mc68336 Freescale Semiconductor, Inc, mc68336 Datasheet - Page 15

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

2.8 Designing for Electromagnetic Compatibility
Because of the fast clock speed and relatively short rise and fall times of MCU signals, the designer must
consider electromagnetic compatibility (EMC) issues. All high-speed digital devices radiate noise, and if
FCC compliance is required, the designer must do everything possible to limit emissions from the MCU. Use
of a four-layer board is probably the best single option the designer has. Although a two-layer board will
work, a multilayer PCB is much more effective at both protecting the MCU from emissions, and reducing
emissions from the MCU. EMC compatibility is a complex topic, and this tutorial can present only a brief
overview of EMC design techniques.
2.8.1 Reducing Power Supply Noise
The MCU is very susceptible to noise created by large or rapid fluctuations in current through a particular
power supply pin. The power supply pins are divided up into V
power the external drivers and pins, while the V
MCU. It is very important to keep the V
to power supply noise than the port drivers. When designing a multilayer board, simply route the power and
ground pins directly to the power and ground planes; when designing a two-layer board, however, it is best
to isolate the power bus that serves the core of the chip from the power bus that serves the port drivers.
Figure 11 shows groups of pins on the MC68331/MC68332 that are powered by the same supply pins. Al-
though only the 132-pin plastic surface mount package for the MC68332 is shown, the groups for the 144-
pin package are the same. Also, the pinout for the MC68331 is the same as for the MC68332, except that
the GPT pins take the place of the TPU pins. In each group, the V
group are shown in bold face type. The V
are powered only by V
When control of noise on the power buses is important, it is possible to isolate sections of the chip that are
particularly noisy. The data and address buses are particularly noisy because they continually change state,
and the same can be true of serial ports and timer pins. The amount of noise generated by a particular pin
is dependent upon the load being driven and the switching frequency. A designer who knows which power
and ground connections serve particular pins can shield other signal conductors from these noisy lines.
MC68331/332
M68331/332TUT/D
POWER SUPPLY
Figure 10 Using LVI Devices with Multiple Power Supplies
DDSYN
DEVICE A
Freescale Semiconductor, Inc.
.
For More Information On This Product,
DDI
Go to: www.freescale.com
DDI
VOLTAGE
/V
INHIBIT
LOW
SSI
and V
DDI
pins free of noise, as the CPU is generally more sensitive
/V
SSI
SSI
pins are labeled as such. EXTAL, XTAL, and XFC
pins power the internal peripherals and core of the
VOLTAGE
INHIBIT
LOW
DDE
/V
DDE
SSE
and V
and V
POWER SUPPLY
SSE
DEVICE B
DDI
pins that power a particular
/V
SSI
. The V
DDE
332TUT DUAL RESET CONN
/V
SSE
pins
15