NXP S08QD Reference guide

DRM088 Designer Reference Manual

Devices Supported:

M9S08QD4

Document Number: DRM088

Rev. 0

07/2008

DRM088, Rev. 0

Freescale Semiconductor 3

Pulse Width Modulation Controlled Fans Using the

M9S08QD4

Designer Reference Manual

by: Eddie Ng

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 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

Revision

Level

Description

Page

Number(s)

Feb, 22 2008 0.B Written by Eddie Ng, Edited by Ping Xu N/A

Revision History

DRM088, Rev. 0

4 Freescale Semiconductor

DRM088, Rev. 0

Freescale Semiconductor 5

Contents

Chapter 1

Introduction

1.1 Introduction ................................................................................................................................... 11

1.2 Freescale’s Low Cost MCU Advantages and Features ................................................................. 11

1.3 BLDC Fan Reference Design Targets ...........................................................................................13

1.4 Bi-Phase BLDC Motor ................................................................................................................... 13

Chapter 2

Motor Control

2.1 Commutation ................................................................................................................................. 15

2.2 Rotor Position Control ................................................................................................................... 15

2.3 Commutation Waveforms .............................................................................................................. 16

2.4 Speed Control Mechanism ............................................................................................................ 16

2.5 Motor Startup ................................................................................................................................ 18

2.6 Fault Detection and Protection ...................................................................................................... 18

2.7 Use of Table Look-Up (TLU) and Closed Loop Speed Control Mechanism .................................. 19

2.8 Input Source Options .................................................................................................................... 19

Chapter 3

Implementation

3.1 Block Diagram ............................................................................................................................... 21

3.2 Hardware Resources .................................................................................................................... 22

3.3 Control Loop .................................................................................................................................. 22

3.4 Fan Speed Response to PWM Control Input Signal ..................................................................... 23

Appendix A

Schematic

Appendix B

Program Listing

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Tables

Design Targets 13

Measurement of PWM Control Input Signal and Fan Speed (RPM) 26

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Figures

Bi-Phase BLDC Fan Motor Diagram 13

Bi-Phase BLDC Fan Motor 15

Bi-Phase BLDC Motor Commutation Waveform 16

Commutation Waveform using Phase On/Off Delay Time Control Method 17

Commutation Waveform using PWM Control Method 18

MC9S08QD4 DC Fan Design Block Diagram 21

On Board DAC Block Diagram 22

Firmware Control Loop Flowchart 23

PWM Control Input Signal vs. Fan Speed 24

Measured PWM Control Input Signal vs. Fan Speed 25

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Introduction

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Freescale Semiconductor 11

Chapter 1

Introduction

1.1 Introduction

This document describes the implementation of a Brushless DC (BLDC) fan controller using the Freescale

MC9S08QD4 8-bit Microcontroller (MCU). The design reads the standard 4 wire PWM controlled input

signal, which in turn controls the fan speed in a closed-loop feedback system. Complete coding and

schematic are included.

Brushless DC fans are widely used in CPU and graphic display card cooling and system ventilation

applications. The lack of a commutator makes the brushless DC fan more reliable than the conventional

DC fan. Microcontroller (MCU) based, intelligent, variable-speed brushless DC fans are needed to avoid

overheating and fulfill the rapidly changing electronics products requirement. Characteristics of flash

MCU based BLDC such as the MC9S08QD4 include variable speed control, low acoustic noise,

reliability, long lifetime, low power consumption, protection features, easy to maintain/upgrade and

communication interface capability.

There are several advantages of a MCU based closed-loop feedback design over traditional solutions:

• A targeted air flow can be achieved by constantly renewed fan speed adjustment based on

environment changes of the target system, such as temperature.

• Fan characteristics and behavior can be updated and changed easily for different end users by

modifying the Table Look-Up (TLU) in the flash memory of the MCU.

• Fault detection can be easily implemented by the MCU. For example, if the MCU detects a motor

jam or the air flow being blocked, the motor driver can be stopped completely to avoid further

damage. The automatic restart feature is also available.

• Digital feedback and output acknowledgment can be generated under a faulty situation.

• Sophisticated speed control algorithms can be easily implemented and modified in the flash based

MCU, if needed.

The MCU chosen for this purpose must be low cost and physically small in order to integrate it into the

fan controller Printed Circuit Board (PCB). The MC9S08QD4 is ideal for this application.

1.2 Freescale’s Low Cost MCU Advantages and Features

The Freescale MC9S08QD4 MCU is a low cost, small pin count device well suited for home appliances

and small geometry applications, such as the brushless control DC (BLDC) fan application. This device is

composed of standard on-chip modules, including a small and highly efficient HCS08 8-bit CPU core, 256

bytes RAM, 4 Kilobytes flash, two 16-bit modulo timers, four channels 10-bit ADC and keyboard

interrupt. The device is available in small 8-pin PDIP and SOIC packages.

Introduction

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12 Freescale Semiconductor

Features of the MC9S08QD4 include:

• 8-bit 16 MHz HCS08 CPU (Central Processor Unit)

— HC08 instruction set with added BGND instruction

— Background debugging system

— Breakpoint capability to allow single breakpoint setting during in-circuit debugging

— Support for up to 32 interrupt/reset sources

• Memory

— 4096 bytes flash

— 256 bytes RAM

• Internal Clock Source module (ICS)

— Containing a frequency-locked-loop (FLL) controlled by internal or external reference

— Precision trimming of internal reference allows 0.2% resolution and 2% deviation over

temperature and voltage

• System protection

— Watchdog Computer Operating Properly (COP) reset with option to run from dedicated 32 kHz

internal clock source or bus clock

— Low-voltage detection with reset or interrupt

— Illegal opcode detection with reset

— Illegal address detection with reset

— Flash block protect

• ADC

— 4-channel, 10-bit analog-to-digital converter with automatic compare function, asynchronous

clock source and internal bandgap reference channel

— On-chip temperature sensor for ADC temperature compensation, by calibration at 3 points,

-40°C, 25°C and 105°C; the temperature accuracy is up to ±2.5°C

— ADC is hardware triggerable using the RTI counter

•TIM1

— 2-channel timer/pulse-width modulator

— Each channel can be used for input capture, output compare, buffered edge-aligned PWM, or

buffered center-aligned PWM

•TIM2

— 1-channel timer/pulse-width modulator

— Each channel can be used for input capture, output compare, buffered edge-aligned PWM, or

buffered center-aligned PWM

•KBI

— 4-pin keyboard interrupt module with software selectable polarity on edge or edge/level modes

• Package Options

—8-pin PDIP

— 8-pin narrow body small outline integrated circuit (SOIC) package

— All package options are RoHS compliant

Introduction

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Freescale Semiconductor 13

1.3 BLDC Fan Reference Design Targets

1.4 Bi-Phase BLDC Motor

In the conventional DC fan motor, the stator of the permanent magnet is composed of two or more

permanent magnet pole pieces, and the rotor is composed of windings that are connected to a mechanical

brush (commutator). The opposite polarities of the energized winding and the stator magnet attract the

rotor, and the rotor will rotate until it is aligned with the stator. Just as the rotor reaches alignment, the

brushes move across the commutator contacts and energize the next winding. As a result, the fan rotor is

rotated continuously.

The brushless DC (BLDC) fan motor, however, is a rotating electric machine in which the stator is a classic

two-phase stator and the rotor has surface-mounted permanent magnets. In this respect, the BLDC fan is

equivalent to a reverse-conventional DC fan; the current polarity is altered by the electrical commutator

instead of mechanical brushes. The BLDC has no brushes on the rotor and the commutation is performed

electronically at certain rotor positions. An example of a BLDC fan motor is illustrated in Figure 1-1.

Figure 1-1. Bi-Phase BLDC Fan Motor Diagram

Table 1-1. Design Targets

Item Requirement

Motor Type Bi-Phase BLDC Motor

Fan Dimensions (HxLxW) 80mm x 80mm x 25mm

Operating Voltage 12V

Current Rating 0.17V (max.)

Speed 400 to 2400 RPM

Closed Loop Feedback Yes

Fault Detection Lock Detection

Other Features Automatic Restart

Introduction

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Motor Control

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Freescale Semiconductor 15

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 (shown in Figure 2-1) induces magnetic fields that push the equal polarity rotor magnets away from the

energized coils. At the same time the opposite polarity rotor magnets are pulled toward the coils. When

rotation starts, it 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 move the rotor by 180 degrees or one motor phase. One mechanical

revolution is contributed by four commutation steps.

Figure 2-1. Bi-Phase BLDC Fan Motor

2.2 Rotor Position Control

The key idea to prevent a motor lockup involves rotor position detection. The time to switch the

commutation is critical. Energizing coil-pair 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. Positioned between the coils at 45 degrees to the stator coils, as shown in Figure 2-1, the hall

sensor can effectively detect the rotor position. In this case, the hall sensor outputs toggle when the rotor

magnets are aligned to the coils. Commutation will switch at this time from one coil-pair to the next.

Motor Control

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16 Freescale Semiconductor

2.3 Commutation Waveforms

In general, in a bi-phase motor design, alternate coils are tied together and give a single connection to the

driver. In this design, the driver connection for coil A and coil C is called L1 (see Figure 2-1). Similarly,

the driver connection for coil B and coil D is called L2. Driving to either of the connections will energize

a coil-pair. The commutation waveform is shown in Figure 2-2. The coil driving period is aligned with the

hall sensor output. When the sensor outputs toggle, coil driving is stopped, and the coils are de-energized

for a period of time before the next coil-pair is energized.

Figure 2-2. Bi-Phase BLDC Motor Commutation Waveform

2.4 Speed Control Mechanism

Motor speed is normally defined as the mechanical Round Per Minute (rpm). In electrical terms, one

commutation contributes to 90 degrees of a revolution. Four commutation cycles complete one revolution

of the motor. Thus, control of the time taken per commutation can effectively control the overall speed of

the motor.

There are two methods to control the speed of the motor: one is called phase on/off delay time control

method, and the other is called PWM control method.

The phase on/off delay time control method is used to adjust the off time at phase switching. The rotation

speed can be adjusted by the length of the off time. The off time is inversely proportional to the energy

provided to the motor. That is, the longer the off time, the less energy is supplied to the stator coils, which

results in a lower speed of the motor. There is one condition that must be satisfied: it needs to be

synchronized to the feedback signal from the hall sensor (i.e. the commutation). The commutation

waveform is shown in Figure 2-3 to illustrate on the phase on/off delay time control method.

90

o

of rotation

Motor Control

DRM088, Rev. 0

Freescale Semiconductor 17

Figure 2-3. Commutation Waveform using Phase On/Off Delay Time Control Method

Although the implementation of this speed control method is relatively simple, there are some

disadvantages in this method, such as the lower accuracy and higher acoustic noise level. Therefore, it may

not fit for the high-end motor system requirement. The accuracy of this type of the speed control method

is around ±10%, which depends on the maximum speed of BLDC, the maximum bus speed of the MCU,

and the number of bit in the timer. For example, if we drive the higher speed of the BLDC fan with the

lower speed of the MCU with a lower resolution timer, lower accuracy of the speed will occur. The PWM

control method is a better alternative for providing higher accuracy and a lower acoustic noise motor

system.

For the PWM control method, the speed is controlled by changing the duty cycle of the PWM. This drives

the two winding coils in the BLDC fan independently instead of changing the off-time period. The two

winding coils are turned on alternately, but the voltage of the coil is controlled by the duty cycle of the

PWM drive signal. The PWM frequency is a major design consideration. The PWM frequency can be

between 18 kHz to 60 kHz typically.

The PWM frequency range is chosen based on acoustic noise and efficiency. The acoustic noise will occur

if the PWM frequency is falling into the audible range of the human ear. In this method, the PWM

frequency should be set above 18 kHz, which is higher than the audible range of the human ear. This means

that the fan can be operated in a quieter manner. Plus, the use of a higher PWM frequency will also be more

efficient. Because the high speed PWM (18 kHz – 60 kHz) is much higher than the commutation period,

the current ripple is smaller. This is because the averaged DC voltage that is applied in the winding coil is

more stable when compared to the phase on/off time delay method. Figure 2-4 illustrates on the PWM

control method. In this reference design, PWM control method is demonstrated to control the BLDC Fan

speed.

Motor Control

DRM088, Rev. 0

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Figure 2-4. Commutation Waveform using PWM Control Method

2.5 Motor Startup

In this DC fan application, it is desirable to only allow the motor to operate uni-directionally, allowing the

airflow to the target system to always be in one direction. With the bi-phase motor design, it is difficult to

guarantee the direction of rotation. Commutation order or the coil energizing sequence is the same for both

directions of rotation. The rotor position or axis must initially be known in order to guarantee the direction

of rotation.

When the first commutation step is activated where the adjacent coil-pair to the initial axis is energized,

the rotor starts to move. Since the adjacent coil-pairs are connected together and energized at the same

time, there are equal pulling/pushing forces induced on the rotor in both directions. There is a chance for

the rotor to startup in either direction. It is necessary to monitor the initial direction of rotation. If the

direction is not correct, the motor must be locked back to the startup axis again, and the commutation step

must be repeated.

The direction of rotation can be detected by the hall sensor output. If the initial rotor axis is known, the

output edge polarity, rising edge or falling edge, determines the direction of rotation. In the modern

bi-phase motor design, the direction of rotation is normally defined by the manufacturer. The stator design

is not symmetric, so the motor will have a high tendency to rotate in one direction than the other. However,

the direction of rotation cannot be guaranteed without proper monitoring techniques in place.

2.6 Fault Detection and Protection

Motor fault is identified when the rotor is not moving, which is normally the case when the rotor is

jammed, which may be caused by blocked airflow. During each commutation step, the hall sensor output

is monitored. If it is not toggled within a defined duration (which is software configurable), commutation

sequence is terminated and all coils are de-energized. A “rotor locked” signal is then fed back and sent to

the external control system via the PWM 4-wire “Sense” pin.

Given a defined duration (which is also software configurable), the fan system goes through an automatic

restart sequence to re-initialize the whole fan system first and then to re-start the fan.

Motor Control

DRM088, Rev. 0

Freescale Semiconductor 19

2.7 Use of Table Look-Up (TLU) and Closed Loop Speed Control

Mechanism

In this reference design, Table Look-Up (TLU) is implemented so the end user can change their fan speed

profile instantly by reprogramming the TLU inside the built-in MCU flash memory. Based on the same set

of hardware, different behavior of BLDC fans can be tailor made via different TLU stored in the flash

memory in order to meet various end customer requirements.

To implement the TLU, the MCU first reads the DC input voltage via a ADC pin. It then converts the DC

voltage into a digital value. By the TLU, the digital value then points to a pre-defined output fan speed

(stored in the TLU) and the MCU changes the duty cycle of the PWM according to the pre-defined output

fan speed to alter the actual BLDC fan speed.

In order to increase the precision of the BLDC fan speed, a closed loop system is used. First, the actual fan

speed is monitored by the hall signal from time to time. The reading is then fed back and compared to the

pre-defined output fan speed stored in the TLU. If the actual fan speed is faster or slower than the

pre-defined speed, the duty cycle of the PWM will be adjusted according to the result from the comparison.

2.8 Input Source Options

In this reference design, there are four types of speed input options that can be configured by:

• A DC voltage source varied by input voltage via voltage divider (Jumper P4, Pin1 connected to

Pin2)

• A DC voltage which is varied through thermal sensor depending on temperature (Jumper P4, Pin2

connected to Pin3)

• A DC voltage generated through VR circuitry (Jumper P6, Pin1 connected to Pin2)

• A PWM signal generated from an external source either goes through a DAC circuitry to convert

the PWM signal into a DC voltage (Jumper P6, Pin2 connected to Pin3) or directly connects to the

MCU KBI pin to measure the PWM signal and then calculate the PWM input duty cycle for fan

speed.

It consumes quite a lot of MCU resources calculated in the PWM input duty cycle. However, it can save

the cost of DAC circuitry on the PCB.

In this reference design, the PWM input source is chosen as it is most commonly used in the BLDC fan

application, and the DAC circuitry is implemented for the PWM input signal.

Motor Control

DRM088, Rev. 0

20 Freescale Semiconductor

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