at all of its harmonics.
In this device Spread Spectrum is enabled externally through
pin 15 (SSCG#) or internally via SMBus Byte 0 Bit 0 and 6.
Spread spectrum is enabled externally when the SSCG# pin
is low. This pin has an internal device pull up resistor, which
causes its state to default to a HIGH (spread spectrum
modulation disabled) unless externally forced to a low. It may
also be enabled by programming SMBus Byte 0 Bit 0 LOW (to
enable SMBus control of the function) and then programming
SMBus byte 0 bit 6 low to set the feature active.
Note:
5.
When SSCG is enabled, the device will down spread the clock over a range that is 1% of its resting frequency. This means that for a 100-MHz output clock
frequency will sweep through a spectral range from 99 to 100 MHz.
Table 6. Spectrum Spreading Selection Table[5]
Output Clock Frequency
% of Frequency Spreading
Mode
SMBus Byte 0 Bit 5 = 0
SMBus Byte 0 Bit 5 = 1
33.3 MHz (XIN)
1.0% (–1.0% + 0%)
0.5% (–0.5% + 0%)
Down Spread
66.6 MHz (XIN*2)
1.0% (–1.0% + 0%)
0.5% (–0.5% + 0%)
Down Spread
100.0 MHz (XIN*3)
1.0% (–1.0% + 0%)
0.5% (–0.5% + 0%)
Down Spread
133.3 MHz (XIN*4)
1.0% (–1.0% + 0%)
0.5% (–0.5% + 0%)
Down Spread
S p r ead of f
S p r ead o n
C e n t er F r equ enc y ,
S p r e ad of f
C ent er F r e que nc y ,
S p r ead o n
Figure 1. Spread Spectrum
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