Freescale Semiconductor M68HC08 - Manual (Page 21)

Using the ASB520 MC68HC908QT2 Based Infrared Remote Control Reference PC Board DRM045
MOTOROLA Design Considerations 21
Designer Reference Manual — DRM045
Section 5. Design Considerations
5.1 Introduction
Microcontroller systems, in general, have a number of important design
considerations related to PC board layout and grounding considerations.
These design considerations are discussed in
5.2 Grounding , 5.3 Infrared
and Visible LED Drive Circuit , and 5.4 Switch Circuitry . A description of two
of the reference board’s major circuits are included in 5.3 Infrared and Visible
LED Drive Circuit and 5.4 Switch Circuitry .
5.2 Grounding
PC board layout is an important design consideration. In particular, ground
planes and how grounds are tied together influence noise immunity. To
maximize oscillator noise immunity, it is a good practice have ground plane
under the resonator, X1. One good grounding practice is to carry all of the
ground connections, in a star configuration, to a single point which could be the
power supply’s bulk capacitor or in this case, the battery ground terminal.
5.3 Infrared and Visible LED Drive Circuit
Figure 5-1 is the driver schematic, used to drive both LEDs on the PC board.
Because the microcontroller will not source or sink enough current to drive the
two LEDs, a discrete driver is necessary. This circuit uses two transistors. The
first transistor is a small signal NPN, followed by a small signal PNP transistor.
Figure 5-1. Infrared LED Drive Circuit
D2
RED
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Freescale Semiconductor, Inc.
For More Information On This Product,
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