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 311

6. Decide on a programming language. You can write the code in assembly lan-

guage or in a high-level language such as C or PicBasic Pro. We recommend

PicBasic Pro for most applications. Assembly language is a better option only

when extremely fast execution speed is required or if memory capacity is a

limiting factor. C might be a better choice if the solution requires complicated

calculations, algorithms, or data structures.

7. Draw the schematic. Draw a detailed schematic showing required compo-

nents, input and output interface circuitry, and wire connections. If using the

PIC16F84, Figure 7.4 serves as a good starting point.

8. Draw a program flowchart. A flowchart is a graphical representation of the

required functionality of your software. Figure 7.20 illustrates a set of build-

ing blocks that can be used to construct a flowchart. The flow control block

is used as a destination label for a goto branch or a loop (e.g., For . . . Next or

While . . . Wend ). The functional block represents one or more instructions that

perform some task. The decision block is used to represent logic decisions.

Design Example 7.2 illustrates how a typical flowchart is constructed.

9. Write the code. Implement the flowchart in software by writing code to cre-

ate the desired functionality.

10. Build and test the system. Compile your code into machine code and down-

load the resulting hex file to the microcontroller. This can be done using a pro-

gramming device available from the manufacturer (e.g., the PicStart Plus serial

programmer available from Microchip). Internet link 7.11 points to the detailed

list of steps required to create, compile, and download code using PicBasic

Pro, MPLAB software, and the PicStart Plus programmer. The procedure is

also presented and used in Lab Exercise 9. After downloading the code, assem-

ble the system hardware, including the microcontroller and interface circuitry.

Then, fully test the system for the desired functionality. We recommend you do

steps 9 and 10 incrementally as you build functionality, carefully testing each

addition before continuing. For example, make sure you can read and process

an input first. Then add and test additional inputs and incrementally add and

test output functionality. In other words, do not try to write the complete pro-

gram on the first attempt!

For more advice on designing, building, and testing microcontroller-based sys-

tems, including advice on how to select a power supply, see Section 7.10 .

Internet Lin

7.11How to

program a PIC

Lab Exercise

Lab 9Pro-

gramming a PIC

microcontroller—

part I

Figure 7.20 Software flowchart building blocks.

flow control

label block

functional block

(set of instructions)

decision block

(If . . . Then . . . Else)

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312 CHAPTER 7 Microcontroller Programming and Interfacing

A few years ago, we assigned an interesting class project in our Introduction to Mecha-

tronics and Measurement Systems course at Colorado State University. Internet Link 7.12

points to a generic project description that we currently use in the course. Internet Link 7.13

points to the specific guidelines for a combination security device project described here.

Lab Exercise 15 also introduces the course project and provides useful reference informa-

tion for powering a PIC project, using batteries, using relays vs. transistors, soldering, and

dealing with other practical considerations.

We now illustrate the procedure just presented to generate a basic design. Then we

describe some of the student solutions, because an interesting part of the project was the

creativity that the class groups exhibited in solving this problem.

1 . Define the problem. The goal of the project is to use the PIC16F84 microcontroller

in the design of a combination security lock device. The device requirements include

switches to enter a combination, a pushbutton switch to process the combination,

LEDs and a buzzer to indicate the success of a combination attempt, a digital display

to indicate the number of failed combination attempts, and an actuator to perform

a useful output function. In the simple design presented here, we use three toggle

switches to enter the combination allowing for eight possible combinations.

2. Draw a functional diagram. The figure below illustrates the major components in

our basic design example.

PIC

microcontroller

green red

LED indicators

LED display

enter

button

combination

switches

speake

motor

3. Identify I/O requirements. The inputs and outputs are all digital for this problem.

Inputs:

■ three switches for the combination

■ one pushbutton switch to serve as an enter key

Outputs:

■ two LEDs to indicate the combination status

■ one seven-segment LED digit display

■ one small speaker

■ one small DC motor

Internet Lin

7.12PIC-

based class

design project

description

7.13Combina-

tion security

device project

description

Lab Exercise

Lab 15A

microcontroller-

based design

project

PIC Solution to an Actuated Security Device

DESIGN

EXAMPLE 7.2

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7.9 Method to Design a Microcontroller-Based System 313

4 . Select appropriate microcontroller models. If we drive the seven-segment display

with four digital outputs (see Design Example 7.1), the required number of digital

I/O lines is 12. This is the only I/O we require (i.e., we do not need A/D, D/A,

or serial ports). The PIC16F84 is adequate for this problem because it provides

13 digital I/O pins.

5 . Identify necessary interface circuits. A 7447 interface is required to decode the

four digital outputs to control the seven-segment display (see Design Example 7.1).

A small audio speaker can be driven directly through a series capacitor, as recom-

mended in the description of the Sound statement in the PicBasic Pro compiler

manual. The PIC cannot source enough current to drive the motor directly, so a

digital output is used to bias a power transistor connected to the motor. The only I/O

pin we will not use is RA4, the special purpose Schmitt trigger input and open drain

output pin.

6 . Decide on a programming language. We use PicBasic Pro. This or some other high-

level language should always be your first choice, unless you have memory or speed

constraints.

7 . Draw the schematic. The following figure shows the necessary components. It does

not matter which I/O pins are used for the different functions, but we kept them orga-

nized according to function.

PIC16F84

RA2

RA3

RA4

MCLR

RB0

RB1

RB2

RB3

RA1

RA0

OSC1

OSC2

RB7

RB6

RB5

RB4

5 V

22 pF

4 MHz

1 k

5 V

0.1 μF

5 V

supply

enter

button

5 V

1 k

330

green LED

330

red LED

0.1 μF

, 8 Ω,

0.25 W speaker

1.5–3 V

DC motor

330

switch 3

5 V

switch 2

5 V

switch 1

7447

decoder

7-segmen

LED

display

TIP31C

flyback

diode

8 . Draw a program flowchart. The next figure illustrates the logic and flow neces-

sary to perform the desired functions. The program checks the state of the switches

when the pushbutton switch is pressed and compares the switch states to a pre-

stored combination. If the combination is valid, a green LED and a DC motor turn

on and stay on while the pushbutton switch is held down. If the combination is

(continued )

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314 CHAPTER 7 Microcontroller Programming and Interfacing

invalid, a red LED turns on, a buzzer sounds for 3 sec, and the digital display of

failures is incremented by 1. When a valid combination is entered, the counter dis-

play resets to 0.

start

declare variables and constants

initialize the valid combination and the outputs

loop

wait for the enter button to be pressed

read the states of the three switches

combo

valid?

reset the counter

update the counter display

turn on the green LED and the motor

turn on the red LED

increment the counter

sound the buzzer for

3 seconds

turn off the LEDs and the motor

wait for the enter button to be released

9 . Write the code. The PicBasic Pro code to implement the logic and flow illustrated by

the flowchart follows. Note that multiple PicBasic Pro statements can be included on

a single line if they are separated by colons (e.g., the multiple Low statements). Also,

a long statement can be continued on a second line by ending the first line with an

underscore (e.g., the long If statement). Because the byte variable number_invalid is

constrained to vary between 0 and 9, its four least significant bits represent the number

in the binary coded decimal form required by the display decoder.

(continued )

' project.bas

' This program checks the state of three switches when a pushbutton switch is

' pressed and compares the switch states to a prestored combination. If the

' combination is valid, a green LED and a DC motor turn on and stay on while the

' pushbutton switch is held down. If the combination is invalid, a red LED turns

' on, a buzzer sounds for 3 seconds, and the displayed digit (representing the

' number of failed attempts) increments by one. When a valid combination is

' entered, the counter display resets to zero.

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7.9 Method to Design a Microcontroller-Based System 315

' Declare all variables

switch_1 Var PORTA.0 ' first combination switch

switch_2 Var PORTA.1 ' second combination switch

switch_3 Var PORTA.2 ' third combination switch

enter_button Var PORTA.3 ' combination enter key

green_led Var PORTB.0 ' green LED indicating a valid combination

red_led Var PORTB.1 ' red LED indicating an invalid combination

speaker Var PORTB.2 ' speaker signal for sounding an alarm

motor Var PORTB.3 ' signal to bias the motor power transistor

a Var PORTB.4 ' bit 0 for the 7447 BCD input

b Var PORTB.5 ' bit 1 for the 7447 BCD input

c Var PORTB.6 ' bit 2 for the 7447 BCD input

d Var PORTB.7 ' bit 3 for the 7447 BCD input

combination Var BYTE ' stores the valid combination in the 3 LSBs

number_invalid Var BYTE ' counter used to keep track of the number of bad

' combinations

' Initialize the valid combination and turn off all output functions

combination = %101 ' valid combination (switch 3:on, switch

' 2:off, switch 1:on)

Low green_led : Low red_led ' make sure the LEDs are off

Low motor ' make sure the motor is off

Low a : Low b : Low c : Low d ' display zero on the digit display

number_invalid = 0 ' reset the number of invalid combinations

' to zero

' Beginning of the main polling loop

loop:

' Wait for the pushbutton switch to be pressed

If (enter_button == 0) Then loop

' Read switches and compare their states to the valid combination

If ((switch_1 == combination.0) AND (switch_2 == combination.1)_

AND (switch_3 == combination.2)) Then

' Turn on the green LED

High green_led

' Turn on the motor

High motor

' Reset the combination attempt counter

number_invalid = 0

Else

' Turn on the red LED

High red_led

' Sound the alarm

Sound speaker, [80,100]

' Increment the combination attempt counter and check for overflow

number_invalid = number_invalid + 1

(continued )

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316 CHAPTER 7 Microcontroller Programming and Interfacing

If (number_invalid > 9) Then

number_invalid = 0

Endif

' Update the invalid combination attempt counter digit display

a = number_invalid.0 : b = number_invalid.1 : c = number_invalid.2

d = number_invalid.3

' Wait for the pushbutton switch to be released

loop2: If (enter_button == 1) Then loop2

' Turn off the LEDs and the motor

Low green_led : Low red_led

Low motor

' Loop back to the beginning of the polling loop to continue the process

Goto loop

End ' end of program

(continued )

10. Build and test the system. Now that we have a detailed schematic and a complete

program, all that remains is to build and test the system. When first testing the system,

comment out secondary parts of the code (by placing comment apostrophes in front

of selected lines to temporarily disable them), in order to test the remaining parts. In

the example, we could test the combination input and green LED but comment out the

motor driver, the alarm, and the count and digital display. This way, we could ensure

that the basic I/O and logic of the program function properly when the programmed

PIC is inserted in the circuit. Then, additional functionality can be added a piece at a

time to achieve the complete solution. We recommend you create a first prototype on

a solderless breadboard until all of the bugs have been worked out. Then, a more per-

manent version can be created on a protoboard or printed circuit board.

When we assigned this design problem, as a class project, we had 30 groups of

three or four students creating unique designs. Some of the more interesting designs

included a wall safe, where the students fabricated a section of drywall with a face

plate containing three light switches. Externally it appeared to be a set of switches to

control lights in a room, but when the switches were set in the correct combination

and a small pushbutton switch on the side was pressed, a solenoid released a spring-

loaded door exposing a hidden wall safe. Another design was a rocket launcher.

When the correct switch combination was entered, interesting sound effects were

created (by using various For . . . Next loops and the PicBasic Pro Sound statement),

and then the digital display performed a countdown. When the count reached 0,

the device used a relay to fire a model rocket, which rose several hundred feet and

landed softly with parachute assist. This was demonstrated to the whole class and a

curious crowd on the campus grounds outside our building. The most popular design

was affectionately called the Beer-Bot shown in the following image. This device

dispensed a glass of liquid to the user if he or she knew the correct combination.

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7.9 Method to Design a Microcontroller-Based System 317

When the correct combination was entered, the platform (lower right) translated

out of the device with the aid of a DC motor driving a rack and pinion mechanism.

The end of travel was detected by a limit switch. The platter was spring loaded so a

simple switch could detect when a glass of adequate weight had been placed on it.

Then the platter retracted and a pump was turned on to draw fluid from a concealed

reservoir. When the liquid reached a certain level, a circuit was completed between

two metal leads (top right) that pivot into the glass when the platter is retracted.

At this point, the pump was turned off and the platter extended to present the full

glass to the user, accompanied by delightful sound effects. Video Demo 7.6 shows a

demonstration of the Beer-Bot in action, and Internet Link 7.14 points to numerous

video demonstrations of other student design project solutions. These clips repre-

sent some of the best student projects at Colorado State University since 2001.

Video Demo

7.6Beer-Bot—

secure liquid

dispensing

system

Internet Lin

7.14PIC

microcontroller

student design

projects

Below, we present the complete hardware and software solutions to the three

threaded design examples (A, B, and C). Details for various parts of the solution are

presented throughout the book. Refer to the list of Threaded Design Examples on

page xiii for page number references to the various solution portions presented. All

electrical components and devices used to build all of the Threaded Design Example

projects are listed with ordering information at Internet Link 1.4.

Internet Lin

1.4Threaded

Design Example

components

THREADED DESIGN EXAMPLE

DC motor power-op-amp speed controller—Full solution A.4

The figure below shows the functional diagram for Threaded Design Example A (see Sec-

tion 1.3 and Video Demo 1.6). Here, we include the entire solution to this problem. Some of

the details can be found in Threaded Design Example A.2 (the potentiometer interface), A.3

(the power amp motor driver), and A.5 (the D/A converter).

(continued )

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318 CHAPTER 7 Microcontroller Programming and Interfacing

potentiometer

PIC microcontroller

with analog-to-digital

converter

power

amp

motor

A/D D/A

light-

emitting

diode

digital-to-

analog

converter

The entire circuit schematic and software listing for a PIC16F88 are shown below. The

code is commented, so you should be able to follow the logic as it relates to the functional

diagram and circuit schematic. Specific information for the TLC7524C D/A converter can be

found at Internet Link 7.15.

5 V

PIC16F88

AN1

RA0

RA7

CLKO

330

pot

10 k

1 k

0.1 F

RA2

RA3

RA4

RA5

5 V

–9 V

R179-6V

DC motor

J3-1J3-6

diagnostic

LED

PORTB

TLC7524C

D/A converter

4–11

13–6

2, 3

5 V

1, 14

REF

OPA 547

+9 V

3, 4

Video Demo

1.6DC motor

power-op-amp

speed controller

Internet Lin

7.15TLC7524C

D/A converter

' poweramp.bas (PIC16F88 microcontroller)

' Design Example

' Power amp motor driver controlled by a potentiometer

' A potentiometer is attached to an A/D input in the PIC. The PIC

' outputs the corresponding voltage as a digital word to a TI TLC7524

' external D/A converter, which is attached to a TI OPA547 power-op-amp

' circuit. The amplifier circuit can provide up to 500 mA of current

' to a DC motor (e.g., R179-6V-ENC-MOTOR)

(continued )

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7.9 Method to Design a Microcontroller-Based System 319

' Configure the internal 8MHz internal oscillator

DEFINE OSC 8

OSCCON.4 = 1 : OSCCON.5 = 1 : OSCCON.6 = 1

' Turn on and configure AN1 (the A/D converter on pin 18)

ANSEL.1 = 1 : TRISA.1 = 1

ADCON1.7 = 1 ' have the 10 bits be right-justified

DEFINE ADC_BITS 10 ' AN1 is a 10-bit A/D

' Define I/O pin names

led Var PORTA.2 ' diagnostic LED

da_cs Var PORTA.3 ' external D/A converter chip select (low: activate)

da_wr Var PORTA.4 ' external D/A converter write (low: write)

' Declare Variables

key_value Var BYTE ' code byte from the keypad

ad_word Var WORD ' word from the A/D converter (10 bits padded with 6 0's)

ad_byte Var BYTE ' byte representing the pot position

' Define constants

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

' Initialize I/O

TRISB = 0 ' initialize PORTB pins as outputs

High da_wr ' initialize the A/D converter write line

Low da_cs ' activate the external D/A converter

' Main program (loop)

main:

' Read the potentiometer voltage with the A/D converter

ADCIN 1, ad_word

' Scale the A/D word value down to a byte

ad_byte = ad_word/4

' Send the potentiometer byte to the external D/A

PORTB = ad_byte

Low da_wr

Pauseus 1 ' wait 1 microsec for D/A to settle

High da_wr

' Blink the LED to indicate voltage output

Gosub blink

Goto main ' continue polling keypad buttons

End ' end of main program

' Subroutine to blink the speed control indicator LED

blink:

Low led

Pause blink_pause

High led

Pause blink_pause

Return

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320 CHAPTER 7 Microcontroller Programming and Interfacing

THREADED DESIGN EXAMPLE

B.2 Stepper motor position and speed controller—Full solution

The figure below shows the functional diagram for Threaded Design Example B (see Section

1.3 and Video Demo 1.7). Here, we show the complete solution to this problem.

stepper

motor

driver

potentiometer

microcontroller

A/D

light-

emitting

diode

stepper

motor

mode button

PIC

position buttons

The circuit schematic and software listing are shown below. The code is commented, so

you should be able to follow the logic as it relates to the functional diagram and circuit sche-

matic. A special integrated circuit available from E-Lab (see Internet Link 7.16) called the

EDE1200 is used to generate the proper coil sequences for the stepper motor (see Threaded

Design Example B.3).

PIC16F84

RA2

RA3

RA4

MCLR

RB0

RB1

RB2

RB3

RA1

RA0

OSC1

OSC2

RB7

RB6

RB5

RB4

5 V

22 pF

4 MHz

5 V

SPEED

5 V

ADC0831

EDE1200

5 V

3,4,6,

8,10,14

SPEED control LED

ULN2003A

5 V

1N4732

5 V

1–2 common (red)

coil 1 (yellow)

coil 2 (orange)

coil 3 (brown)

coil 4 (black)

3–4 common (green)

OSC

DIR

STEP

IN1

IN2

IN3

IN4

1 kΩ

330 Ω

10 Ω

pot

0.1 μF

in+

in-

CLK

ref

GND

Video Demo

1.7Stepper

motor position

and speed

controller

Internet Lin

7.16 EDE1200

unipolar stepper

motor driver

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