Freescale Semiconductor, Inc.
Appendix: CGM Practical Aspects
spectrum. This is especially the case for the power supply pins
close to the E port, when the E clock and/or the calibration clock
are used.
• On the general VDD power supply input, a ‘T’ low pass filter LCL
can be used (e.g. 10µH-47µF-10µH). The ‘T’ is preferable to the
‘Π’ version as the exhibited impedance is more constant with
respect to the VDD current. Like many modular micro controllers,
HC12 devices have a power consumption which not only varies
with clock edges but also with the functioning modes.
• Keep high speed clock and bus trace length to a minimum. The
higher the clock speed, the shorter the trace length. If noisy
signals are sent over long tracks, impedance adjustments should
be considered at both ends of the line (generally, simple resistors
suffice).
• Bus drivers like the CAN physical interface should be installed
close to their connector, with dedicated filtering on their power
supply.
• Mount components as close to the board as possible. Snip excess
lead length as close to the board as possible. Preferably use
Surface Mount Devices (SMDs).
• Mount discrete components as close to the chip that uses them as
possible.
• Do not cross sensitive signals ON ANY LAYER. If a sensitive
signal must be crossed by another signal, do it as many layers
away as possible and at right angles.
• Always keep PCBs clean. Solder flux, oils from fingerprints,
humidity and general dirt can conduct electricity. Sensitive circuits
can easily be disrupted by small amounts of leakage.
• Choose PCB coatings with care. Certain epoxies, paints, gelatins,
plastics and waxes can conduct electricity. If the manufacturer
cannot provide the electrical characteristics of the substance, do
not use it.
Advance Information
396
68HC(9)12D60 — Rev 4.0
Appendix: CGM Practical Aspects
MOTOROLA
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