mc68336 Freescale Semiconductor, Inc, mc68336 Datasheet - Page 17

• Manufacturer Part Number: mc68336
• Description: An Introduction To The Mc68331 And Mc68332
• Manufacturer: Freescale Semiconductor, Inc

Another way to control power supply noise created by the MCU is to put a small inductor in series with the
power supply lines for the port drivers. This method can help control noise on the power traces of the PCB.
However, it should be used only as a last resort, because it can introduce other noise problems. Also, a
series inductor in the power supply line will probably have little effect on radiated noise, which is generally
a result of the port driver switching speed. Limiting instantaneous current change by putting an inductor in
series with the power supply pin for the port will not appreciably affect the current through a particular driver
because the integrated circuit generally has enough internal capacitance to support an instantaneous cur-
rent surge while the driver switches.
The V
supply noise present on V
supply power to V
2.8.2 A Few Suggestions for Reducing Emissions
In general, follow standard design practices for EMC. A list of techniques that are often used in board design
follows. These techniques are guidelines for good design, not strict rules, and are not specific to designs
that incorporate the MC68331 or MC68332.
MC68331/332
M68331/332TUT/D
• Minimize the number of devices on the board. Capacitive coupling tends to occur around the holes
• Use a canned oscillator instead of a crystal, to reduce emissions from the oscillator. If a crystal circuit
• Use a four layer PCB. As a general rule, a multilayer board is at least ten times better than a two layer
• Plot thick layout lines with the layout program, then cut the actual traces on the board thin.
• If a trace that conducts a high frequency signal must be routed on the surface of the PCB, route ground
• Round off PCB trace corners as much as possible to reduce the amount of excess capacitance that
• Make spacing between adjacent active traces greater than the trace width to minimize crosstalk.
• Put a chassis ground ring on the periphery of each layer of the PCB, to intercept the field coming off
• Use ferrite chokes when troubleshooting. Placing a choke around a signal line and the return conduc-
that connect a particular layer of the board to the power and ground planes.
must be used, locate it as centrally as possible.
board for both emissions and immunity. To reduce emissions even further, enclose the signal traces
between the power and ground planes because the added capacitance between the signal trace and
ground results in a lower characteristic impedance.
traces parallel to it to reduce radiation and crosstalk. Connect the ground traces to ground planes at
varied intervals not to exceed the wavelength/4 at the highest frequency or harmonic expected.
is introduced to the trace at corners.
the board. Interconnect these rings with small ceramic capacitors.
tor carrying a differential signal causes fields developed in the ferrite core by the opposing currents to
cancel. Ferrite chokes can also be used on input/output lines. Because board-mounted chokes in-
crease the number of holes connecting to the supply planes, they should be used only as a last resort.
DDSYN
and V
DDSYN
STBY
supply pins should be separated and isolated from V
, even when using an external oscillator and bypassing the internal PLL.
DDSYN
Figure 12 Proper Placement of a Bypass Capacitor
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will translate into shifts in the system clock generated from the PLL. Always
VDD PIN
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GND PLANE
POWER PLANE
(.1 F TYPICAL)
BYPASS
CAP
DD
with a low pass filter. Any
332TUT VDD LAYOUT
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