Freescale Semiconductor DRM079 User Manual

Variable Speed DC Fan Control
using the MC9RS08KA2

Designer Reference Manual

RS08
Microcontrollers

DRM079
Rev. 0
5/2006

freescale.com

Variable Speed DC Fan Control using the MC9RS08KA2

Designer Reference Manual

by: Vincent Ko

Freescale Semiconductor, Inc.
Hong Kong

To provide the most up-to-date information, the revision of our documents on the World Wide Web will be
the most current. Your printed copy may be an earlier revision. To verify that you have the latest
information available, refer to http://www.freescale.com

The following revision history table summarizes changes contained in this document. For your
convenience, the page number designators have been linked to the appropriate location.

Revision History

Date

05/2006 0 Initial release N/A

Freescale Semiconductor 3

Revision

Level

Description

Variable Speed DC Fan Control using the MC9RS08KA2, Rev. 0

Page

Number(s)

Revision History

Variable Speed DC Fan Control using the MC9RS08KA2, Rev. 0

4 Freescale Semiconductor

Table of Contents

Chapter 1

Introduction

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.2 Freescale’s New Generation Ultra Low Cost MCU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3 DC Fan Reference Design Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.4 Bi-Phase BLDC Motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Chapter 2

Motor Control

2.1 Commutation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2 Rotor Position Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3 Commutation Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.4 Speed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.5 Motor Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.6 Fault Detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Chapter 3

Implementation

3.1 Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.2 Hardware Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.3 Control Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.4 Temperature Sensor Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.4.1 Temperature Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Appendix A.

Schematic

Appendix B.

Program Listing

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Table of Contents

Variable Speed DC Fan Control using the MC9RS08KA2, Rev. 0

6 Freescale Semiconductor

Chapter 1 Introduction

1.1 Introduction

This document describes the implementation of a DC brushless fan controller using the Freescale ultra
low cost MC9RS08KA2 8-bit microcontroller (MCU). The design contains a temperature sensor the MCU
reads with control on fan speed against the ambient temperature. Complete coding and schematic are
included.

VARIABLE RESISTOR
(TO EMULATE A TEMPERATURE SENSOR)

MC9RS08KA2
MCU IN 8-PIN NARROW BODY
SOIC PACKAGE

BUZZER

Figure 1-1. The MC9RS08KA2 DC Fan Reference Design

The DC fan used is a brushless DC motor fan. It is widely used in chip cooling or system ventilation
applications. In the market, most of the DC fans are of the constant air flow design. As the high
performance electronic products continue to increase, cooling requirement becomes more and more
sophisticated. MCU approach provides a cost effective solution to this application. There are several
advantages of a MCU based design over traditional solutions.

1. Instead of having a constant air flow the MCU provides enough processing power to modify the fan
speed according to environment changes such as the temperature of the target system.

2. Fault detection can easily be implemented by the MCU. For example, the MCU can detect for the
air flow blocking or motor jam, the motor driver can be stopped completely to avoid further damage.

3. Buzzer alarm or digital output acknowledgement can be generated under the faulty situation.

The MCU chosen for this purpose must be low cost and it must provide small geometry package to
integrate into the fan controller printed circuit board (PCB). The MC9RS08KA2 is ideal for this application.

Variable Speed DC Fan Control using the MC9RS08KA2, Rev. 0

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Introduction

1.2 Freescale’s New Generation Ultra Low Cost MCU

The MC9RS08KA2 microcontroller unit (MCU) is an extremely low cost, small pin count device for home
appliances, toys, and small geometry applications, such as a DC fan controller. This device is composed
of standard on-chip modules including a very small and highly efficient RS08 CPU core, 62 bytes RAM,
2K bytes FLASH, an 8-bit modulo timer, keyboard interrupt, and analog comparator. The device is
available in small 6- and 8-pin packages.

Features of the MC9RS08KA2 include:

• 8-bit RS08 core
– Up to 10 MHz (bus frequency) at 1.8V for 100 ns minimum instruction time
– RS08 instruction set
– Supports tiny/short address mode
– 14-byte fast-access RAM
– Allows emulation of HC08/HCS08 zero-offset index addressing mode instructions

• Third-generation Flash and RAM (extremely fast, byte writable programming)
– 63 Byte RAM
– 2K Byte Flash

• Flexible clock options

• 4 Bidirectional I/O lines with software selectable pull-up (eliminates need for external resistors)

• Analog comparator

• Real time interrupt

• 8-bit timer with 8-bit prescale

• System protection
– Resets in instance of runaways or corrupted code
– Low voltage detection
– Illegal opcode and illegal address detection
– Flash security feature

• Single wire debugging and emulation interface; eliminates need for expensive emulation tools or
development hardware

1.3 DC Fan Reference Design Targets

Table 1-1. Design Targets

Item Requirement

Motor Type Bi-phase BLDC motor

Fan Dimensions 60mm x 60mm x 25mm

Operating Voltage 12V

Current Rating 0.18A (max.)

Speed 1000 to 4000 RPM

Temperature Feedback Yes

Fault Detection Air flow blocking (motor jam)

Fault Notification Buzzer alarm

Variable Speed DC Fan Control using the MC9RS08KA2, Rev. 0

8 Freescale Semiconductor

Bi-Phase BLDC Motor

1.4 Bi-Phase BLDC Motor

The brushless DC motor (BLDC) design for DC fan is commonly consist of a permanent magnet attached
on the rotor and the stator phase coil windings are mounted on the motor shaft as illustrated in Figure 1-2.
The BLDC has no brushes on the rotor and the commutation is performed electronically at certain rotor
positions.

Hall Effect
Sensor

Stator
Coil

Fan Hub

Axle

Permanent
Magnets

Figure 1-2. Bi-Phase BLDC Motor Diagram

Variable Speed DC Fan Control using the MC9RS08KA2, Rev. 0

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Introduction

Variable Speed DC Fan Control using the MC9RS08KA2, Rev. 0

10 Freescale Semiconductor

Commutation

Chapter 2 Motor Control

2.1 Commutation

The typical bi-phase BLDC has one pole-pair per phase. Each commutation rotates the rotor by 90
degrees and four commutation steps complete a mechanical revolution. Each pole-pair is implemented
by two coils, with four coils in total for a bi-phase motor. Energizing a pair of coils, either coil A & C or coil
B & D as shown in Figure 2-1, induces magnetic fields that push the equal polarity rotor magnets away
from the energized coils and at the same time the opposite polarity rotor magnets are pulled toward the
coils. Rotation starts and this is called a commutation step. When the rotor magnetic pole is aligned with
the energized coils, the coils are deactivated and the previously un-energized pair of coils are then
energized. As the magnetic field switches to the next motor position or pole, the inertia of the rotor keeps
the motor running. As a result, two commutation steps moves the rotor by 180 degrees or one motor
phase. One mechanical revolution is contributed by four commutation steps.

To avoid conflict to the magnetic field, adjacent coils cannot be energized at the same time. Dead-time,
where all coils are un-energized must be added between each commutation step.

N

L2

S

L1

HALL

Coil A

Coil D

Coil B

L1

Coil C

S

L2

N

Figure 2-1. Bi-phase BLDC Motor Schematic

2.2 Rotor Position Control

The key idea to prevent a motor lockup concerns rotor position detection. The time to switch the
commutation is critical. Energizing coil-pair for too long will kill the rotor inertia and the motor stops
running. This is called motor lockup. Switching the commutation too soon will lose control to the rotor and
eventually stall the motor. The rotor position in this design is determined by a hall sensor which will
respond to the change in magnetic field. Hall sensor output toggles when the magnetic field changes its
polarity. Positioning the hall sensor between the coils at 45 degree to the stator coils, as shown in

Figure 2-1, can effectively detect the rotor position. In this case the hall sensor output toggles when the

rotor magnets is aligned to the coils. Commutation should switch at this time from one coil-pair to the next
coil-pair.

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