ICS342
FIELD PROGRAMMABLE DUAL OUTPUT SS VERSACLOCK SYNTHESIZER EPROM CLOCK SYNTHESIZER
IDT® / ICS™
FIELD PROGRAMMABLE DUAL OUTPUT SS VERSACLOCK SYNTHESIZER 3
ICS342 REV N 090613
3) To minimize EMI, the 33 series termination resistor (if
needed) should be placed close to the clock output.
4) An optimum layout is one with all components on the
same side of the board, minimizing vias through other signal
layers. Other signal traces should be routed away from the
ICS342. This includes signal traces just underneath the
device, or on layers adjacent to the ground plane layer used
by the device.
ICS342 Configuration Capabilities
The architecture of the ICS342 allows the user to easily
configure the device to a wide range of output frequencies,
for a given input reference frequency.
The frequency multiplier PLL provides a high degree of
precision. The M/N values (the multiplier/divide values
available to generate the target VCO frequency) can be set
within the range of M = 1 to 2048 and N = 1 to 1024.
The ICS342 also provides separate output divide values,
from 2 through 20, to allow the two output clock banks to
support widely differing frequency values from the same
PLL.
Each output frequency can be represented as:
IDT VersaClock Software
IDT applies years of PLL optimization experience into a user
friendly software that accepts the user’s target reference
clock and output frequencies and generates the lowest jitter,
lowest power configuration, with only a press of a button.
The user does not need to have prior PLL experience or
determine the optimal VCO frequency to support multiple
output frequencies.
VersaClock software quickly evaluates accessible VCO
frequencies with available output divide values and provides
an easy to understand, bar code rating for the target output
frequencies. The user may evaluate output accuracy,
performance trade-off scenarios in seconds.
Spread Spectrum Modulation
The ICS342 utilizes frequency modulation (FM) to distribute
energy over a range of frequencies. By modulating the
output clock frequencies, the device effectively lowers
energy across a broader range of frequencies; thus,
lowering a system’s electromagnetic interference (EMI). The
modulation rate is the time from transitioning from a
minimum frequency to a maximum frequency and then back
to the minimum.
Spread Spectrum Modulation can be applied as either
“center spread” or “down spread”. During center spread
modulation, the deviation from the target frequency is equal
in the positive and negative directions. The effective
average frequency is equal to the target frequency. In
applications where the clock is driving a component with a
maximum frequency rating, down spread should be applied.
In this case, the maximum frequency, including modulation,
is the target frequency. The effective average frequency is
less than the target frequency.
The ICS342 operates in both center spread and down
spread modes. For center spread, the frequency can be
modulated between +/- 0.125% to +/-2.0%. For down
spread, the frequency can be modulated between -0.25% to
-4.0%.
Both output frequency banks will utilize identical spread
spectrum percentage deviations and modulation rates, if a
common VCO frequency can be identified.
Spread Spectrum Modulation Rate
The spread spectrum modulation frequency applied to the
output clock frequency may occur at a variety of rates. For
applications requiring the driving of “down-circuit” PLLs,
Zero Delay Buffers, or those adhering to PCI standards, the
spread spectrum modulation rate should be set to 30-33
kHz. For other applications, a 120 kHz modulation option is
available.
Using VersaClock Products with an Input Clock
Source
In order to ensure proper startup with an input clock rather
than a crystal, the supply voltage must be within the
operating range (3.3V ±10%) and the input signal must be
stable and free from glitching. The input clock must provide
pulses of at least 20ns, and no more than 500ns, for at least
160 clock cycles without any interruptions to the clock or
power during this period. It may take up to 4ms for output
frequencies to reach their target frequency values.
An alternative method is to have the PDTS
pin asserted low
while power supplies and clock sources stabilize.Once the
power supply and input clock source are constant and within
the acceptable frequency range, bring PDTS
high. This
approach is preferred if the clock source is derived from
another PLL, or the source oscillator produces
unpredictable output pulses prior to stabilization. No
considerations need to be taken when using a crystal input
source with VersaClock products.
OutputFreq
REFFreq
OutputDivide
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M
N
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