Alciatore D.G., Histand M.B. Introduction to Mechatronics and Measurement Systems

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7.9 Method to Design a Microcontroller-Based System 331

' Display current position and error on the LCD

Lcdout $FE, 1, "pos:", DEC motor_pos, " error:", DEC error

Lcdout $FE, $C0, "exit: stop button"

Wend

Goto position ' continue the polling loop

Return ' end of subroutine, not reached (see the #-key If above)

' Subroutine to get the encoder position (enc_pos) from the counter

get_encoder:

' Command the PIC16F84 to transmit the low byte

High enc_start

' Receive the high byte

SERIN enc_serial, enc_mode, enc_pos.HighBYTE

' Command the PIC16F84 to transmit the high byte

Low enc_start

' Receive the low byte

SERIN enc_serial, enc_mode, enc_pos.LowBYTE

Return

' Subroutine to calculate the PWM on and off pulse widths based on the desired

' motor speed (motor_speed)

pwm_periods:

' Be careful to avoid integer arithmetic and

' WORD overflow [max=65535] problems

If (pwm_period >= 655) Then

on_time = pwm_period/100 * motor_speed

off_time = pwm_period/100 * (100-motor_speed)

Else

on_time = pwm_period*motor_speed / 100

off_time = pwm_period*(100-motor_speed) / 100

Endif

Return

' Subroutine to output a full PWM pulse based on the data from pwm_periods

pwm_pulse:

' Send the ON pulse

High motor_pwm

Pauseus on_time

' Send the OFF pulse

Low motor_pwm

Pauseus off_time

Return

(continued )

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332 CHAPTER 7 Microcontroller Programming and Interfacing

' Subroutine for speed control of the motor

speed:

' Display the speed control menu on the LCD

Gosub speed_menu

' Wait for a keypad button to be pressed

Serin key_serial, key_mode, key_value

' Take the appropriate action based on the key pressed

If (key_value == key_1) Then

' Slow the speed by 10%

If (motor_speed > 0) Then ' don't let speed go negative

motor_speed = motor_speed - 10

Endif

Else

If (key_value == key_2) Then

' Reverse the motor direction

motion_dir = ~motion_dir

Else

If (key_value == key_3) Then

' Increase the speed by 10%

If (motor_speed < 100) Then ' don't let speed exceed 100

motor_speed = motor_speed + 10

EndIf

Else

If (key_value == key_pound) Then

Gosub main_menu

Return

Else

If (key_value == key_0) Then

' Run the motor until the stop button is pressed

Gosub run_motor

Else

' Wrong key pressed

Goto speed

Endif : Endif : Endif : Endif : Endif

Goto speed ' continue the polling loop

Return ' end of subroutine, not reached (see the #-key If above)

' Subroutine to display the speed control menu on the LCD

speed_menu:

Lcdout $FE, 1, "speed:", DEC motor_speed, " dir:", DEC motion_dir

Lcdout $FE, $C0, "1:- 2:dir 3:+ 0:start #:<"

Return

(continued )

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7.9 Method to Design a Microcontroller-Based System 333

' Subroutine to run the motor at the desired speed and direction until the

' stop button is pressed. The duty cycle of the PWM signal is the

' motor_speed percentage

run_motor:

' Display the current speed and direction

Lcdout $FE, 1, "speed:", DEC motor_speed, " dir:", DEC motion_dir

Lcdout $FE, $C0, "exit: stop button"

' Set the motor direction

motor_dir = motion_dir

' Output the PWM signal

Gosub pwm_periods ' calculate the on and off pulse widths

While (stop_button == 0) ' until the stop button is pressed

Gosub pwm_pulse ' send out a full PWM pulse

Wend

' Return to the speed menu

Gosub speed_menu

Return

' Subroutine to wait for the stop button to be pressed and released

' (used during program debugging)

button_press:

While (stop_button == 0) : Wend ' wait for button press

Pause 50 ' wait for switch bounce to settle

While (stop_button == 1) : Wend wait for button release

Pause 50 ' wait for switch bounce to settle

Return

' Subroutine to allow the user to adjust the proportional and derivative

' gains used in position control mode

adjust_gains:

' Display the gain values and menu on the LCD

Gosub gains_menu

' Wait for a keypad button to be pressed

Serin key_serial, key_mode, key_value

' Take the appropriate action based on the key pressed

If (key_value == key_1) Then

' Increase the proportional gain by 10%

If (kp < 100) Then

kp = kp + 10

Endif

Else

(continued )

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334 CHAPTER 7 Microcontroller Programming and Interfacing

If (key_value == key_4) Then

' Decrease the proportional gain by 10%

If (kp > 0) Then ' don't allow negative gain

kp = kp - 10

Endif

Else

If (key_value == key_3) Then

' Increase the number of PWM cycles sent each position control loop

pwm_cycles = pwm_cycles + 5

Else

If (key_value == key_6) Then

' Decrease the number of PWM cycles sent each position control loop

If (kp > 5) Then ' maintain positive number of pulses

pwm_cycles = pwm_cycles - 5

Endif

Else

If (key_value == key_pound) Then

Gosub main_menu

Return

Else

Goto adjust_gains

Endif : Endif : Endif : Endif : Endif

Goto adjust_gains ' continue the polling loop

Return ' end of subroutine, not reached (see the #-key If above)

' Subroutine to display the position control gain, the number of PWM

' cycles/loop, and the adjustment menu on the LCD

gains_menu:

Lcdout $FE, 1, "kp:", DEC kp, " PWM:", DEC pwm_cycles

Lcdout $FE, $C0, "1:+P 4:-P 3:+C 6:-C #:<"

Return

slave PIC code:

' dc_enc (PIC16F84 microcontroller)

' Design Example

' Position and Speed Control of a dc Servo Motor.

' Slave program to send encoder data, upon request, to the a PIC16F88

' microcontroller running dc_motor.bas

' Define I/O pin names and constants

enc_start Var PORTA.0 ' signal line used to start encoder data transmission

enc_serial Var PORTA.1 ' serial line used to get encoder data from the 16F84

enc_sel Var PORTA.2 ' encoder data byte select (0:high 1:low)

enc_oe Var PORTA.3 ' encoder output enable latch signal (active low)

led Var PORTA.4 ' diagnostic LED (open drain output: 1:OC, 0:ground)

enc_mode Con 2 ' 9600 baud mode for serial connection to encoder IC

blink_pause Con 200 ' 1/5 second (200 ms) pause between LED blinks

(continued )

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7.10 Practical Considerations 335

' Turn off the diagnostic LED

High led

' Wait to ensure the PIC16F88 is initialized

PAUSE 500

' Initialize I/O signals

High enc_oe ' disable encoder output

Low enc_sel ' select the encoder counter high byte initially

' (to prevent transparent latch on low byte)

' Blink the LED to indicate proper operation

Gosub blink : Gosub blink : Gosub blink

' Send dummy byte (66) to ensure proper communication

SEROUT enc_serial, enc_mode, [66]

' Main loop

start:

' Wait for the start signal from the PIC16F88 to go high

While (enc_start == 0) : Wend

' Enable the encoder output (latch the counter values)

Low enc_oe

' Send out the high byte of the counter

SEROUT enc_serial, enc_mode, [PORTB]

' Wait for the start signal from the PIC16F88 to go low

While (enc_start == 1) : Wend

' Send out the low byte of the counter

High enc_sel

SEROUT enc_serial, enc_mode, [PORTB]

' Disable the encoder output

High enc_oe

Low enc_sel

goto start ' wait for next request

End ' end of main program (not reached)

' Subroutine to blink the diagnostic LED on and back off again

blink:

Low led

Pause blink_pause

High led

Pause blink_pause

Return

Lab Exercise

Lab 9Program-

ming a PIC

microcontroller—

Part I

Lab 10Program-

ming a PIC

microcontroller—

Part II

Lab 11Pulse-

width-modulation

motor speed

control with a PIC

■ CLASS DISCUSSION ITEM 7.10

Negative logic LED

Why does the blink subroutine in the slave PIC code in Threaded Design Example

C.3 use negative logic (i.e., Low turns the LED on, and High turns it off)?

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336 CHAPTER 7 Microcontroller Programming and Interfacing

7.10 PRACTICAL CONSIDERATIONS

Throughout this chapter we’ve seen how to design and program microcontroller-

based systems. As with most things in the “real world,” when you actually construct

circuits and implement software, things don’t always work perfectly. Lab Exercises

9 through 11 provide experiences that will help you develop some of the skills

necessary to be successful. The best advice anyone could give concerning design-

ing a complicated microcontroller-based system is to follow a methodical design

procedure as documented and demonstrated in Section 7.9 . It also helps to have

and follow a detailed procedure for the specific development system you are using.

Internet Link 7.11 provides the detailed procedure necessary for working with

Microchip PIC microcontrollers programmed in PicBasic Pro, using the PicStart

Plus programming hardware.

7.10.1 PIC Project Debugging Procedure

The following check list and items of advice can be helpful when debugging soft-

ware and trying to get a PIC circuit to function properly:

1 . Before writing and testing the entire code for your project, always start with

a very simple program (e.g., the flash.bas program in Section 7.5.1 ) on your

PIC(s) to first make sure all necessary components and wiring are in place to

allow the PIC(s) to run.

2 . Once the PIC is known to be running properly, incrementally add and test por-

tions of your code, one functional component at a time. In other words, modu-

larize your software and independently develop and test each module (i.e.,

don’t write the entire program at once, expecting it to work).

3 . Use LEDs to indicate status and location within your program when it is run-

ning and to indicate input and output states.

4 . Be aware of the different characteristics of the I/O pins on the PIC. Refer to

Section 7.8 to see how to properly interface to the different pins for different

purposes.

5 . Be aware that PicBasic Pro commands totally occupy the processor while they

are running (e.g., the line after a SOUND command is not reached or processed

until the SOUND command has completely finished its action).

6 . If you are using a PIC with no internal oscillator (e.g., the PIC16F84), make

sure your circuit includes the proper clock crystal and capacitor components. If

you are using a PIC with an internal oscillator (e.g., the PIC16F88), make sure

you have included the necessary initialization code (e.g., see the code template

for the PIC16F88 available in Internet Link 7.17).

7 . Make sure you carefully select all of the configuration bit settings when down-

loading code to a PIC, so the oscillator, timers, and selected pin functions are

defined properly. This can be done manually each time a PIC is reprogrammed

(as documented in Internet Link 7.11), or you can define all of the settings in

your code as described in Internet Link 7.18.

Internet Lin

7.11How to

program a PIC

Internet Lin

7.17PIC16F88

code template

7.18Setting

configuration bits

in code

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Figure 7.21 Low-cost power supply options.

wall transformer

potted

power supply

4 AA batteries

in series

9V battery

voltage regulator

7.10 Practical Considerations 337

8 . Follow all of the recommendations in Section 2.10.2 for prototyping circuits.

9 . Always follow the PIC programming procedure in Internet Link 7.11 to ensure

that you don’t miss any important steps or details.

Much more advice for developing and debugging PIC-based systems can be

found at Internet Link 7.19, which provides a summary of “lessons learned” by

many past students in our mechatronics course at Colorado State University.

7.10.2 Power Supply Options for PIC Projects

There are a number of ways to provide the DC power required by a PIC and any

ancillary digital integrated circuits. Actuators may also be powered by the same DC

supply if their drive voltages match that of the digital circuitry, and if the current

demands don't exceed the supply's capacity. We begin by assuming that TTL digi-

tal ICs are being used, which require a regulated 5 V DC source. If CMOS is used

exclusively, there are fewer restrictions on the regulation of the DC voltage.

Figure 7.21 shows several low-cost options for powering systems requiring a

5 V supply. These and other options include:

1 . A 6 V, 9 V, or 12 V wall transformer with a 5 V regulator

2 . A potted power supply with AC input and 5 V regulated output

3 . Four AA batteries (6 V) in series with a 5 V regulator

4 . A 9 V battery with a 5 V regulator

5 . A rechargeable battery (or batteries in series) with a 5 V regulator

6 . A full-featured instrumentation power supply

Internet Lin

7.19Lessons

learned by past

students

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338 CHAPTER 7 Microcontroller Programming and Interfacing

Other alternatives for powering projects include a computer power supply, or

large batteries (e.g., car or motorcycle lead-acid batteries), especially if you have

high current demands.

A wall transformer (6 V, 9 V, or 12 V) will provide current up to its rating and

must be used with a 5 V regulator to control the level of the output voltage. Be

sure that the current rating of the wall transformer exceeds (with a healthy mar-

gin) the maximum current your circuit and actuators will draw. A potted power

supply also has AC inputs and may provide one or more regulated DC outputs at

its rated current. No voltage regulator is required if a 5 V output is provided. Four

AA batteries can be connected in series with the 6 V output regulated down to

5 V with a voltage regulator. A 9 V battery must also be connected to a 5 V regulator.

The battery options provide portability for your design but may not be able to supply

enough current. Section 7.10.3 presents more information on different types of bat-

teries and their characteristics. Generally, actuators such as motors and solenoids, as

well as high-current LEDs, can draw substantial current. You should test your batter-

ies to ensure that they will provide sufficient supply (do not simply assume that they

will). Digital circuitry, on the other hand, usually draws very little current.

Figure 7.22 shows an example of a full-featured instrumentation power supply.

This particular model (HP 6235A) is a triple-output power supply, with three adjust-

able voltage outputs, each independently current rated. A full-featured instrumenta-

tion power supply provides the easiest solution, but these are expensive, heavy, and

generally not portable.

Except for the 5 V potted supply and the adjustable instrumentation power

supply, voltage regulators are required to convert the output voltage down to the

5 V level. If your system is entirely CMOS, the regulation of the DC voltage is not

required. Figure 7.23 illustrates a standard 7805 5 V voltage regulator and shows

how it is properly connected to your unregulated power-supply output and your sys-

tem. There must be a common ground from the power supply to your system. The

mounting hole on the heat sink allows you to easily connect to the common ground.

Figure 7.22 Example of a full-featured instrumentation power supply.

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Figure 7.23 7805 voltage regulator connections.

heat sink

(COMMON)

INPUT

OUTPUT

COMMON

mounting

hole

input

(7V - 35V)

output

(5V regulated)

7.10 Practical Considerations 339

Table 7.7 provides a summary of how the various power supply options compare

in terms of current ratings, size, and cost. Figure 7.24 shows an example specifica-

tion sheet for an enclosed power supply. Before selecting or purchasing a supply for

your design, it is important to first review the specifications, especially the current

rating (2.5 A in this case).

7.10.3 Battery Characteristics

This section presents some of the important terms, considerations, and specifica-

tions in the proper selection of a battery as a power source. The most important

specification for a battery (besides its rated voltage) is the amp-hour capacity. This

is defined as the current a battery can provide for one hour before it reaches its end-

of-life point. The current that a battery can deliver is limited by its equivalent series

resistance, which is the internal resistance that can be considered in series with an

“ideal voltage source.” The load current times the internal resistance results in a

voltage drop reducing the effective voltage of the battery. Furthermore, there will

Table 7.7 5 V power supply options summary

Device Typical current Relative size Relative cost

instrumentation power

supply

1 A – 5 A large very expensive

(∼ $1,000) but many

features

small potted, open frame,

or enclosed power supply

1 A – 10 A medium moderately expensive

(∼ $20–$100)

wall transformer 1 A small cheap

9 V battery 100 mA small cheap

4 AA batteries 100 mA small cheap

rechargeable battery See Section 7.10.3 small moderate

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Figure 7.24 Specifications for an example

closed-frame power supply.

340 CHAPTER 7 Microcontroller Programming and Interfacing

be power dissipated by the internal resistance, which at high currents may result in

considerable heat production.

Batteries are composed of cells, the electrochemical construct that supplies

voltage and current. Cells can be combined in series or parallel within a battery

for larger current and voltage capacities. The voltage of a cell will differ among

the types of batteries, due to their chemistry. Primary cell batteries are not

rechargeable and are meant for one-time use. Devices that are used infrequently

or that require very low drain currents are good candidates for primary cells.

Secondary cell batteries are rechargeable, and their effectiveness may be replen-

ished many times. Devices that require daily use with higher drain currents are good

candidates for secondary cells.

The plot of a battery discharge curve is important in determining the stability

of the voltage output. Figure 7.25 shows a typical shape for a discharge curve. One

desires a broad plateau representing a long effective life.

The salient factors a designer must consider in the selection of a power source

for a mechatronic design are:

■ Voltage required by the load

■ Current required by the load

■ Duty cycle of the system

■ Cost

■ Size and weight (specific energy)

■ Need for rechargeability

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