AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 16

• Manufacturer Part Number: AN2321
• Description: Designing for Board Level Electromagnetic Compatibility
• Manufacturer: Freescale Semiconductor / Motorola

AN2321/D
Diode termination
Microcontroller
Circuits
16
Figure 16
termination, except that the resistors are replaced by diodes, which has a lower
power consumption. The D1 and D2 configuration is used to limit overshoots of
the reflected signal from the load. The diodes do not affect the line impedance,
unlike the thevenin termination. Schottky and fast switching diodes are good
choice to use for this type of termination.
The advantage of this termination is that Z
can be used in conjunction with other types of terminations. This termination is
commonly used inside MCUs for protection of I/O ports.
Nowadays, many IC manufacturers continually reduce the die size of the
microcontroller to achieve more parts per silicon wafer. Reduced die size
usually results in faster transistors. Consequently, although the MCU clock rate
may not increase, the rise and fall times will increase and the harmonic content
will therefore move up the frequency scale. In many cases, the reduction in die
size may not be communicated to the user, hence, a MCU may initially be fine
in a circuit, then sometime during its production life cycle EMC problems could
arise. The best way to adapt to a possible die shrink is to design a clean circuit
from the start.
Many real-time applications require high-speed MCUs, the designer must take
care of their circuit design and PCB layout to reduce potential EMC problems.
The electrical power required by MCUs increase as their processing power
increases. It is not uncommon to have a supply circuit (e.g. a regulator) very
close to the microcontroller, with a separate bypass capacitor to reduce its
effect on the DC supply to other circuits.
MCUs usually have an on-chip oscillator, that requires its own crystal or
resonator connection, and avoids using clocks from other clock driver circuits.
This independent clock allows better immunity from noise radiated from other
parts of the system. The MCU is usually the highest power demand device at
the clock frequency, hence locating the clock close to the MCU ensures
minimum drive demand at the clock frequency.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
For More Information On This Product,
shows the diode termination method. It is similar to the thevenin
Go to: www.freescale.com
SOURCE
DRIVER
Figure 16. Diode Termination
Z
S
Z
0
0
does not need to be known, and it
V
CC
D1
D2
Z
L
LOAD
C
d
MOTOROLA