May 2008
rev 0.5
ASM3P622S00B/E
Spread Spectrum Frequency Generation
The clocks in digital systems are typically square waves
with a 50% duty cycle and as frequencies increase the
edge rates also get faster. Analysis shows that a square
wave is composed of fundamental frequency and
harmonics. The fundamental frequency and harmonics
generate the energy peaks that become the source of
EMI. Regulatory agencies test electronic equipment by
measuring the amount of peak energy radiated from the
equipment. In fact, the peak level allowed decreases as
the frequency increases. The standard methods of
reducing EMI are to use shielding, filtering, multi-layer
PCBs etc. These methods are expensive. Spread
spectrum clocking reduces the peak energy by reducing
the Q factor of the clock. This is done by slowly
modulating the clock frequency. The ASM3P622S00B/E
uses the center modulation spread spectrum technique in
which the modulated output frequency varies above and
below the reference frequency with
a
specified
modulation rate. With center modulation, the average
frequency is the same as the unmodulated frequency and
there is no performance degradation
Zero Delay and Skew Control
All outputs should be uniformly loaded to achieve Zero
Delay between input and output. Since the CLKOUT pin
is the internal feedback to the PLL, its relative loading can
adjust the input-output delay.
For applications requiring zero input-output delay, all
outputs, including CLKOUT, must be equally loaded.
Even if CLKOUT is not used, it must have a capacitive
load equal to that on other outputs, for obtaining zero-
input-output delay.
Timing-Safe™ technology
Timing-Safe™ technology is the ability to modulate a
clock source with Spread Spectrum technology and
maintain synchronization with any associated data path.
Low Frequency Timing-Safe™ Peak EMI Reduction IC
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Notice: The information in this document is subject to change without notice.
PULSECORE [ PulseCore Semiconductor ]
