PCS3P622Z05B, PCS3P622Z05C, PCS3P622Z09B, PCS3P622Z09C
CLKIN
PLL
PCS3P622Z05B/C
CLKIN
PLL MUX
DLY_CTRL
CLKOUT1
CLKOUT2
CLKOUT3
CLKOUT4
PCS3P622Z09B/C
S2 Select
Input
S1 Decoding
Figure 1. General Block Diagrams
DLY_CTRL
CLKOUTA1
CLKOUTA2
CLKOUTA3
CLKOUTA4
CLKOUTB1
CLKOUTB2
CLKOUTB3
CLKOUTB4
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 PCS3P622Z05/09 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 DLY_CTRL 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 DLY_CTRL, must be equally loaded.
Even if DLY_CTRL 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.
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