Section 5. design considerations, 1 introduction, 2 grounding – Freescale Semiconductor M68HC08 User Manual

Page 21: 3 infrared and visible led drive circuit, Introduction, Grounding, Infrared and visible led drive circuit, Provided in, Section 5. design, Considerations

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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

R4

1K

R3
47

R6
470

Vbat

R2

330

JP4

IR_DISC

1

2

1 2

R5

47K

To PTA0

Q2

2N3906

Q1

2N3904

D1

IR_LED

R1
47K

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Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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