Freescale Semiconductor MC68HC908QT2, M68HC08 manual Design Considerations, Grounding

Semiconductor, Inc.

Freescale Semiconductor, Inc.

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 5-1is 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.

Freescale

	1	JP4	2
To PTA0		1 2

IR_DISC

R5

47K

R4

1K

Vbat

R1

47K

R2Q2

3302N3906

Q1

2N3904

R6	R3
470	47
D2	D1

RED IR_LED

Figure 5-1. Infrared LED Drive Circuit

Using the ASB520 MC68HC908QT2 Based Infrared Remote Control Reference PC Board		DRM045
MOTOROLA	Design Considerations	21
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