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MPC958 Datasheet(PDF) 4 Page - Motorola, Inc

Part No. MPC958
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Maker  MOTOROLA [Motorola, Inc]
Homepage  http://www.freescale.com

MPC958 Datasheet(HTML) 4 Page - Motorola, Inc

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Power Supply Filtering
The MPC958 is a mixed analog/digital product and as
such it exhibits some sensitivities that would not necessarily
be seen on a fully digital product. Analog circuitry is naturally
susceptible to random noise, especially if this noise is seen
on the power supply pins. The MPC958 provides a separate
power supply for the phase–locked loop (VCCA) of the
device. The purpose of this design technique is to try and
isolate the high switching noise digital outputs from the
relatively sensitive internal analog phase–locked loop. In a
controlled environment such as an evaluation board this level
of isolation is sufficient. However, in a digital system
environment where it is more difficult to minimize noise on the
power supplies a second level of isolation may be required.
The simplest form of isolation is a power supply filter on the
VCCA pin for the MPC958.
Figure 3 illustrates a typical power supply filter scheme.
The MPC958 is most susceptible to noise with spectral
content in the 1KHz to 1MHz range. Therefore the filter
should be designed to target this range. The key parameter
that needs to be met in the final filter design is the DC voltage
drop that will be seen between the VCC supply and the VCCA
pin of the MPC958. From the data sheet the IVCCA current
(the current sourced through the VCCA pin) is typically 15mA
(20mA maximum), assuming that a minimum of 3.0V must be
maintained on the VCCA pin very little DC voltage drop can
be tolerated when a 3.3V VCC supply is used. The resistor
shown in Figure 3 must have a resistance of 10–15
Ω to meet
the voltage drop criteria. The RC filter pictured will provide a
broadband filter with approximately 100:1 attenuation for
noise whose spectral content is above 20KHz. As the noise
frequency crosses the series resonant point of an individual
capacitor it’s overall impedance begins to look inductive and
thus increases with increasing frequency. The parallel
capacitor combination shown ensures that a low impedance
path to ground exists for frequencies well above the
bandwidth of the PLL. It is recommended that the user start
with an 8–10
Ω resistor to avoid potential VCC drop problems
and only move to the higher value resistors when a higher
level of attenuation is shown to be needed.
Figure 3. Power Supply Filter
Although the MPC958 has several design features to
minimize the susceptibility to power supply noise (isolated
power and grounds and fully differential PLL) there still may
be applications in which overall performance is being
degraded due to system power supply noise. The power
supply filter schemes discussed in this section should be
adequate to eliminate power supply noise related problems
in most designs.
Driving Transmission Lines
The MPC958 clock driver was designed to drive high
speed signals in a terminated transmission line environment.
To provide the optimum flexibility to the user the output
drivers were designed to exhibit the lowest impedance
possible. With an output impedance of approximately 20
the drivers can drive either parallel or series terminated
transmission lines. For more information on transmission
lines the reader is referred to application note AN1091 in the
Timing Solutions brochure (BR1333/D).
In most high performance clock networks point–to–point
distribution of signals is the method of choice. In a
point–to–point scheme either series terminated or parallel
terminated transmission lines can be used. The parallel
technique terminates the signal at the end of the line with a
Ω resistance to VCC/2. This technique draws a fairly high
level of DC current and thus only a single terminated line can
be driven by each output of the MPC958 clock driver. For the
series terminated case however there is no DC current draw,
thus the outputs can drive multiple series terminated lines.
Figure 4 illustrates an output driving a single series
terminated line vs two series terminated lines in parallel.
When taken to its extreme the fanout of the MPC958 clock
driver is effectively doubled due to its capability to drive
multiple lines.
Figure 4. Single versus Dual Transmission Lines
RS = 36Ω
ZO = 50Ω
RS = 36Ω
ZO = 50Ω
RS = 36Ω
ZO = 50Ω
The waveform plots of Figure 5 show the simulation
results of an output driving a single line vs two lines. In both
cases the drive capability of the MPC958 output buffers is
more than sufficient to drive 50
Ω transmission lines on the
incident edge. Note from the delay measurements in the
simulations a delta of only 43ps exists between the two
differently loaded outputs. This suggests that the dual line
driving need not be used exclusively to maintain the tight
output–to–output skew of the MPC958. The output waveform
in Figure 5 shows a step in the waveform, this step is caused
Freescale Semiconductor, Inc.
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