MPC930

Manufacturer Part NumberMPC930
ManufacturerFreescale Semiconductor, Inc
MPC930 datasheet
 


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MPC930 MPC931
Although the MPC930/931 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.
Using the Power Management Features of the
MPC930/931
The MPC930/931 clock driver offers two different features
that designers can take advantage of for managing power
dissipation in their designs. The first feature allows the user
to turn off outputs which drive portions of the system which
may go idle in a sleep mode. The Shut_Dn pins allow for
three different combinations of output shut down schemes.
The schemes are summarized in the function tables in the
data sheet. The MPC930/931 synchronizes the shut down
signals internal to the chip and applies them in a manner
which eliminates the possibility of creating runt pulse on the
outputs. The device waits for the output to go into the “LOW”
state prior to disabling. When the outputs are re–enabled the
device waits and re–enables the output such that the
transition is synchronous and in the proper phase
relationship to the outputs which remained active.
The Power_Dn pin offers another means of implementing
power management schemes into a design. To use this
feature the device must be set up in its normal operating
mode with the Power_Dn pin “LOW”, in addition the user
must use the internal feedback option. If the external
feedback option were used the output frequency reduction
would change the feedback frequency and the PLL will lose
lock. When the Power_Dn pin is driven “HIGH” the
MPC930/931 synchronizes the signal to the internal clock
and then seemlessly reduces the frequency of the outputs by
one half. The Power_Dn signal is synchronized to the
slowest internal VCO clock. It waits until both VCO clocks are
in the “LOW” state and then switches from the nominal speed
VCO clock to the half speed VCO clock. This will in turn
cause the current output pulse to stretch to reflect the
reduction in output frequency. When the Power_Dn pin is
brought back “LOW” the device will again wait until both of
the VCO clocks are “LOW” and then switch to the nominal
VCO clock. This will cause the current output pulses, and all
successive pulses, to shrink to match the higher output
frequency. Both the power up and power down features are
illustrated in the timing diagrams of in this data sheet.
Timing diagrams for both of the power management
features are shown in Figure 3 and Figure 4 on page 3.
Using the On–Board Crystal Oscillator
The MPC930 features an on–board crystal oscillator to
allow for seed clock generation as well as final distribution.
The on–board oscillator is completely self contained so that
the only external component required is the crystal. As the
oscillator is somewhat sensitive to loading on its inputs the
MOTOROLA
user is advised to mount the crystal as close to the
MPC930/931 as possible to avoid any board level parasitics.
To facilitate co–location surface mount crystals are
recommended, but not required.
The oscillator circuit is a series resonant circuit as
opposed to the more common parallel resonant circuit, this
eliminates the need for large on–board capacitors. Because
the design is a series resonant design for the optimum
frequency accuracy a series resonant crystal should be used
(see specification table below). Unfortunately most off the
shelf crystals are characterized in a parallel resonant mode.
However a parallel resonant crystal is physically no different
than a series resonant crystal, a parallel resonant crystal is
simply a crystal which has been characterized in its parallel
resonant mode. Therefore in the majority of cases a parallel
specified crystal can be used with the MPC930 with just a
minor frequency error due to the actual series resonant
frequency of the parallel resonant specified crystal. Typically
a parallel specified crystal used in a series resonant mode
will exhibit an oscillatory frequency a few hundred ppm lower
than the specified value. For most processor implement–
ations a few hundred ppm translates into kHz inaccuracies, a
level which does not represent a major issue.
Table 4. Crystal Specifications
Parameter
Crystal Cut
Resonance
Frequency Tolerance
Frequency/Temperature Stability
Operating Range
Shunt Capacitance
Equivalent Series Resistance (ESR)
Correlation Drive Level
Aging
* See accompanying text for series versus parallel resonant
discussion.
The MPC930 is a clock driver which was designed to
generate outputs with programmable frequency relationships
and not a synthesizer with a fixed input frequency. As a result
the crystal input frequency is a function of the desired output
frequency. For a design which utilizes the external feedback
to the PLL the selection of the crystal frequency is straight
forward; simply chose a crystal which is equal in frequency to
the fed back signal. To determine the crystal required to
produce the desired output frequency for an application
which utilizes internal feedback the block diagram of
Figure 15 should be used. The P and the M values for the
MPC930/931 are also included in Figure 15. The M values
can be found in the configuration tables included in this
applications section.
10
Value
Fundamental AT Cut
Series Resonance*
75ppm at 25 C
150pm 0 to 70 C
0 to 70 C
5–7pF
50 to 80Ω Max
100µW
5ppm/Yr (First 3 Years)
TIMING SOLUTIONS
BR1333 — Rev 6