COP8SA-DM National Semiconductor, COP8SA-DM Datasheet - Page 176

• Manufacturer Part Number: COP8SA-DM
• Description: MODULE DEBUGGING FOR COP8SA
• Manufacturer: National Semiconductor

Specifications of COP8SA-DM

• Module/board Type: Debugger Module
• For Use With/related Products: Cop 8
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *COP8SA-DM

Multilayer Board
The best way to provide a ground plane is through the use of a multilayer printed circuit
board. The large area and the proximity of the V
and GND planes provide additional
CC
decoupling for the power, and provide effective return paths for both power and signals.
The problem with the use of a multilayer board, particularly in consumer related
industries, is cost. Due to the volumes involved, an addition of several dollars to the cost
of an item may be prohibitive.
2.23.4 Decoupling
Control of the emitted noise can be accomplished by several techniques, including
decoupling, reduced power supplies, and limitation of signal strength by the addition of
series resistance.
It is important to take the time to properly design the decoupling for CMOS processors.
Two decoupling techniques can and should be used to minimize both voltage and current
switching noise in the system.
Capacitive Decoupling
Capacitive decoupling is commonly used to control voltage noise on the V
and GND
CC
lines of the board, but if the decoupling is properly designed and is kept as close as
possible to the power pins of the device, it can also reduce the effective loop area and thus
the antenna efficiency. Capacitive decoupling can prevent high-frequency current
transients from being seen by the power supply.
One factor of capacitive decoupling which is often overlooked is the frequency response
of the capacitors. Each capacitor, dependent on value, lead length, and dielectric
material, possesses a series resonant frequency beyond which the device has inductive
characteristics. This inductance inhibits the capacitor from responding quickly to the
current needs of the processor and forces the current to use the longer path back to the
main power supply.
These inductive characteristics can be countered by the addition of extra capacitors of
different values in parallel with the original device. As the value of the capacitor
decreases (for capacitors of similar manufacture), the resonant frequency increases.
Placing multiple decoupling capacitors across the power pins of the processor can
effectively improve the high frequency performance of the decoupling network.
Capacitance values are normally selected which are separated by a decade. However, it
is best to check the specifications of the capacitors which are used.
Inductive Decoupling
Another very effective method of decoupling which is rarely used is inductive decoupling.
The proper placement of ferrite beads between the decoupling capacitors and the
processor can significantly reduce the current noise on the power pins.
The use of inductive decoupling, which will increase the series impedance of the power
supply, appears to be contradictory to the effect of capacitive decoupling. However, the
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COP8SAx7 MICROCONTROLLER