Atmel AVR042, Примечание к применению

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AVR Hardware Design Considerations [APPLICATION NOTE]
Atmel-2521M-AVR-Hardware-Design-Considerations_ApplicationNote_092014
3
1 Power Supply
Two aspects should be considered when designing the power supply for the discrete/digital elements of an Atmel
AVR: ESD protection and noise emission. Both these topics are treated in details in the AVR040 application note ,
and only a short summary is included in this document.
1.1 Digital Supply
Looking at the datasheet for an Atmel AVR microcontroller, one can be fooled to believe that power supply is not
critical. The device has a very wide voltage range, and draws only a few mA supply current. But as with all digital
circuits, the supply current is an average value. The current is drawn in very short spikes on the clock edges, and
if I/O lines are switching, the spikes will be even higher. The current pulses on the power supply lines can be
several hundred mA if all eight I/O lines of an I/O port changes value at the same time. If the I/O lines are not
loaded, the pulse will only be a few ns.
This kind of current spike cannot be delivered over long power supply lines; the main source is (or should be) the
decoupling capacitor.
Figure 1-1. Incorrect Decoupling
Figure 1-1 shows an example of insufficient decoupling. The capacitor is placed too far away from the
microcontroller, creating a large high current loop. The power and ground planes here are parts of the high
current loop. As a result of this, noise is spread more easily to other devices on the board, and radiated emission
from the board is increased even further. The whole ground plane will act as an antenna for the noise, instead of
only the high current loop. This will be the case if the power and ground pins are connected directly to the planes
(typical for hole-mounted components) and the decoupling capacitor is connected the same way. The same is
often seen for boards with surface-mount components if the integrated circuits are placed on one side of the
board and the decoupling capacitors are placed on the other.
Figure 1-2 shows a better placement of the capacitor. The lines that are part of the high current loop are not part
of the power or ground planes. This is important, as the power and ground planes otherwise will spread a lot of
noise. Further, the figure shows another improvement of the decoupling. A series ferrite bead is inserted to
reduce the switching noise on the power plane. The series resistance of the ferrite bead must of course be low
enough to ensure that there will be no significant DC voltage drop.