What’s a Microcontroller?
Student Guide
VERSION 3.0
Page 2 · What’s a Microcontroller?
WARRANTY
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product. If you discover a defect, Parallax will, at its option, repair or replace the merchandise, or refund the
purchase price. Before returning the product to Parallax, call for a Return Merchandise Authorization (RMA)
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of the problem. Parallax will return your product or its replacement using the same shipping method used to ship the
product to Parallax.
14-DAY MONEY BACK GUARANTEE
If, within 14 days of having received your product, you find that it does not suit your needs, you may return it for a
full refund. Parallax will refund the purchase price of the product, excluding shipping/handling costs. This guarantee
is void if the product has been altered or damaged. See the Warranty section above for instructions on returning a
product to Parallax.
COPYRIGHTS AND TRADEMARKS
This documentation is Copyright 2003-2009 by Parallax Inc. By downloading or obtaining a printed copy of this
documentation or software you agree that it is to be used exclusively with Parallax products. Any other uses are not
permitted and may represent a violation of Parallax copyrights, legally punishable according to Federal copyright or
intellectual property laws. Any duplication of this documentation for commercial uses is expressly prohibited by
Parallax Inc. Duplication for educational use, in whole or in part, is permitted subject to the following conditions: the
material is to be used solely in conjunction with Parallax products, and the user may recover from the student only the
cost of duplication. Check with Parallax for approval prior to duplicating any of our documentation in part or whole
for any other use.
BASIC Stamp, Board of Education, Boe-Bot, Stamps in Class, and SumoBot are registered trademarks of Parallax
Inc. HomeWork Board, PING))), Parallax, the Parallax logo, Propeller, and Spin are trademarks of Parallax Inc. If
you decide to use any of these words on your electronic or printed material, you must state that “(trademark) is a
(registered) trademark of Parallax Inc.” upon the first use of the trademark name. Other brand and product names
herein are trademarks or registered trademarks of their respective holders.
ISBN 9781928982524
3.0.0-09.12.09-HKTP
DISCLAIMER OF LIABILITY
Parallax Inc. is not responsible for special, incidental, or consequential damages resulting from any breach of
warranty, or under any legal theory, including lost profits, downtime, goodwill, damage to or replacement of
equipment or property, or any costs of recovering, reprogramming, or reproducing any data stored in or used with
Parallax products. Parallax is also not responsible for any personal damage, including that to life and health, resulting
from use of any of our products. You take full responsibility for your BASIC Stamp application, no matter how lifethreatening it may be.
ERRATA
While great effort is made to assure the accuracy of our texts, errors may still exist. Occasionally an errata sheet with
a list of known errors and corrections for a given text will be posted on the related product page at
www.parallax.com. If you find an error, please send an email to editor@parallax.com.
Table of Contents
Preface.........................................................................................................................7
About Version 3.0 ...........................................................................................................7
Audience .........................................................................................................................8
Support Forums ..............................................................................................................8
Resources for Educators ................................................................................................9
Foreign Translations .....................................................................................................10
About the Author ...........................................................................................................10
Special Contributors .....................................................................................................10
Chapter 1 : Getting Started......................................................................................11
How Many Microcontrollers Did You Use Today? ........................................................11
The BASIC Stamp 2 – Your New Microcontroller Module ............................................11
Amazing Inventions with BASIC Stamp Microcontrollers .............................................12
Hardware and Software ................................................................................................15
Activity #1 : Getting the Software..................................................................................15
Activity #2 : Using the Help File for Hardware Setup ....................................................21
Summary ......................................................................................................................23
Chapter 2 : Lights On – Lights Off ..........................................................................27
Indicator Lights .............................................................................................................27
Making a Light-Emitting Diode (LED) Emit Light ..........................................................27
Activity #1 : Building and Testing the LED Circuit.........................................................28
Activity #2 : On/Off Control with the BASIC Stamp.......................................................37
Activity #3 : Counting and Repeating............................................................................43
Activity #4 : Building and Testing a Second LED Circuit ..............................................46
Activity #5 : Using Current Direction to Control a Bicolor LED .....................................50
Summary ......................................................................................................................57
Chapter 3 : Digital Input – Pushbuttons.................................................................61
Found on Calculators, Handheld Games, and Applicances .........................................61
Receiving vs. Sending High and Low Signals ..............................................................61
Activity #1 : Testing a Pushbutton with an LED Circuit .................................................61
Activity #2 : Reading a Pushbutton with the BASIC Stamp ..........................................65
Activity #3 : Pushbutton Control of an LED Circuit .......................................................70
Activity #4 : Two Pushbuttons Controlling Two LED Circuits........................................73
Activity #5 : Reaction Timer Test ..................................................................................79
Summary ......................................................................................................................87
Chapter 4 : Controlling Motion................................................................................93
Microcontrolled Motion..................................................................................................93
On/Off Signals and Motor Motion .................................................................................93
Introducing the Servo....................................................................................................93
Page 4 · What’s a Microcontroller?
Activity #1 : Connecting and Testing the Servo ............................................................95
Activity #2 : Servo Control Test Program....................................................................102
Activity #3 : Control Servo Hold Time .........................................................................112
Activity #4 : Controlling Position with your Computer .................................................118
Activity #5 : Converting Position to Motion .................................................................126
Activity #6 : Pushbutton-Controlled Servo ..................................................................129
Summary ....................................................................................................................134
Chapter 5 : Measuring Rotation............................................................................ 139
Adjusting Dials and Monitoring Machines...................................................................139
The Variable Resistor Under the Dial – a Potentiometer............................................139
Activity #1 : Building and Testing the Potentiometer Circuit .......................................141
Activity #2 : Measuring Resistance by Measuring Time .............................................143
Activity #3 : Reading the Dial with the BASIC Stamp .................................................150
Activity #4 : Controlling a Servo with a Potentiometer ................................................156
Summary ....................................................................................................................164
Chapter 6 : Digital Display..................................................................................... 169
The Everyday Digital Display......................................................................................169
What’s a 7-Segment Display? ....................................................................................169
Activity #1 : Building and Testing the 7-Segment LED Display ..................................171
Activity #2 : Controlling the 7-Segment LED Display..................................................175
Activity #3 : Displaying Digits......................................................................................178
Activity #4 : Displaying the Position of a Dial..............................................................185
Summary ....................................................................................................................191
Chapter 7 : Measuring Light.................................................................................. 195
Devices that Contain Light Sensors ...........................................................................195
Introducing the Phototransistor...................................................................................198
Activity #1 : Building and Testing the Light Meter.......................................................199
Activity #2 : Tracking Light Events..............................................................................202
Activity #3 : Graphing Light Measurements (Optional) ...............................................211
Activity #4 : Simple Light Meter ..................................................................................214
Activity #5 : On/Off Phototransistor Output.................................................................225
Activity #6 : For Fun—Measure Outdoor Light with an LED .......................................235
Summary ....................................................................................................................239
Chapter 8 : Frequency and Sound ....................................................................... 245
Your Day and Electronic Beeps..................................................................................245
Microcontrollers, Speakers, and On/Off Signals.........................................................245
Activity #1 : Building and Testing the Speaker ...........................................................246
Activity #2 : Action Sounds .........................................................................................248
Activity #3 : Musical Notes and Simple Songs............................................................253
Activity #4 : Microcontroller Music ..............................................................................258
Activity #5 : Ringtones with RTTTL.............................................................................271
Summary ....................................................................................................................283
Chapter 9 : Electronic Building Blocks ................................................................287
Those Little Black Chips .............................................................................................287
Expand your Projects with Peripheral Integrated Circuits...........................................288
Activity #1 : Control Current Flow with a Transistor ....................................................289
Activity #2 : Introducing the Digital Potentiometer ......................................................292
Summary ....................................................................................................................302
Chapter 10 : Prototyping Your Own Inventions ..................................................307
Apply what You Know to Other Parts and Components .............................................307
Prototyping a Micro Security System ..........................................................................308
Activity #1 : From Idea to Proof of Concept ................................................................308
Activity #2 : Build and Test Each Circuit Individually ..................................................311
Activity #3 : Organize Coding Tasks Into Small Pieces ..............................................313
Activity #4 : Document Your Code! .............................................................................317
Activity #5 : Give Your App Amazing New Functionality.............................................319
Activity #6 : How to Jump Over Design Hurdles .........................................................320
Activity #7 : What’s Next? ...........................................................................................327
Summary ....................................................................................................................331
Complete Kit Options ..................................................................................................334
Bonus Activity: Ohm’s Law, Voltage, and Current ......................................................336
Index ........................................................................................................................345
Page 6 · What’s a Microcontroller?
Preface · Page 7
Preface
This text answers the question “What’s a microcontroller?” by showing students how
they can design their own customized, intelligent inventions with Parallax Inc.’s BASIC
Stamp® microcontroller module. The activities in this text incorporate a variety of fun
and interesting experiments designed to appeal to a student’s imagination by using
motion, light, sound, and tactile feedback to explore new concepts. These activities
introduce students to a variety of basic principles in the fields of computer programming,
electricity and electronics, mathematics, and physics. Many of the activities facilitate
hands-on presentation of design practices used by engineers and technicians in the
creation of modern machines and appliances, while using common inexpensive parts.
What’s a Microcontroller? is the gateway text in to the Stamps in Class program. To see
the full series, which includes such titles as Robotics with the Boe-Bot, Smart Sensors and
Applications, Process Control, and more, visit www.parallax.com/Education.
ABOUT VERSION 3.0
This is the first revision of this title since 2004. The major changes include:
•
•
•
•
Replacement of the cadmium sulfide photoresistor with an RoHS-compliant light
sensor of a type that will be more common in product design going forward. This
required rewrites of Chapters 7 and 10, and adjustments in other chapters.
Improved activities and illustrations of servo control in Chapter 4.
Moving the “Setup and Testing” portion of Chapter 1 and the Hardware and
Troubleshooting appendices to the Help file. This was done to support both
serial and USB hardware connections, and other programming connections as
our products and technologies continue to expand. This also allows for the
dynamic maintenance of the Hardware and Troubleshooting material.
Removal of references to the Parallax CD, which has been removed from our
kits, reducing waste and ensuring that customers download the most recent
BASIC Stamp Editor software and USB drivers available for their operating
systems.
In addition, small errata items noted in the previous version (2.2) have been corrected.
The material still aims for the same goals, and all of the same programming concepts and
commands are covered, along with a few new ones. Finally, page numbers have been
changed so the PDF page and the physical page numbers are the same, for ease of use.
Page 8 · What’s a Microcontroller?
AUDIENCE
This text is designed to be an entry point to technology literacy, and an easy learning
curve for embedded programming and device design. The text is organized so that it can
be used by the widest possible variety of students as well as independent learners.
Middle-school students can try the examples in this text in a guided tour fashion by
simply following the check-marked instructions with instructor supervision. At the other
end of the spectrum, pre-engineering students’ comprehension and problem-solving skills
can be tested with the questions, exercises and projects (with solutions) in each chapter
summary. The independent learner can work at his or her own pace, and obtain
assistance through the Stamps in Class forum cited below.
SUPPORT FORUMS
Parallax maintains free, moderated forums for our customers, covering a variety of
subjects:
• Propeller Chip: for all discussions related to the multicore Propeller
microcontroller and development tools product line.
• BASIC Stamp: Project ideas, support, and related topics for all of the Parallax
BASIC Stamp models.
• SX Microcontrollers: Technical assistance for all SX chip products, including
the SX/B Compiler, and SX-Key Tool.
• Sensors: Discussion relating to Parallax’s wide array of sensors, and interfacing
sensors with Parallax microcontrollers.
• Stamps in Class: Students, teachers, and customers discuss Parallax’s education
materials and school projects here.
• Robotics: For all Parallax robots and custom robots built with Parallax
processors and sensors.
• The Sandbox: Topics related to the use of Parallax products but not specific to
the other forums.
• Completed Projects: Post your completed projects here, made from Parallax
products.
• HYDRA System and Propeller Game Development: Discussion and technical
assistance for the HYDRA Game Development Kit and related Propeller
microcontroller programming.
Preface · Page 9
RESOURCES FOR EDUCATORS
We have a variety of resources for this text designed to support educators.
Stamps in Class “Mini Projects”
To supplement our texts, we provide a bank of projects for the classroom. Designed to
engage students, each “Mini Project” contains full source code, “How it Works”
explanations, schematics, and wiring diagrams or photos for a device a student might like
to use. Many projects feature an introductory video, to promote self-study in those
students most interested in electronics and programming. Just follow the Stamps in Class
“Mini Projects” link at www.parallax.com/Education.
Educators Courses
These hands-on, intensive 1 or 2 day courses for instructors are taught by Parallax
engineers or experienced teachers who are using Parallax educational materials in their
classrooms. Visit www.parallax.com/Education → Educators Courses for details.
Parallax Educator’s Forum
In this free, private forum, educators can ask questions and share their experiences with
using Parallax products in their classrooms. Supplemental Education Materials are also
posted here. To enroll, email education@parallax.com for instructions; proof of status as
an educator will be required.
Supplemental Educational Materials
Select Parallax educational texts have an unpublished set of questions and solutions
posted in our Parallax Educators Forum; we invite educators to copy and modify this
material at will for the quick preparation of homework, quizzes, and tests. PowerPoint
presentations and test materials prepared by other educators may be posted here as well.
Copyright Permissions for Educational Use
No site license is required for the download, duplication and installation of Parallax
software for educational use with Parallax products on as many school or home
computers as needed. Our Stamps in Class texts and BASIC Stamp Manual are all
available as free PDF downloads, and may be duplicated as long as it is for educational
use exclusively with Parallax products and the student is charged no more than the cost of
duplication. The PDF files are not locked, enabling selection of texts and images to
prepare handouts, transparencies, or PowerPoint presentations.
Page 10 · What’s a Microcontroller?
FOREIGN TRANSLATIONS
Many of our Stamps in Class texts have been translated into other languages; these texts
are free downloads and subject to the same Copyright Permissions for Educational Use as
our original versions. To see the full list, click on the Tutorials & Translations link at
www.parallax.com/Education. These were prepared in coordination with the Parallax
Volunteer Translator program. If you are interested in participating in our Volunteer
Translator program, email translations@parallax.com.
ABOUT THE AUTHOR
Andy Lindsay joined Parallax Inc. in 1999, and has since authored eight books and
numerous articles and product documents for the company. The last three versions of
What’s a Microcontroller? were designed and updated based on observations and
educator feedback that Andy collected while traveling the nation and abroad teaching
Parallax Educator Courses and events. Andy studied Electrical and Electronic
Engineering at California State University, Sacramento, and is a contributing author to
several papers that address the topic of microcontrollers in pre-engineering curricula.
When he’s not writing educational material, Andy does product and application
engineering for Parallax.
SPECIAL CONTRIBUTORS
The Parallax team assembled to prepare this edition includes: excellent department
leadership by Aristides Alvarez, lesson design and technical writing by Andy Lindsay;
cover art by Jen Jacobs; graphic illustrations by Rich Allred and Andy Lindsay; technical
review by Jessica Uelmen; technical nitpicking, editing, and layout by Stephanie Lindsay.
Special thanks go to Ken Gracey, founder of the Stamps in Class program, and to Tracy
Allen and Phil Pilgrim for consulting in the selection of the light sensor used in this
version to replace the cadmium-sulfide photoresistor.
Many people contributed to the development of What’s a Microcontroller? and assisted
with previous editions, to whom we are still grateful. Parallax wishes to again thank
Robert Ang for his thorough review and detailed input, and the late veteran engineer and
esteemed customer Sid Weaver for his insightful review. Thanks also to Stamps in Class
authors Tracy Allen (Applied Sensors) and Martin Hebel (Process Control) for their
review and recommendations. Andy Lindsay wishes to thank his father Marshall and
brother-in-law Kubilay for their expert musical advice and suggestions.
Getting Started · Page 11
Chapter 1: Getting Started
HOW MANY MICROCONTROLLERS DID YOU USE TODAY?
A microcontroller is a kind of miniature computer that you can find in all kinds of
devices. Some examples of common, every-day products that have microcontrollers
built-in are shown in Figure 1-1. If it has buttons and a digital display, chances are it also
has a programmable microcontroller brain.
Figure 1-1
Everyday Examples of
Devices that Contain
Microcontrollers
Try making a list and counting how many devices with microcontrollers you use in a
typical day. Here are some examples: if your clock radio goes off, and you hit the snooze
button a few times in the morning, the first thing you do in your day is interact with a
microcontroller. Heating up some food in the microwave oven and making a call on a
cell phone also involve interacting with microcontrollers. That’s just the beginning.
Here are a few more examples: turning on the television with a handheld remote, playing
a handheld game, and using a calculator. All those devices have microcontrollers inside
them that interact with you.
THE BASIC STAMP 2 – YOUR NEW MICROCONTROLLER MODULE
Parallax Inc.’s BASIC Stamp® 2 module shown in Figure 1-2 has a microcontroller built
onto it; it is the largest black chip. The rest of the components on the BASIC Stamp
module are also found in consumer appliances you use every day. All together, they are
correctly called an embedded computer system. This name is almost always shortened to
just “embedded system.” Frequently, such modules are commonly just called
“microcontrollers.”
The activities in this text will guide you through building circuits similar to the ones
found in consumer appliances and high-tech gadgets. You will also write computer
programs that the BASIC Stamp module will run. These programs will make the BASIC
Stamp module monitor and control these circuits so that they perform useful functions.
Page 12 · What’s a Microcontroller?
Figure 1-2
BASIC Stamp 2
Microcontroller
Module
®
In this text, “BASIC Stamp” refers to the BASIC Stamp 2 microcontroller module.
Designed and manufactured by Parallax Incorporated, this module’s name is commonly
abbreviated BS2, and it’s the first in the series of modules shown in Figure 1-3. Each of the
other modules is slightly different, featuring higher speed, more memory, additional
functionality, or some combination of these extra features. To learn more, follow the
“Compare BASIC Stamp Modules” link at www.parallax.com/basicstamp.
Figure 1-3
BASIC Stamp 2
Models, left to
right: BS2, BS2e,
BS2sx, BS2p24,
BS2p40, BS2pe,
BS2px
AMAZING INVENTIONS WITH BASIC STAMP MICROCONTROLLERS
Consumer appliances aren’t the only things that contain microcontrollers. Robots,
machinery, aerospace designs and other high-tech devices are also built with
microcontrollers. Let’s take a look at some examples that were created with BASIC
Stamp modules.
Robots have been designed to do everything from helping students learn more about
microcontrollers, to mowing the lawn, to solving complex mechanical problems. Figure
1-4 shows two example robots. On each of these robots, students use the BASIC Stamp 2
to read sensors, control motors, and communicate with other computers. The robot on
the left is Parallax Inc.’s Boe-Bot® robot. The projects in the Robotics with the Boe-Bot
text can be tackled using the Boe-Bot after you’ve worked through the activities in this
text. The one on the right is called an underwater ROV (remotely operated vehicle) and
it was constructed and tested at a MATE (Marine Advanced Technology Education)
Getting Started · Page 13
Summer Teachers Institute. Operators view a TV displaying what the ROV sees through
a video camera and control it with a combination of hand controls and a laptop. Its
BASIC Stamp measures depth and temperature, controls the vertical thrust motor, and
exchanges information with the laptop. MATE coordinates regional and international
ROV competitions for students at levels ranging from middle school to university.
Figure 1-4
Educational Robots
Boe-Bot robot (left)
ROV at MATE Summer
Teachers Institute (right,
www.marinetech.org)
Other robots solve complex problems, such as the autonomous remote flight robot shown
at the left of Figure 1-5. This robot was built and tested by mechanical engineering
students at the University of California, Irvine. They used a BASIC Stamp module to
help it communicate with a satellite global positioning system (GPS) so that the robot
could know its position and altitude. The BASIC Stamp also read level sensors and
controlled the motor settings to keep the robot flying properly. The mechanical millipede
robot on the right of Figure 1-5 was developed by a professor at Nanyang Technical
University, Singapore. It has more than 50 BASIC Stamp modules on board, and they all
communicate with each other in an elaborate network that help control and orchestrate
the motion of each set of legs. Robots like this not only help us better understand designs
in nature, but they may eventually be used to explore remote locations, or even other
planets.
Figure 1-5
Research Robots that
Contain Microcontrollers
Autonomous flying robot
at UC Irvine (left) and
Millipede Project at
Nanyang University
(right)
Page 14 · What’s a Microcontroller?
With the help of microcontrollers, robots can also take on day-to-day tasks, such as
mowing the lawn. The BASIC Stamp module inside the robotic lawn mower shown in
Figure 1-6 helps it stay inside the boundaries of the lawn, and it also reads sensors that
detect obstacles and controls the motors that make it move.
Figure 1-6
BASIC Stamp 2
Microcontroller Module
Microcontrollers are also used in scientific, high technology, and aerospace projects.
The weather station shown on the left of Figure 1-7 is used to collect environmental data
related to coral reef decay. The BASIC Stamp module inside it gathers this data from a
variety of sensors and stores it for later retrieval by scientists. The submarine in the
center is an undersea exploration vehicle, and its thrusters, cameras and lights are all
controlled by BASIC Stamp microcontrollers. The rocket shown on the right was part of
a competition to launch a privately owned rocket into space. Nobody won the
competition, but this rocket almost made it! The BASIC Stamp controlled just about
every aspect of the launch sequence.
Figure 1-7
Environmental and Aerospace
Microcontroller Examples
Ecological data collection by
EME Systems (left), undersea
research by Harbor Branch
Institute (center), and JP
Aerospace test launch (right)
From common household appliances all the way through scientific and aerospace
applications, the microcontroller basics you will need to get started on projects like these
are introduced here. By working through the activities in this book, you will get to
Getting Started · Page 15
experiment with and learn how to use a variety of building blocks found in all these hightech inventions. You will build circuits for displays, sensors, and motion controllers.
You will learn how to connect these circuits to the BASIC Stamp 2 module, and then
write computer programs that make it control displays, collect data from the sensors, and
control motion. Along the way, you will learn many important electronic and computer
programming concepts and techniques. By the time you’re done, you might find yourself
well on the way to inventing a device of your own design.
HARDWARE AND SOFTWARE
Getting started with BASIC Stamp microcontroller modules is similar to getting started
with a brand-new PC or laptop. The first things that most people have to do is take it out
of the box, plug it in, install and test some software, and maybe even write some software
of their own using a programming language. If this is your first time using a BASIC
Stamp module, you will be doing all these same activities. If you are in a class, your
hardware may already be all set up for you. If this is the case, your teacher may have
other instructions. If not, this chapter will take you through all the steps of getting your
new BASIC Stamp microcontroller up and running.
ACTIVITY #1: GETTING THE SOFTWARE
The BASIC Stamp Editor (version 2.5 or higher) is the software you will use in most of
the activities and projects in this text. You will use this software to write programs that
the BASIC Stamp module will run. You can also use this software to display messages
sent by the BASIC Stamp that help you understand what it senses.
Computer System Requirements
You will need a personal computer to run the BASIC Stamp Editor software. Your
computer will need to have the following features:
•
•
•
Microsoft Windows 2000 or newer operating system
An available serial or USB port
Internet access and an Internet browser program
Downloading the Software from the Internet
It is important to always use the latest version of the BASIC Stamp Editor software if
possible. The first step is to go to the Parallax web site and download the software.
Page 16 · What’s a Microcontroller?
9 Using a web browser, go to www.parallax.com/basicstampsoftware (Figure 1-8).
Figure 1-8
The BASIC Stamp Editor
software page at
www.parallax.com/
basicstampsoftware
This is the place to
download the latest
version of the software.
9 Click on the Click Here to Download button to download the latest version of the
BASIC Stamp Windows Editor software (Figure 1-9).
Figure 1-9
The Download button on
the BASIC Stamp Editor
Software page.
Click on the button to
start the download.
Getting Started · Page 17
9 A File Download window will open, asking you if you want to run or to save this
file (Figure 1-10). Click on the Save button.
Figure 1-10
File Download Window
Click Save, then save the
file to your computer.
9 Use the Save in field to choose a place on your computer to save the installer file,
then click the Save button (Figure 1-11).
Figure 1-11
Save As Window
Choose a place to save
the software installer on
your computer, then click
Save.
Page 18 · What’s a Microcontroller?
9 When you see “Download Complete,” click the Run button (Figure 1-12.)
9 Follow the prompts that appear. You may see messages from your operating
system asking you to verify that you wish to continue with installation. Always
agree that you want to continue.
Figure 1-12
Download Complete
Message
Click Run.
If prompted, always
confirm you want to
continue.
9 The BASIC Stamp Editor Installer window will open (Figure 1-13). Click Next
and follow the prompts, accepting all defaults.
Figure 1-13
BASIC Stamp Editor
Installer Window
Click Next.
Getting Started · Page 19
9 IMPORTANT: When the “Install USB Driver” message appears (Figure 1-14),
leave the checkmark in place for the Automatically install/update driver
(recommended) box, and then click Next.
Figure 1-14
Install USB Driver
Message
Leave the box
checked, and click
Next.
9 When the “Ready to Install the Program” message appears (Figure 1-15), click
the Install button. A progress bar may appear, and this could take a few minutes.
Figure 1-15
Ready to Install the
Program
Click Install to
continue.
Page 20 · What’s a Microcontroller?
At this point, an additional window may appear behind the current window while the
USB drivers are updating. This window will eventually close on its own when the driver
installation is complete. If you don’t see this window, it does not indicate a problem.
About USB drivers. The USB drivers that install with the BASIC Stamp Windows Editor
installer by default are necessary to use any Parallax hardware connected to your
computer’s USB port. VCP stands for Virtual COM Port, and it will allow your computer’s
USB port to look and be treated as a standard RS232 serial port by Parallax hardware.
USB Drivers for Different Operating Systems The USB VCP drivers included in the
BASIC Stamp Windows Editor software are for certain Windows operating systems only. For
more information, visit www.parallax.com/usbdrivers.
9 When the window tells you that installation has been successfully completed,
click Finish (Figure 1-16).
Figure 1-16
BASIC Stamp
Editor Installation
Completed
Click Finish.
Getting Started · Page 21
ACTIVITY #2: USING THE HELP FILE FOR HARDWARE SETUP
In this section you will run the BASIC Stamp Editor’s Help file. Within the Help file, you
will learn about the different BASIC Stamp programming boards available for the Stamps
in Class program, and determine which one you are using. Then, you will follow the steps
in the Help to connect your hardware to your computer and test your BASIC Stamp
programming system.
Running the BASIC Stamp Editor for the first time
9 If you see the BASIC Stamp Editor icon on your computer desktop, double-click
it (Figure 1-17).
9 Or, click on your computer’s Start menu, then choose All Programs Parallax Inc BASIC Stamp Editor 2.5 BASIC Stamp Editor 2.5.
Figure 1-17
BASIC Stamp Editor
Desktop Icon
Double-click to launch
the program.
9 On the BASIC Stamp Editor’s toolbar, click Help on the toolbar (Figure 1-18)
and then select BASIC Stamp Help… from the drop-down menu.
Figure 1-18
Opening the Help Menu
Click Help, then choose
BASIC Stamp Help from
the drop-down menu.
Page 22 · What’s a Microcontroller?
Figure 1-19: BASIC Stamp Editor Help
9 Click on the Getting Started with Stamps in Class link on the bottom of the
Welcome page, as shown in the lower right corner of Figure 1-19.
Getting Started · Page 23
Following the Directions in the Help File
From here, you will follow the directions in the Help file to complete these tasks:
•
•
•
•
•
•
Identify which BASIC Stamp development board you are using
Connect your development board to your computer
Test your programming connection
Troubleshoot your programming connection, if necessary
Write your first PBASIC program for your BASIC Stamp
Power down your hardware when you are done
When you have completed the activities in the Help file, return to this book and continue
with the Summary below before moving on to Chapter 2.
What do I do if I get stuck?
If you run into problems while following the directions in this book or in the Help file, you
have many options to obtain free Technical Support:
•
•
•
•
Forums: sign up and post a message in our free, moderated Stamps in Class
forum at forums.parallax.com.
Email: send an email to support@parallax.com.
Telephone: In the Continental United States, call toll-free to 888-99-STAMP
(888-997-8267). All others call (916) 624-8333.
More resources: Visit www.parallax.com/support.
SUMMARY
This chapter guided you through the following:
• An introduction to some devices that contain microcontrollers
• An introduction to the BASIC Stamp module
• A tour of some interesting inventions made with BASIC Stamp modules
• Where to get the free BASIC Stamp Editor software you will use in just about all
of the experiments in this text
• How to install the BASIC Stamp Editor software
• How to use the BASIC Stamp Editor’s Help and the BASIC Stamp Manual
• An introduction to the BASIC Stamp module, Board of Education, and
HomeWork Board
• How to set up your BASIC Stamp hardware
• How to test your software and hardware
Page 24 · What’s a Microcontroller?
•
•
•
•
•
How to write and run a PBASIC program
Using the DEBUG and END commands
Using the CR control character and DEC formatter
A brief introduction to ASCII code
How to disconnect the power to your Board of Education or HomeWork Board
when you’re done
Questions
1. What is a microcontroller?
2. Is the BASIC Stamp module a microcontroller, or does it contain one?
3. What clues would you look for to figure out whether or not an appliance like a
clock radio or a cell phone contains a microcontroller?
4. What does an apostrophe at the beginning of a line of PBASIC program code
signify?
5. What PBASIC commands did you learn in this chapter?
6. Let’s say you want to take a break from your BASIC Stamp project to go get a
snack, or maybe you want to take a longer break and return to the project in a
couple days. What should you always do before you take your break?
Exercises
1. Explain what the asterisk does in this command:
DEBUG DEC 7 * 11
2. Guess what the Debug Terminal would display if you ran this command:
DEBUG DEC 7 + 11
3. There is a problem with these two commands. When you run the code, the
numbers they display are stuck together so that it looks like one large number
instead of two small ones. Modify these two commands so that the answers
appear on different lines in the Debug Terminal.
DEBUG DEC 7 * 11
DEBUG DEC 7 + 11
Getting Started · Page 25
Projects
1. Use DEBUG to display the solution to the math problem: 1 + 2 + 3 + 4.
2. Save FirstProgramYourTurn.bs2 under another name. If you were to place the
DEBUG command shown below on the line just before the END command in the
program, what other lines could you delete and still have it work the same?
Modify the copy of the program to test your hypothesis (your prediction of what
will happen).
DEBUG "What's 7 X 11?", CR, "The answer is: ", DEC 7 * 11
Solutions
Q1. A microcontroller is a kind of miniature computer found in electronic products.
Q2. The BASIC Stamp module contains a microcontroller chip.
Q3. If the appliance has buttons and a digital display, these are good clues that it has
a microcontroller inside.
Q4. A comment.
Q5. DEBUG and END
Q6. Disconnect the power from the BASIC Stamp project.
E1. It multiplies the two operands 7 and 11, resulting in a product of 77. The asterisk
is the multiply operator.
E2. The Debug Terminal would display: 18
E3. To fix the problem, add a carriage return using the CR control character and a
comma.
DEBUG DEC 7 * 11
DEBUG CR, DEC 7 + 11
P1. Here is a program to display a solution to the math problem: 1+2+3+4.
' What's a Microcontroller - Ch01Prj01_Add1234.bs2
'{$STAMP BS2}
'{$PBASIC 2.5}
DEBUG "What's 1+2+3+4?"
DEBUG CR, "The answer is: "
DEBUG DEC 1+2+3+4
END
Page 26 · What’s a Microcontroller?
P2. The last three DEBUG lines can be deleted. An additional CR is needed after the
"Hello" message.
' What's a Microcontroller - Ch01Prj02_ FirstProgramYourTurn.bs2
' BASIC Stamp sends message to Debug Terminal.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Hello, it's me, your BASIC Stamp!", CR
DEBUG "What's 7 X 11?", CR, "The answer is: ", DEC 7 * 11
END
The output from the Debug Terminal is:
Hello, it's me, your BASIC Stamp!
What's 7 X 11?
The answer is: 77
This output is the same as it was with the previous code. This is an example of
using commas to output a lot of information, using only one DEBUG command
with multiple elements in it.
Lights On – Lights Off · Page 27
Chapter 2: Lights On – Lights Off
INDICATOR LIGHTS
Indicator lights are so common that most people tend not to give them much thought.
Figure 2-1 shows three indicator lights on a laser printer. Depending on which light is
on, the person using the printer knows if it is running properly or needs attention. Here
are just a few examples of devices with indicator lights: car stereos, televisions, DVD
players, disk drives, printers, and alarm system control panels.
Figure 2-1
Indicator Lights
Indicator lights are
common on many
everyday devices.
Turning an indicator light on and off is a simple matter of connecting and disconnecting
it from a power source. In some cases, the indicator light is connected directly to the
battery or power supply, like the power indicator light on the Board of Education. Other
indicator lights are switched on and off by a microcontroller inside the device. These are
usually status indicator lights that tell you what the device is up to.
MAKING A LIGHT-EMITTING DIODE (LED) EMIT LIGHT
Most of the indicator lights you see on devices are called light emitting diodes. You will
often see a light emitting diode referred to in books and circuit diagrams by the letters
LED. The name is usually pronounced as three letters: “L-E-D.” You can build an LED
circuit and connect power to it, and the LED emits light. You can disconnect the power
from an LED circuit, and the LED stops emitting light.
Page 28 · What’s a Microcontroller?
An LED circuit can be connected to the BASIC Stamp, and the BASIC Stamp can be
programmed to connect and disconnect the LED circuit’s power. This is much easier
than manually changing the circuit’s wiring or connecting and disconnecting the battery.
The BASIC Stamp can also be programmed to do the following:
•
•
•
•
Turn an LED circuit on and off at different rates
Turn an LED circuit on and off a certain number of times
Control more than one LED circuit
Control the color of a bicolor (two color) LED circuit
ACTIVITY #1: BUILDING AND TESTING THE LED CIRCUIT
It’s important to test components individually before building them into a larger system.
This activity focuses on building and testing two different LED circuits. The first circuit
is the one that makes the LED emit light. The second circuit is the one that makes it not
emit light. In the activity that comes after this one, you will build the LED circuit into a
larger system by connecting it to the BASIC Stamp. You will then write programs that
make the BASIC Stamp cause the LED to emit light, then not emit light. By first testing
each LED circuit to make sure it works, you can be more confident that it will work when
you connect it to a BASIC Stamp.
Introducing the Resistor
A resistor is a component that “resists” the flow of electricity. This flow of electricity is
called current. Each resistor has a value that tells how strongly it resists current flow.
This resistance value is called the ohm, and the sign for the ohm is the Greek letter
omega: Ω. Later in this book you will see the symbol kΩ, meaning kilo-ohm, or one
thousand ohms. The resistor you will be working with in this activity is the 470 Ω resistor
shown in Figure 2-2. The resistor has two wires (called leads and pronounced “leeds”),
one coming out of each end. There is a ceramic case between the two leads, and it’s the
part that resists current flow. Most circuit diagrams that show resistors use the jagged
line symbol on the left to tell the person building the circuit that he or she must use a 470
Ω resistor. This is called a schematic symbol. The drawing on the right is a part drawing
used in some beginner level Stamps in Class texts to help you identify the resistor in your
kit, and where to place it when you build the circuit.
Lights On – Lights Off · Page 29
Gold
Silver
or
Blank
470 Ω
Yellow
Violet
Brown
Figure 2-2
470 Ω Resistor Part Drawing
Schematic symbol (left) and Part
Drawing (right)
Resistors like the ones we are using in this activity have colored stripes that tell you what
their resistance values are. There is a different color combination for each resistance
value. For example, the color code for the 470 Ω resistor is yellow-violet-brown.
There may be a fourth stripe that indicates the resistor’s tolerance. Tolerance is measured
in percent, and it tells how far off the part’s true resistance might be from the labeled
resistance. The fourth stripe could be gold (5%), silver (10%) or no stripe (20%). For the
activities in this book, a resistor’s tolerance does not matter, but its value does.
Each color bar that tells you the resistor’s value corresponds to a digit, and these
colors/digits are listed in Table 2-1. Figure 2-3 shows how to use each color bar with the
table to determine the value of a resistor.
Table 2-1
Resistor Color
Code Values
Digit
Color
0
1
2
3
4
5
6
7
8
9
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Gray
White
Tolerance
Code
First Digit
Number of Zeros
Second Digit
Figure 2-3
Resitor Color
Codes
Page 30 · What’s a Microcontroller?
Here is an example that shows how Table 2-1 and Figure 2-3 can be used to figure out a
resistor value by proving that yellow-violet-brown is really 470 Ω:
•
•
•
The first stripe is yellow, which means the leftmost digit is a 4.
The second stripe is violet, which means the next digit is a 7.
The third stripe is brown. Since brown is 1, it means add one zero to the right of
the first two digits.
Yellow-Violet-Brown = 4-7-0 = 470 Ω.
Introducing the LED
A diode is a one-way current valve, and a light emitting diode (LED) emits light when
current passes through it. Unlike the color codes on a resistor, the color of the LED
usually just tells you what color it will glow when current passes through it. The
important markings on an LED are contained in its shape. Since it is a one-way current
valve, make sure to connect it the right way in your circuit or it won’t work as intended.
Figure 2-4 shows an LED’s schematic symbol and part drawing. An LED has two
terminals. One is called the anode, and the other is called the cathode. In this activity,
you will have to build the LED into a circuit, paying attention to make sure the leads
connected to the anode and cathode are connected to the circuit properly. On the part
drawing, the anode lead is labeled with the plus-sign (+). On the schematic symbol, the
anode is the wide part of the triangle. In the part drawing, the cathode lead is the
unlabeled pin, and on the schematic symbol, the cathode is the line across the point of the
triangle.
Figure 2-4
LED Part Drawing and Schematic
Symbol
Part Drawing (above) and schematic
symbol (below).
+
LED
The LED’s part drawings in later
pictures will have a + next to the
anode leg.
Lights On – Lights Off · Page 31
When you start building your circuit, make sure to check it against the schematic symbol
and part drawing. For the part drawing, note that the LED’s leads are different lengths.
The longer lead is connected to the LED’s anode, and the shorter lead is connected to its
cathode. Also, if you look closely at the LED’s plastic case, it’s mostly round, but there
is a small flat spot right near the shorter lead that that tells you it’s the cathode. This
really comes in handy if the leads have been clipped to the same length.
LED Test Circuit Parts
(1) LED – Green
(1) Resistor – 470 Ω (yellow-violet-brown)
Identifying the parts: In addition to the part drawings in Figure 2-2 and Figure 2-4, you can
use the photo on the last page of the book to help identify the parts in the kit needed for this
and all other activities.
Building the LED Test Circuit
You will build a circuit by plugging the LED and resistor leads into small holes called
sockets on the prototyping area shown in Figure 2-5. This prototyping area has black
sockets along the top and along the left. The black sockets along the top have labels
above them: Vdd (+5 V), Vin (the unregulated voltage straight from your battery or
power supply), and Vss (0 V, also called ground). These are called the power terminals,
and they will be used to supply your circuits with electricity. The black sockets on the
left have labels like P0, P1, up through P15. These are sockets that you can use to
connect your circuit to the BASIC Stamp module’s input/output pins.
Vdd
Vin
Vss
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Figure 2-5
Prototyping Area
Power terminals (black sockets along
top), I/O pin access (black sockets
along the side), and solderless
breadboard (white sockets)
Page 32 · What’s a Microcontroller?
Input/output pins are usually called I/O pins, and after connecting your circuit to one or
more of these I/O pins, you can program your BASIC Stamp to monitor the circuit (input) or
send on or off signals to the circuit (output). You will try this in the next activity.
The white board with lots of holes in it is called a solderless breadboard. You will use
this breadboard to connect components to each other and build circuits. This breadboard
has 17 rows of sockets. In each row, there are two five-socket groups separated by a
trench in the middle. All the sockets in a 5-socket group are connected together. So, if
you plug two wires into the same 5-socket group, they will make electrical contact. Two
wires in the same row but on opposite sides of the center trench will not be connected.
Many devices are designed to be plugged in over this trench, such as the pushbutton we
will use in Chapter 3.
More about breadboarding: To learn about the history of breaboards, how modern
breadboards are constructed, and how to use them, see the video resources at
www.parallax.com/go/WAM.
Figure 2-6 shows a circuit schematic, and a picture of how that circuit will look when it is
built on the prototyping area. Each 5-socket group can connect up to five leads, or wires,
to each other. For this circuit, the resistor and the LED are connected because each one
has a lead plugged into the same 5-socket group. Note that one lead of the resistor is
plugged into Vdd (+5 V) so the circuit can draw power. The other resistor lead connects
to the LED’s anode lead. The LED’s cathode lead is connected to Vss (0 V, ground)
completing the circuit.
You are now ready to build the circuit shown in Figure 2-6 (below) by plugging the LED
and resistor leads into sockets on the prototyping area. Follow these steps:
9 Disconnect power from your Board of Education or HomeWork Board.
9 Use Figure 2-4 to decide which lead is connected to the LED’s cathode. Look
for the shorter lead and the flat spot on the plastic part of the LED.
9 Plug the LED’s cathode into one of the black sockets labeled Vss on the
prototyping area.
9 Plug the LED’s anode (the other, longer lead) into the socket shown on the
breadboard portion of the prototyping area.
9 Plug one of the resistor’s leads into the same 5-socket group as the LED’s anode.
This will connect those two leads together.
9 Plug the resistor’s other lead into one of the sockets labeled Vdd.
Lights On – Lights Off · Page 33
Direction does matter for the LED, but not for the resistor. If you plug the LED in
backward, the LED will not emit light when you connect power. The resistor just resists the
flow of current. There is no backwards or forwards for a resistor.
9 Reconnect power to your Board of Education or HomeWork Board.
9 Check to make sure your green LED is emitting light. It should glow green.
Vdd
Vdd
X3
470 Ω
LED
Vss
Vin
Vss
+
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Figure 2-6
LED On, wired directly to power
Schematic (left) and Wiring Diagram
(right).
Note that one resistor lead and the
green LED’s anode lead are
plugged into the same 5-socket
group. This electrically connects the
two components.
If your green LED does not emit light when you connect power to the board:
9 Some LEDs are brightest when viewed from above. Try looking straight down
onto the dome part of the LED’s plastic case from above.
9 If the room is bright, try turning off some of the lights, or use your hands to cast
a shadow on the LED.
If you still do not see any green glow, try these steps:
9 Double check to make sure the LED’s cathode and anode are connected
properly. If not, simply remove the LED, give it a half-turn, and plug it back in.
It will not hurt the LED if you plug it in backwards, it just doesn’t emit light.
When you have it plugged in the right direction, it should emit light.
9 Double check to make sure you built your circuit exactly as shown in Figure 2-6.
Page 34 · What’s a Microcontroller?
9 If you are using a What’s a Microcontroller kit that somebody used before you,
the LED may be damaged, so try a different one.
9 If you are in a lab class, check with your instructor.
Still stuck? Try these free online resources:
Visit the Stamps In Class moderated forums: If you don’t have an instructor or friend who
can help, you can always check with the Stamps in Class forum at
http://forums.parallax.com. If you don’t get your questions answered there, you can contact
Parallax Technical Support department by following the Support link at www.parallax.com.
How the LED Test Circuit Works
The Vdd and Vss terminals supply electrical pressure in the same way that a battery
would. The Vdd sockets are like the battery’s positive terminal, and the Vss sockets are
like the battery’s negative terminal. Figure 2-7 shows how applying electrical pressure to
a circuit using a battery causes electrons to flow through it. This flow of electrons is
called electric current, or often just current. Electric current is limited by the resistor.
This current is what causes the diode to emit light.
+
-
N
+++
+++
+++
_
--- - -N
-N - N
-
+
+
=
N
Figure 2-7
LED On Circuit Electron Flow
-
-
N N
-
-
-
-
-
-
-
-
-
The minus signs with the circles
around them are used to show
electrons flowing from the battery’s
negative terminal to its positive
terminal.
Lights On – Lights Off · Page 35
Chemical reactions inside the battery supply the circuit with current. The battery’s negative
terminal contains a compound that has molecules with extra electrons (shown Figure 2-7 by
minus-signs). The battery’s positive terminal has a chemical compound with molecules that
are missing electrons (shown by plus-signs). When an electron leaves a molecule in the
negative terminal and travels through the wire, it is called a free electron (also shown by
minus-signs). The molecule that lost that extra electron no longer has an extra negative
charge; it is now called neutral (shown by an N). When an electron gets to the positive
terminal, it joins a molecule that was missing an electron, and now that molecule is neutral
too.
Figure 2-8 shows how the flow of electricity through the LED circuit is described using
schematic notation. The electrical pressure across the circuit is called voltage. The + and
– signs are used to show the voltage applied to a circuit. The arrow shows the current
flowing through the circuit. This arrow is almost always shown pointing the opposite
direction of the actual flow of electrons. Benjamin Franklin is credited with not having
been aware of electrons when he decided to represent current flow as charge passing from
the positive to negative terminal of a circuit. By the time physicists discovered the true
nature of electric current, the convention was already well established.
Voltage
+
Vdd
Resistance
Figure 2-8
LED-On Circuit Schematic Showing
Conventional Voltage and Current
Flow
Current
LED
Voltage
-
The + and – signs show voltage
applied to the circuit, and the arrow
shows current flow through the
circuit.
Vss
A schematic drawing (like Figure 2-8) is a picture that explains how one or more circuits
are connected. Schematics are used by students, electronics hobbyists, electricians,
engineers, and just about everybody else who works with circuits.
Appendix B: More about Electricity contains some glossary terms and an activity you can
try to get more familiar with measurements of voltage, current and resistance.
Page 36 · What’s a Microcontroller?
Your Turn – Modifying the LED Test Circuit
In the next activity, you will program the BASIC Stamp to turn the LED on, then off,
then on again. The BASIC Stamp will do this by switching the LED circuit between two
different connections, Vdd and Vss. You just finished working with the circuit where the
resistor is connected to Vdd, and the LED emits light. Make the changes shown in Figure
2-9 to verify that the LED will turn off (not emit light) when the resistor’s lead is
disconnected from Vdd and connected to Vss.
9 Disconnect power from your Board of Education or HomeWork Board.
9 Unplug the resistor lead that’s plugged into the Vdd socket, and plug it into a
socket labeled Vss as shown in Figure 2-9.
9 Reconnect power to your Board of Education or HomeWork Board.
9 Check to make sure your green LED is not emitting light. It should not glow
green.
Why does the LED not glow? Since both ends of the circuit are connected to the same
voltage (Vss), there isn’t any electrical pressure across the circuit. So, no current flows
through the circuit, and the LED stays off.
Vdd
X3
470 Ω
Vss
LED
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vin
Vss
+
Figure 2-9
LED Off Circuit
Schematic (left) and
wiring diagram (right).
Lights On – Lights Off · Page 37
ACTIVITY #2: ON/OFF CONTROL WITH THE BASIC STAMP
In Activity #1, two different circuits were built and tested. One circuit made the LED
emit light while the other did not. Figure 2-10 shows how the BASIC Stamp can do the
same thing if you connect an LED circuit to one if its I/O pins. In this activity, you will
connect the LED circuit to the BASIC Stamp and program it to turn the LED on and off.
You will also experiment with programs that make the BASIC Stamp do this at different
speeds.
SOUT
1
SIN
2
ATN
3
VSS
4
P0
5
P1
6
P2
7
BS2
Vdd
Vss
24
VIN
SOUT
23
VSS
SIN
2
22
RES
ATN
3
21
VDD (+5V)
VSS
4
20
P15
P0
5
19
P14
P1
6
18
P13
P2
7
1
BS2
Vdd
Vss
24
VIN
23
VSS
22
RES
21
VDD (+5V)
20
P15
19
P14
18
P13
8
17
P12
P3
8
17
P12
P4
9
16
P11
P4
9
16
P11
P5
10
15
P10
P5
10
15
P10
P6
11
14
P9
P6
11
14
P9
P7
12
13
P8
P7
12
13
P8
P3
BS2-IC
BS2-IC
Figure 2-10
BASIC Stamp
Switching
The BASIC
Stamp can be
programmed to
internally
connect the
LED circuit’s
input to Vdd or
Vss.
There are two big differences between changing the connection manually and having the
BASIC Stamp do it. First, the BASIC Stamp doesn’t have to cut the power to the
development board when it changes the LED circuit’s supply from Vdd to Vss. Second,
while a human can make that change several times a minute, the BASIC Stamp can do it
thousands of times per second!
LED Test Circuit Parts
Same as Activity #1.
Connecting the LED Circuit to the BASIC Stamp
The LED circuit shown in Figure 2-11 is wired almost the same as the circuit in the
previous exercise. The difference is that the resistor’s lead that was manually switched
between Vdd and Vss is now connected to a BASIC Stamp I/O pin.
9 Disconnect power from your Board of Education or HomeWork Board.
9 Modify the circuit you were working with in Activity #1 so that it matches
Figure 2-11.
Page 38 · What’s a Microcontroller?
Vdd
P14
X3
470 Ω
LED
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vin
Vss
+
Figure 2-11
BASIC Stamp
Controlled LED Circuit
The LED circuit’s
input is now
connected to a BASIC
Stamp I/O pin instead
of Vdd or Vss.
Resistors are essential. Always remember to use a resistor. Without it, too much current
will flow through the circuit, and it could damage any number of parts in your circuit, BASIC
Stamp, or Board of Education or HomeWork Board.
Turning the LED On/Off with a Program
The example program makes the LED blink on and off one time per second. It
introduces several new programming techniques at once. After running it, you will
experiment with different parts of the program to better understand how it works.
Example Program: LedOnOff.bs2
9
9
9
9
9
Enter the LedOnOff.bs2 code into the BASIC Stamp Editor.
Reconnect power to your Board of Education or HomeWork Board.
Run the program.
Verify that the LED flashes on and off once per second.
Disconnect power when you are done with the program.
Lights On – Lights Off · Page 39
'What's a Microcontroller - LedOnOff.bs2
'Turn an LED on and off. Repeat 1 time per second indefinitely.
'{$STAMP BS2}
'{$PBASIC 2.5}
DEBUG "The LED connected to P14 is blinking!"
DO
HIGH 14
PAUSE 500
LOW 14
PAUSE 500
LOOP
How LedOnOff.bs2 Works
The command DEBUG "The LED connected to P14 is blinking!" makes this
statement appear in the Debug Terminal. The command HIGH 14 causes the BASIC
Stamp to internally connect I/O pin P14 to Vdd. This turns the LED on.
The command PAUSE 500 causes the BASIC Stamp to do nothing for ½ a second while
the LED stays on. The number 500 tells the PAUSE command to wait for 500/1000 of a
second. The number that follows PAUSE is called an argument. Arguments give PBASIC
commands the information that they need to execute. If you look up PAUSE in the BASIC
Stamp Manual, you will discover that it calls this number the Duration argument. The
name Duration was chosen for this argument to show that the PAUSE command pauses for
a certain “duration” of time, in milliseconds.
What’s a Millisecond? A millisecond is 1/1000 of a second. It is abbreviated as ms. It
takes 1000 ms to equal one second.
The command LOW 14 causes the BASIC Stamp to internally connect I/O pin P14 to Vss.
This turns the LED off. Since LOW 14 is followed by another PAUSE 500, the LED stays
off for half a second.
The reason the code repeats itself over and over again is because it is nested between the
PBASIC keywords DO and LOOP. Figure 2-12 shows how a DO…LOOP works. By placing
the code segment that turns the LED on and off with pauses between DO and LOOP, it tells
the BASIC Stamp to execute those four commands over and over again. The result is
Page 40 · What’s a Microcontroller?
that the LED flashes on and off, over and over again. It will keep flashing until you
disconnect power, press and hold the Reset button, or until the battery runs out. Code
that repeats a set of commands indefinitely is called an infinite loop.
DO
HIGH 14
PAUSE 250
LOW 14
PAUSE 250
Figure 2-12
DO…LOOP
The code between the keywords DO
and LOOP gets executed over and
over endlessly.
LOOP
A Diagnostic Test for your Computer
Although it’s not common, there are some computer systems, such as certain laptops and
docking stations, that will halt the PBASIC program after the first time through a
DO...LOOP. These computers have a non-standard serial port design. By placing a DEBUG
command in the program LedOnOff.bs2, the open Debug Terminal prevents this from
possibly happening. You will next re-run this program without the DEBUG command to
see if your computer has this non-standard serial port problem. It is not likely, but it
would be important for you to know.
9 Open LedOnOff.bs2.
9 Delete the entire DEBUG command.
9 Run the modified program while you observe your LED.
If the LED blinks on and off continuously, just as it did when you ran the original
program with the DEBUG command, your computer will not have this problem.
If the LED blinked on and off only once and then stopped, you have a computer with a
non-standard serial port design. If you disconnect the serial cable from your board and
press the Reset button, the BASIC Stamp will run the program properly without freezing.
In programs you write yourself, you will always need to add a single DEBUG command,
such as:
DEBUG "Program Running!"
Lights On – Lights Off · Page 41
…right after the compiler directives. It will open the Debug Terminal and keep the COM
port open. This will prevent your programs from freezing after one pass through the
DO...LOOP, or any of the other looping commands you will be learning in later chapters.
You will see this command in some of the example programs that would not otherwise
need a DEBUG instruction. So, you should be able to run all of the remaining programs in
this book even if your computer failed the diagnostic test, but in that case be sure to add a
short DEBUG command when you start writing your own programs.
Your Turn – Timing and Repetitions
By changing the PAUSE command’s Duration argument you can change the amount of
time the LED stays on and off. For example, by changing both the Duration arguments to
250, it will cause the LED to flash on and off twice per second. The DO…LOOP in your
program will now look like this:
DO
HIGH 14
PAUSE 250
LOW 14
PAUSE 250
LOOP
9 Open LedOnOff.bs2 and save a copy of it as LedOnOffYourTurn.bs2.
9 Change both of the PAUSE commands’ Duration arguments from 500 to 250, and
re-run the program.
If you want to make the LED blink on and off once every three seconds, with the low
time twice as long as the high time, you can program the PAUSE command after the
HIGH 14 command so that it takes one second using PAUSE 1000. The PAUSE command
after the LOW 14 command will have to be PAUSE 2000.
DO
HIGH 14
PAUSE 1000
LOW 14
PAUSE 2000
LOOP
9 Modify and re-run the program using the code snippet above.
Page 42 · What’s a Microcontroller?
A fun experiment is to see how short you can make the pauses and still see that the LED
is flashing. When the LED is flashing very fast, but it looks like it’s just on, it’s called
persistence of vision.
Here is how to test to see what your persistence of vision threshold is:
9 Try modifying both of your PAUSE command’s Duration arguments so that they
are 100.
9 Re-run your program and check for flicker.
9 Reduce both Duration arguments by 5 and try again.
9 Keep reducing the Duration arguments until the LED appears to be on all the
time with no flicker. It will be dimmer than normal, but it should not appear to
flicker.
One last thing to try is to create a one-shot LED flasher. When the program runs, the
LED flashes only once. This is a way to look at the functionality of the DO…LOOP. You
can temporarily remove the DO…LOOP from the program by placing an apostrophe to the
left of both the DO and LOOP keywords as shown below.
' DO
HIGH 14
PAUSE 1000
LOW 14
PAUSE 2000
' LOOP
9 Modify and re-run the program using the code snippet above.
9 Explain what happened, why did the LED only flash once?
Commenting a line of code: Placing an apostrophe to the left of a command changes it
into a comment. This is a useful tool because you don’t actually have to delete the
command to see what happens if you remove it from the program. It is much easier to add
and remove an apostrophe than it is to delete and re-type the commands.
Lights On – Lights Off · Page 43
ACTIVITY #3: COUNTING AND REPEATING
In the previous activity, the LED circuit either flashed on and off all the time, or it
flashed once and then stopped. What if you want the LED to flash on and off ten times?
Computers (including the BASIC Stamp) are great at keeping running totals of how many
times something happens. Computers can also be programmed to make decisions based
on a variety of conditions. In this activity, you will program the BASIC Stamp to stop
flashing the LED on and off after ten repetitions.
Counting Parts and Test Circuit
Use the example circuit shown in Figure 2-11 on page 38.
How Many Times?
There are many ways to make the LED blink on and off ten times. The simplest way is to
use a FOR...NEXT loop. The FOR...NEXT loop is similar to the DO...LOOP. Although
either loop can be used to repeat commands a fixed number of times, FOR...NEXT is
easier to use. This is sometimes called a counted or finite loop.
The FOR...NEXT loop depends on a variable to track how many times the LED has
blinked on and off. A variable is a word of your choosing that is used to store a value.
The next example program chooses the word counter to “count” how many times the
LED has been turned on and off.
Picking words for variable names has several rules:
1.
The name cannot be a word that is already used by PBASIC. These words are
called reserved words, and some examples that you should already be familiar with
are DEBUG, PAUSE, HIGH, LOW, DO, and LOOP. You can see the full Reserved Word
List in the BASIC Stamp Manual.
2.
The name cannot contain a space.
3.
Even though the name can contain letters, numbers, or underscores, it must begin
with a letter.
4.
The name must be less than 33 characters long.
Example Program: LedOnOffTenTimes.bs2
The program LedOnOffTenTimes.bs2 demonstrates how to use a FOR...NEXT loop to
blink an LED on and off ten times.
9 Your test circuit from Activity #2 should be built (or rebuilt) and ready to use.
Page 44 · What’s a Microcontroller?
9
9
9
9
9
Enter the LedOnOffTenTimes.bs2 code into the BASIC Stamp Editor.
Connect power to your Board of Education or HomeWork Board.
Run the program.
Verify that the LED flashes on and off ten times.
Run the program a second time, and verify that the value of counter shown in
the Debug Terminal accurately tracks how many times the LED blinked. Hint:
instead of clicking Run a second time, you can press and release the Reset button
on your Board of Education or HomeWork Board.
' What's a Microcontroller - LedOnOffTenTimes.bs2
' Turn an LED on and off. Repeat 10 times.
' {$STAMP BS2}
' {$PBASIC 2.5}
counter VAR Byte
FOR counter = 1 TO 10
DEBUG ? counter
HIGH 14
PAUSE 500
LOW 14
PAUSE 500
NEXT
DEBUG "All done!"
END
How LedOnOffTenTimes.bs2 Works
This PBASIC statement:
counter VAR Byte
…tells the BASIC Stamp Editor that your program will use the word counter as a
variable that can store a byte’s worth of information.
Lights On – Lights Off · Page 45
What’s a Byte? A byte is enough memory to store a number between 0 and 255. The
BASIC Stamp has four different types of variables, and each can store a different range of
numbers:
Table 2-2: Variable Types and Values they can Store
Variable type
Range of Values
Bit
Nib
Byte
Word
0 to 1
0 to 15
0 to 255
0 to 65535
A DEBUG instruction can include formatters that determine how information should be
displayed in the Debug Terminal. Placing the “?”question mark formatter before a
variable in a DEBUG command tells the Debug Terminal to display the name of the
variable and its value. This is how the command:
DEBUG ? counter
…displays both the name and the value of the counter variable in the Debug Terminal.
The FOR...NEXT loop and all the commands inside it are shown below. The statement
FOR counter = 1 to 10 tells the BASIC Stamp that it will have to set the counter
variable to 1, then keep executing commands until it gets to the NEXT statement. When
the BASIC Stamp gets to the NEXT statement, it jumps back to the FOR statement. The
FOR statement adds one to the value of counter. Then, it checks to see if counter is
greater than ten yet. If not, it repeats the process. When the value of counter finally
reaches eleven, the program skips the commands between the FOR and NEXT statements
and moves on to the command that comes after the NEXT statement.
FOR counter = 1 to 10
DEBUG ? counter
HIGH 14
PAUSE 500
LOW 14
PAUSE 500
NEXT
Page 46 · What’s a Microcontroller?
The command that comes after the NEXT statement is:
DEBUG "All done!"
This command is included just to show what the program does after ten times through the
FOR...NEXT loop. It moves on to the command that comes after the NEXT statement.
Your Turn – Other Ways to Count
9 In the program LedOnOffTenTimes.bs2, replace the statement:
FOR counter = 1 to 10
with this:
FOR counter = 1 to 20
9 Re-run the program. What did the program do differently, and was this
expected?
9 Try a second modification to the FOR statement. This time, change it to:
FOR counter = 20 to 120 STEP 10
How many times did the LED flash? What values displayed in the Debug Terminal?
ACTIVITY #4: BUILDING AND TESTING A SECOND LED CIRCUIT
Indicator LEDs can be used to tell the machine’s user many things. Many devices need
two, three, or more LEDs to tell the user if the machine is ready or not, if there is a
malfunction, if it’s done with a task, and so on.
In this activity, you will repeat the LED circuit test in Activity #1 for a second LED
circuit. Then you will adjust the example program from Activity #2 to make sure the
LED circuit is properly connected to the BASIC Stamp. After that, you will modify the
example program from Activity #2 to make the LEDs operate in tandem.
Extra Parts Required
In addition to the parts you used in Activities 1 and 2, you will need these parts:
(1) LED – yellow
(1) Resistor – 470 Ω (yellow-violet-brown)
Lights On – Lights Off · Page 47
Building and Testing the Second LED Circuit
In Activity #1, you manually tested the first LED circuit to make sure it worked before
connecting it to the BASIC Stamp. Before connecting the second LED circuit to the
BASIC Stamp, it’s important to test it too.
9
9
9
9
Disconnect power from your Board of Education or HomeWork Board.
Construct the second LED circuit as shown in Figure 2-13.
Reconnect power to your Board of Education or HomeWork Board.
Did the LED circuit you just added turn on? If yes, then continue. If no,
Activity #1 has some trouble-shooting suggestions that you can repeat for this
circuit.
Vdd
X3
Vdd
470 Ω
P14
470 Ω
LED
LED
Vss
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vin
Vss
+
+
Figure 2-13
Manual Test
Circuit for
Second LED
9 Disconnect power to your Board of Education or HomeWork Board.
9 Modify the second LED circuit you just tested by connecting the LED circuit’s
resistor lead (input) to P15 as shown in Figure 2-14.
Page 48 · What’s a Microcontroller?
Vdd
X3
P15
470 Ω
P14
470 Ω
LED
Vss
LED
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vin
Vss
+
+
Figure 2-14
Connecting
the Second
LED to the
BASIC Stamp
Schematic
(left) and
wiring
diagram
(right).
Using a Program to Test the Second LED Circuit
In Activity #2, you used an example program and the HIGH and LOW commands to control
the LED circuit connected to P14. These commands will have to be modified to control
the LED circuit connected to P15. Instead of using HIGH 14 and LOW 14, you will use
HIGH 15 and LOW 15.
Example Program: TestSecondLed.bs2
9
9
9
9
Enter TestSecondLed.bs2 into the BASIC Stamp Editor.
Connect power to your Board of Education or HomeWork Board.
Run TestSecondLED.bs2.
Make sure the LED circuit connected to P15 is flashing. If the LED connected
to P15 flashes, move on to the next example (Controlling Both LEDs). If the
LED circuit connected to P15 is not flashing, check your circuit for wiring errors
and your program for typing errors and try again.
' What's a Microcontroller - TestSecondLed.bs2
' Turn LED connected to P15 on and off.
' Repeat 1 time per second indefinitely.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
Lights On – Lights Off · Page 49
DO
HIGH 15
PAUSE 500
LOW 15
PAUSE 500
LOOP
Controlling Both LEDs
Yes, you can flash both LEDs at once. One way you can do this is to use two HIGH
commands before the first PAUSE command. One HIGH command sets P14 high, and the
next HIGH command sets P15 high. You will also need two LOW commands to turn both
LEDs off. It’s true that both LEDs will not turn on and off at exactly the same time
because one is turned on or off after the other. However, there is no more than a
millisecond’s difference between the two changes, and the human eye will not detect it.
Example Program: FlashBothLeds.bs2
9 Enter the FlashBothLeds.bs2 code into the BASIC Stamp Editor.
9 Run the program.
9 Verify that both LEDs appear to flash on and off at the same time.
' What's a Microcontroller - FlashBothLeds.bs2
' Turn LEDs connected to P14 and P15 on and off.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
DO
HIGH 14
HIGH 15
PAUSE 500
LOW 14
LOW 15
PAUSE 500
LOOP
Page 50 · What’s a Microcontroller?
Your Turn – Alternate LEDs
You can cause the LEDs to alternate by swapping the HIGH and LOW commands that
control one of the I/O pins. This means that while one LED is on, the other will be off.
9 Modify FlashBothLeds.bs2 so that the commands between the DO and LOOP
keywords look like this:
HIGH 14
LOW 15
PAUSE 500
LOW 14
HIGH 15
PAUSE 500
9 Run the modified version of FlashBothLeds.bs2 and verify that the LEDs flash
alternately on and off.
ACTIVITY #5: USING CURRENT DIRECTION TO CONTROL A BICOLOR
LED
The device shown in Figure 2-15 is a security monitor for electronic keys. When an
electronic key with the right code is used, the LED changes color, and a door opens. This
kind of LED is called a bicolor LED. This activity answers two questions:
1. How does the LED change color?
2. How can you run one with the BASIC Stamp?
Lights On – Lights Off · Page 51
Figure 2-15
Bicolor LED in a Security
Device
When the door is locked,
this bicolor LED glows
red. When the door is
unlocked by an electronic
key with the right code,
the LED turns green.
Introducing the Bicolor LED
The bicolor LED’s schematic symbol and part drawing are shown in Figure 2-16.
Figure 2-16
Bicolor LED
Schematic symbol (left)
and part drawing (right).
The bicolor LED is really just two LEDs in one package. Figure 2-17 shows how you
can apply voltage in one direction and the LED will glow green. By disconnecting the
LED and plugging it back in reversed, the LED will then glow red. As with the other
LEDs, if you connect both terminals of the circuit to Vss, the LED will not emit light.
Page 52 · What’s a Microcontroller?
Figure 2-17
Bicolor LED and
Applied Voltage
Green (left), red
(center) and no
light (right)
Bicolor LED Circuit Parts
(1) LED – bicolor
(1) Resistor – 470 Ω (yellow-violet-brown)
(1) Jumper wire
Building and Testing the Bicolor LED Circuit
Figure 2-18 shows the manual test for the bicolor LED.
9
9
9
9
9
9
9
9
Disconnect power from your Board of Education or HomeWork Board.
Build the circuit shown on the left side of Figure 2-18.
Reconnect power and verify that the bicolor LED is emitting green light.
Disconnect power again.
Modify your circuit so that it matches the right side of Figure 2-18.
Reconnect power.
Verify that the bicolor LED is now emitting red light.
Disconnect power.
What if my bicolor LED’s colors are reversed? Bicolor LEDs are manufactured like the
one in Figure 2-16 as well as with the colors reversed. If your bicolor LED glows red when
it’s connected in the circuit that should make it glow green and vice-versa, your LED’s colors
are reversed. If that’s the case, always plug pin 1 in where the diagrams show pin 2, and
pin 2 where the diagrams show pin 1.
Lights On – Lights Off · Page 53
Vdd
1 Vin 2
Vss
Vdd
X3
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
2 Vin 1
Vss
Figure 2-18
Manual bicolor LED Test
Bicolor LED green (left)
and red (right).
Controlling a bicolor LED with the BASIC Stamp requires two I/O pins. After you have
manually verified that the bicolor LED works using the manual test, you can connect the
circuit to the BASIC Stamp as shown in Figure 2-19.
9 Connect the bicolor LED circuit to the BASIC Stamp as shown in Figure 2-19.
Figure 2-19
Bicolor LED Connected
to BASIC Stamp
Schematic (left) and
wiring diagram (right).
Page 54 · What’s a Microcontroller?
BASIC Stamp Bicolor LED Control
Figure 2-20 shows how you can use P15 and P14 to control the current flow in the
bicolor LED circuit. The upper schematic shows how current flows through the green
LED when P15 is set to Vdd with HIGH and P14 is set to Vss with LOW. This is because
the green LED will let current flow through it when electrical pressure is applied as
shown, but the red LED acts like a closed valve and does not let current through it. The
bicolor LED glows green.
The lower schematic shows what happens when P15 is set to Vss and P14 is set to Vdd.
The electrical pressure is now reversed. The green LED shuts off and does not allow
current through. Meanwhile, the red LED turns on, and current passes through the circuit
in the opposite direction.
HIGH = Vdd P15
1
Current
2
LOW = Vss P14
Figure 2-20
BASIC Stamp bicolor
LED Test
470 Ω
LOW = Vss P15
Current through green
LED (above) and red
LED (below).
1
Current
2
HIGH = Vdd P14
470 Ω
Figure 2-20 also shows the key to programming the BASIC Stamp to make the bicolor
LED glow two different colors. The upper schematic shows how to make the bicolor
LED green using HIGH 15 and LOW 14. The lower schematic shows how to make the
bicolor LED glow red by using LOW 15 and HIGH 14. To turn the LED off, send low
signals to both P14 and P15 using LOW 15 and LOW 14. In other words, use LOW on both
pins.
Lights On – Lights Off · Page 55
The bicolor LED will also turn off if you send high signals to both P14 and P15. Why?
Because the electrical pressure (voltage) is the same at P14 and P15 regardless of whether
you set both I/O pins high or low.
Example Program: TestBiColorLED.bs2
9 Reconnect power.
9 Enter and run TestBiColorLed.bs2 code in the BASIC Stamp Editor.
9 Verify that the LED cycles through the red, green, and off states.
' What's a Microcontroller - TestBiColorLed.bs2
' Turn bicolor LED red, then green, then off in a loop.
' {$STAMP BS2}
' {$PBASIC 2.5}
PAUSE 1000
DEBUG "Program Running!", CR
DO
DEBUG "Green..."
HIGH 15
LOW 14
PAUSE 1500
DEBUG "Red..."
LOW 15
HIGH 14
PAUSE 1500
DEBUG "Off...", CR
LOW 15
LOW 14
PAUSE 1500
LOOP
Your Turn – Lights Display
In Activity #3, a variable named counter was used to control how many times an LED
blinked. What happens if you use the value counter to control the PAUSE command’s
Duration argument while repeatedly changing the color of the bicolor LED?
9 Rename and save TestBiColorLed.bs2 as TestBiColorLedYourTurn.bs2.
9 Add a counter variable declaration before the DO statement:
Page 56 · What’s a Microcontroller?
counter VAR BYTE
9 Replace the test code in the DO...LOOP with this FOR...NEXT loop.
FOR counter = 1 to 50
HIGH 15
LOW 14
PAUSE counter
LOW 15
HIGH 14
PAUSE counter
NEXT
When you are done, your code should look like this:
counter VAR BYTE
DO
FOR counter = 1 to 50
HIGH 15
LOW 14
PAUSE counter
LOW 15
HIGH 14
PAUSE counter
NEXT
LOOP
At the beginning of each pass through the FOR...NEXT loop, the PAUSE value (Duration
argument) is only one millisecond. Each time through the FOR...NEXT loop, the pause
gets longer by one millisecond at a time until it gets to 50 milliseconds. The DO...LOOP
causes the FOR...NEXT loop to execute over and over again.
9 Run the modified program and observe the effect.
Lights On – Lights Off · Page 57
SUMMARY
The BASIC Stamp can be programmed to switch a circuit with a light emitting diode
(LED) indicator light on and off. LED indicators are useful in a variety of places
including many computer monitors, disk drives, and other devices. The LED was
introduced along with a technique to identify its anode and cathode terminals. An LED
circuit must have a resistor to limit the current passing through it. Resistors were
introduced along with one of the more common coding schemes for indicating a resistor’s
value.
The BASIC Stamp switches an LED circuit on and off by internally connecting an I/O
pin to either Vdd or Vss. The HIGH command can be used to make the BASIC Stamp
internally connect one of its I/O pins to Vdd, and the LOW command can be used to
internally connect an I/O pin to Vss. The PAUSE command is used to cause the BASIC
Stamp to not execute commands for an amount of time. This was used to make LEDs
stay on and/or off for certain amounts of time. The amount of time is determined by the
number used in the PAUSE command’s Duration argument.
DO...LOOP can be used to create an infinite loop. The commands between the DO and
LOOP keywords will execute over and over again. Even though this is called an infinite
loop, the program can still be re-started by disconnecting and reconnecting power or
pressing and releasing the Reset button. A new program can also be downloaded to the
BASIC Stamp, and this will erase the program with the infinite loop. Counted loops can
be made with FOR...NEXT, a variable to keep track of how many repetitions the loop has
made, and numbers to specify where to start and stop counting.
Current direction and voltage polarity were introduced using a bicolor LED. If voltage is
applied across the LED circuit, current will pass through it in one direction, and it glows
a particular color. If the voltage polarity is reversed, current travels through the circuit in
the opposite direction and it glows a different color.
Questions
1. What is the name of this Greek letter: Ω, and what measurement does Ω refer
to?
2. Which resistor would allow more current through the circuit, a 470 Ω resistor or
a 1000 Ω resistor?
3. How do you connect two wires using a breadboard? Can you use a breadboard
to connect four wires together?
Page 58 · What’s a Microcontroller?
4. What do you always have to do before modifying a circuit that you built on a
breadboard?
5. How long would PAUSE 10000 last?
6. How would you cause the BASIC Stamp to do nothing for an entire minute?
7. What are the different types of variables?
8. Can a byte hold the value 500?
9. What will the command HIGH 7 do?
Exercises
1. Draw the schematic of an LED circuit like the one you worked with in Activity
#2, but connect the circuit to P13 instead of P14. Explain how you would modify
LedOnOff.bs2 on page 38 so that it will make your LED circuit flash on and off
four times per second.
2. Explain how to modify LedOnOffTenTimes.bs2 so that it makes the LED circuit
flash on and off 5000 times before it stops. Hint: you will need to modify just
two lines of code.
Project
1. Make a 10-second countdown using one yellow LED and one bicolor LED.
Make the bicolor LED start out red for 3 seconds. After 3 seconds, change the
bicolor LED to green. When the bicolor LED changes to green, flash the yellow
LED on and off once every second for ten seconds. When the yellow LED is
done flashing, the bicolor LED should switch back to red and stay that way.
Solutions
Q1. Omega refers to the ohm which measures how strongly something resists current
flow.
Q2. A 470 Ω resistor: higher values resist more strongly than lower values, therefore
lower values allow more current to flow.
Q3. To connect 2 wires, plug the 2 wires into the same 5-socket group. You can
connect 4 wires by plugging all 4 wires into the same 5-socket group.
Q4. Disconnect the power.
Q5. 10 seconds.
Q6. PAUSE 60000
Q7. Bit, Nib, Byte, and Word
Q8. No. The largest value a byte can hold is 255. The value 500 is out of range for a
byte.
Lights On – Lights Off · Page 59
Q9. HIGH 7 will cause the BASIC Stamp to internally connect I/O pin P7 to Vdd.
E1. The PAUSE Duration must be reduced to 500 ms / 4 = 125 ms. To use I/O pin
P13, HIGH 14 and LOW 14 have been replaced with HIGH 13 and LOW 13.
P13
DO
HIGH 13
PAUSE 125
LOW 13
PAUSE 125
LOOP
470 Ω
LED
Vss
E2. The counter variable has to be changed to Word size, and the FOR statement has
to be modified to count from 1 to 5000.
counter VAR Word
FOR counter = 1 to 5000
DEBUG ? counter, CR
HIGH 14
PAUSE 500
LOW 14
PAUSE 500
NEXT
P1. The bicolor LED schematic, on the left, is unchanged from Figure 2-19 on page
53. The yellow LED schematic is based on Figure 2-11 on page 38. For this
project P14 was changed to P13, and a yellow LED was used instead of green.
NOTE: When the BASIC Stamp runs out of commands, it goes into a low power
mode that causes the bicolor LEDs to flicker briefly every 2.3 seconds. The
same applies after the program executes an END command. There’s another
command called STOP that you can add to the end of the program to make it hold
any high/low signals without going into low power mode, which in turn prevents
the flicker.
P13
470 Ω
Yellow
LED
Vss
Page 60 · What’s a Microcontroller?
' What's a Microcontroller - Ch02Prj01_Countdown.bs2
' 10 Second Countdown with Red, Yellow, Green LED
' Red/Green: Bicolor LED on P15, P14. Yellow: P13
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
counter VAR Byte
' Red for three seconds
LOW 15
HIGH 14
PAUSE 3000
' Green for 10 seconds...
HIGH 15
LOW 14
' ...while the yellow LED is flashing
FOR counter = 1 TO 10
HIGH 13
PAUSE 500
LOW 13
PAUSE 500
NEXT
' Red stays on
LOW 15
HIGH 14
' Bicolor LED Red
' Bicolor LED Green
' Yellow LED on
' Yellow LED off
' Bi Color LED Red
Digital Input – Pushbuttons · Page 61
Chapter 3: Digital Input – Pushbuttons
FOUND ON CALCULATORS, HANDHELD GAMES, AND APPLICANCES
How many devices with pushbuttons do you use on a daily basis? Here are a few
examples that might appear in your list: computer, mouse, calculator, microwave oven,
TV remote, handheld game, and cell phone. In each device, there is a microcontroller
scanning the pushbuttons and waiting for the circuit to change. When the circuit changes,
the microcontroller detects the change and takes action. By the end of this chapter, you
will have experience with designing pushbutton circuits and programming the BASIC
Stamp to monitor them and take action when changes occur.
RECEIVING VS. SENDING HIGH AND LOW SIGNALS
In Chapter #2, you programmed the BASIC Stamp to send high and low signals, and you
used LED circuits to display these signals. Sending high and low signals means you used
a BASIC Stamp I/O pin as an output. In this chapter, you will use a BASIC Stamp I/O
pin as an input. As an input, an I/O pin listens for high/low signals instead of sending
them. You will send these signals to the BASIC Stamp using a pushbutton circuit, and
you will program the BASIC Stamp to recognize whether the pushbutton is pressed or not
pressed.
Other terms that mean send, high/low, and receive: Sending high/low signals is
described in different ways. You may see sending referred to as transmitting, controlling, or
switching. Instead of high/low, you might see it referred to as binary, TTL, CMOS, or
Boolean signals. Another term for receiving is sensing.
ACTIVITY #1: TESTING A PUSHBUTTON WITH AN LED CIRCUIT
If you can use a pushbutton to send a high or low signal to the BASIC Stamp, can you
also control an LED with a pushbutton? The answer is yes, and you will use it to test a
pushbutton in this activity.
Introducing the Pushbutton
Figure 3-1 shows the schematic symbol and the part drawing of a normally open
pushbutton. Two of the pushbutton’s pins are connected to each terminal. This means
that connecting a wire or part lead to pin 1 of the pushbutton is the same as connecting it
Page 62 · What’s a Microcontroller?
to pin 4. The same rule applies with pins 2 and 3. The reason the pushbutton doesn’t just
have two pins is because it needs stability. If the pushbutton only had two pins, those
pins would eventually bend and break from all the pressure that the pushbutton receives
when people press it.
1, 4
2, 3
1
4
2
3
Figure 3-1
Normally Open Pushbutton
Schematic symbol (left) and
part drawing (right)
The left side of Figure 3-2 shows how a normally open pushbutton looks when it’s not
pressed. When the button is not pressed, there is a gap between the 1,4 and 2,3 terminals.
This gap makes it so that the 1,4 terminal can not conduct current to the 2,3 terminal.
This is called an open circuit. The name “normally open” means that the pushbutton’s
normal state (not pressed) forms an open circuit. When the button is pressed, the gap
between the 1,4 and 2,3 terminals is bridged by a conductive metal. This is called a
closed circuit, and current can flow through the pushbutton.
1, 4
1, 4
2, 3
2, 3
Figure 3-2
Normally Open Pushbutton
Not pressed (left) and pressed (right)
Test Parts for the Pushbutton
(1) LED – pick a color
(1) Resistor – 470 Ω (yellow-violet-brown)
(1) Pushbutton – normally open
(1) Jumper wire
Building the Pushbutton Test Circuit
Figure 3-3 shows a circuit you can build to manually test the pushbutton.
Digital Input – Pushbuttons · Page 63
Always disconnect power from your Board of Education or BASIC Stamp HomeWork
Board before making any changes to your test circuit. From here onward, the instructions
will no longer say “Disconnect power…” between each circuit modification. It is up to you to
remember to do this.
Always reconnect power to your Board of Education or BASIC Stamp HomeWork Board
before downloading a program to the BASIC Stamp.
9 Build the circuit shown in Figure 3-3.
Vdd
Vdd
Vin
Vss
+
X3
1, 4
2, 3
470 Ω
LED
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Figure 3-3
Pushbutton Test Circuit
Testing the Pushbutton
When the pushbutton is not pressed, the LED will be off. If the wiring is correct, when
the pushbutton is pressed, the LED should be on (emitting light).
Warning Signs: If the “Pwr” LED on the Board of Education flickers, goes dim, or goes out
completely when you reconnect power, it may mean that there is a short circuit from Vdd to
Vss or from Vin to Vss. If this happens, disconnect power immediately and find and correct
the mistake in your circuit.
The LED built into the HomeWork Board is different. It may either be labeled “Power” or
“Running” and it only glows while a program is running. If a program ends, either because it
executes an END command or because it runs out of commands to execute, the LED will
turn off.
9 Verify that the LED in your test circuit is off.
Page 64 · What’s a Microcontroller?
9 Press and hold the pushbutton, and verify that the LED emits light while you are
holding the pushbutton down.
How the Pushbutton Circuit Works
The left side of Figure 3-4 shows what happens when the pushbutton is not pressed. The
LED circuit is not connected to Vdd. It is an open circuit that cannot conduct current.
By pressing the pushbutton, as shown on the right side of the figure, you close the
connection between the terminals with conductive metal. This makes a pathway for
electrons to flow through the circuit and so the LED emits light as a result.
Vdd
Vdd
1, 4
1, 4
2, 3
2, 3
No
Current
470 Ω
Figure 3-4
Pushbutton Not Pressed,
and Pressed
470 Ω
Current
LED
Vss
LED
Pushbutton not pressed:
circuit open and light off
(left)
Pushbutton pressed:
circuit closed and light on
(right)
Vss
Your Turn – Turn the LED off with a Pushbutton
Figure 3-5 shows a circuit that will cause the LED to behave differently. When the
button is not pressed, the LED stays on; when the button is pressed, the LED turns off.
Since this pushbutton connects a conductor across terminals 1,4 and 2,3 when pressed, it
means that electricity can take the path of least resistance through the pushbutton instead
of through the LED. Unlike the potential short circuits discussed in the Warning Signs
box, the short circuit the pressed pushbutton creates across the LED’s terminals does not
damage any circuits and serves a useful purpose.
9 Build the circuit shown in Figure 3-5.
9 Repeat the tests you performed on the first pushbutton circuit you built with this
new circuit.
Digital Input – Pushbuttons · Page 65
Vdd
Vdd
Vin
Vss
X3
1, 4
LED
2, 3
470 Ω
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
+
Figure 3-5
LED that Gets Shorted
by Pushbutton
Can you really do that with the LED? Up until now, the LED’s cathode has always been
connected to Vss. Now, the LED is in a different place in the circuit, with its anode
connected to Vdd. People often ask if this breaks any circuit rules, and the answer is no.
The electrical pressure supplied by Vdd and Vss is 5 volts. The red LED will always use
about 1.7 volts, and the resistor will use the remaining 3.3 volts, regardless of their order.
ACTIVITY #2: READING A PUSHBUTTON WITH THE BASIC STAMP
In this activity, you will connect a pushbutton circuit to the BASIC Stamp and display
whether or not the pushbutton is pressed. You will do this by writing a PBASIC program
that checks the state of the pushbutton and displays it in the Debug Terminal.
Parts for a Pushbutton Circuit
(1) Pushbutton – normally open
(1) Resistor – 220 Ω (red-red-brown)
(1) Resistor – 10 kΩ (brown-black-orange)
(2) Jumper wires
Page 66 · What’s a Microcontroller?
Building a Pushbutton Circuit for the BASIC Stamp
Figure 3-6 shows a pushbutton circuit that is connected to BASIC Stamp I/O pin P3.
9 Build the circuit shown in Figure 3-6.
Vdd
X3
Vdd
P3
220 Ω
10 kΩ
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vin
Vss
Figure 3-6
Pushbutton Circuit
Connected to I/O
Pin P3
On the wiring
diagram, the
220 Ω resistor is
on the left side
connecting the
pushbutton to P3
while the 10 kΩ
resistor is on the
right, connecting
the pushbutton
circuit to Vss.
Figure 3-7 shows what the BASIC Stamp sees when the button is pressed, and when it’s
not pressed. When the pushbutton is pressed, the BASIC Stamp senses that Vdd is
connected to P3. Inside the BASIC Stamp, this causes it to place the number 1 in a part
of its memory that stores information about its I/O pins. When the pushbutton is not
pressed, the BASIC Stamp cannot sense Vdd, but it can sense Vss through the 10 kΩ and
220 Ω resistors. This causes it to store the number 0 in that same memory location that
stored a 1 when the pushbutton was pressed.
Digital Input – Pushbuttons · Page 67
Vdd
220 Ω
10 kΩ
SOUT
1
SIN
2
BS2
24
VIN
23
VSS
ATN
3
22
RES
VSS
4
21
VDD (+5V)
P0
5
20
P15
P1
6
19
P14
P2
7
P3
8
P4
9
P5
1
18
P13
17
P12
16
P11
10
15
P10
P6
11
14
P9
P7
12
13
P8
0
BS2-IC
Vss
Vdd
220 Ω
10 kΩ
SOUT
1
SIN
2
ATN
3
VSS
4
21
VDD (+5V)
P0
5
20
P15
P1
6
19
P14
P2
7
P3
8
BS2
1
0
24
VIN
23
VSS
22
RES
18
P13
17
P12
P4
9
16
P11
P5
10
15
P10
P6
11
14
P9
P7
12
13
P8
Figure 3-7
BASIC Stamp Reading a
Pushbutton
When the pushbutton is
pressed, the BASIC Stamp
reads a 1 (above). When
the pushbutton is not
pressed, the BASIC Stamp
reads a 0 (below).
BS2-IC
Vss
Binary and Circuits: The base-2 number system uses only the digits 1 and 0 to make
numbers, and these binary values can be transmitted from one device to another. The
BASIC Stamp interprets Vdd (5 V) as binary-1 and Vss (0 V) as binary-0. Likewise, when
the BASIC Stamp sets an I/O pin to Vdd using HIGH, it sends a binary-1. When it sets an
I/O pin to Vss using LOW, it sends a binary-0. This is a very common way of communicating
binary numbers used by many computer chips and other devices.
Programming the BASIC Stamp to Monitor the Pushbutton
The BASIC Stamp stores the one or zero it senses at I/O pin P3 in a memory location
called IN3. Here is an example program that shows how this works:
Example Program: ReadPushbuttonState.bs2
This next program makes the BASIC Stamp check the pushbutton every ¼ second and
send the value of IN3 to the Debug Terminal.
Page 68 · What’s a Microcontroller?
Figure 3-8 shows the Debug Terminal while the program is running. When the
pushbutton is pressed, the Debug Terminal displays the number 1, and when the
pushbutton is not pressed, the Debug Terminal displays the number 0.
Figure 3-8
Debug Terminal Displaying
Pushbutton States
The Debug Terminal displays 1 when
the pushbutton is pressed and 0
when it is not pressed.
9 Enter the ReadPushbuttonState.bs2 program into the BASIC Stamp Editor.
9 Run the program.
9 Verify that the Debug Terminal displays the value 0 when the pushbutton is not
pressed.
9 Verify that the Debug Terminal displays the value 1 when the pushbutton is
pressed and held.
' What's a Microcontroller - ReadPushbuttonState.bs2
' Check and send pushbutton state to Debug Terminal every 1/4 second.
' {$STAMP BS2}
' {$PBASIC 2.5}
DO
DEBUG ? IN3
PAUSE 250
LOOP
How ReadPushbuttonState.bs2 Works
The DO...LOOP in the program repeats every ¼ second because of the command PAUSE
250. Each time through the DO...LOOP, the command DEBUG ? IN3 sends the value of
IN3 to the Debug Terminal. The value of IN3 is the state that I/O pin P3 senses at the
instant the DEBUG command is executed.
Digital Input – Pushbuttons · Page 69
Your Turn – A Pushbutton with a Pull-up Resistor
The circuit you just finished working with has a resistor connected to Vss. This resistor
is called a pull-down resistor because it pulls the voltage at P3 down to Vss (0 volts)
when the button is not pressed. Figure 3-9 shows a pushbutton circuit that uses a pull-up
resistor. This resistor pulls the voltage up to Vdd (5 volts) when the button is not pressed.
The rules are now reversed. When the button is not pressed, IN3 stores the number 1,
and when the button is pressed, IN3 stores the number 0.
The 220 Ω resistor is used in the pushbutton example circuits to protect the BASIC Stamp
I/O pin. Although it’s a good practice for prototyping, in most products this resistor is
replaced with a wire (since wires cost less than resistors).
9 Modify your circuit as shown in Figure 3-9.
9 Re-run ReadPushbuttonState.bs2.
9 Using the Debug Terminal, verify that IN3 is 1 when the button is not pressed
and 0 when the button is pressed.
Vdd
Vdd
10 kΩ
P3
220 Ω
Vss
Vin
Vss
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Figure 3-9
Modified Pushbutton
Circuit
Active-low vs. Active-high: The pushbutton circuit in Figure 3-9 is called active-low
because it sends the BASIC Stamp a low signal (Vss) when the button is active (pressed).
The pushbutton circuit in Figure 3-6 is called active-high because it sends a high signal
(Vdd) when the button is active (pressed).
Page 70 · What’s a Microcontroller?
ACTIVITY #3: PUSHBUTTON CONTROL OF AN LED CIRCUIT
Figure 3-10 shows a zoomed-in view of a pushbutton and LED used to adjust the settings
on a computer monitor. This is just one of many devices that have a pushbutton that you
can press to adjust the device and an LED to show you the device’s status.
Figure 3-10
Button and LED on
a Computer Monitor
The BASIC Stamp can be programmed to make decisions based on what it senses. For
example, it can be programmed to decide to flash the LED on/off ten times per second
when the button is pressed.
Pushbutton and LED Circuit Parts
(1) Pushbutton – normally open
(1) Resistor – 10 kΩ (brown-black-orange)
(1) LED – any color
(1) Resistor – 220 Ω (red-red-brown)
(1) Resistor – 470 Ω (yellow-violet-brown)
(2) Jumper wires
Digital Input – Pushbuttons · Page 71
Building the Pushbutton and LED Circuits
Figure 3-11 shows the pushbutton circuit used in the activity you just finished along with
the LED circuit from Chapter 2, Activity #2.
9 Build the circuit shown in Figure 3-11.
P14
470 Ω
LED
Vdd
Vin
X3
Vss
Vdd
P3
220 Ω
10 kΩ
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vss
+
Figure 3-11
Pushbutton and
LED Circuit
Programming Pushbutton Control
The BASIC Stamp can be programmed to make decisions using an IF...THEN...ELSE
statement. The example program you are about to run will flash the LED on and off
when the pushbutton is pressed.
Each time through the DO...LOOP, the
IF...THEN...ELSE statement checks the state of the pushbutton and decides whether or
not to flash the LED.
Example Program: PushbuttonControlledLed.bs2
9 Enter PushbuttonControlledLed.bs2 into the BASIC Stamp Editor and run it.
9 Verify that the LED flashes on and off while the pushbutton is pressed and held
down.
9 Verify that the LED does not flash when the pushbutton is not pressed down.
Page 72 · What’s a Microcontroller?
' What's a Microcontroller - PushbuttonControlledLed.bs2
' Check pushbutton state 10 times per second and blink LED when pressed.
' {$STAMP BS2}
' {$PBASIC 2.5}
DO
DEBUG ? IN3
IF (IN3 = 1) THEN
HIGH 14
PAUSE 50
LOW 14
PAUSE 50
ELSE
PAUSE 100
ENDIF
LOOP
How PushbuttonControlledLed.bs2 Works
This program is a modified version of ReadPushbuttonState.bs2 from the previous
activity. The DO...LOOP and DEBUG ? IN3 commands are the same. The PAUSE 250
was replaced with an IF...THEN...ELSE statement. When the condition after the IF is
true (IN3 = 1), the commands that come after the THEN statement are executed. They
will be executed until the ELSE statement is reached, at which point the program skips to
the ENDIF and moves on. When the condition after the IF is not true (IN3 = 0), the
commands after the ELSE statement are executed until the ENDIF is reached.
You can make a detailed list of what a program should do, to either help you plan the
program or to describe what it does. This kind of list is called pseudo code, and the
example below uses pseudo code to describe how PushbuttonControlledLed.bs2 works.
Digital Input – Pushbuttons · Page 73
•
•
Do the commands between here and the Loop statement over and over again
o Display the value of IN3 in the Debug Terminal
o If the value of IN3 is 1, Then
ƒ Turn the LED on
ƒ Wait for 1/20 of a second
ƒ Turn the LED off
ƒ Wait for 1/20 of a second
o Else, (if the value of IN3 is 0)
ƒ do nothing, but wait for the same amount of time it would have
taken to briefly flash the LED (1/10 of a second).
Loop
Your Turn – Faster/Slower
9 Save the example program under a different name.
9 Modify the program so that the LED flashes twice as fast when you press and
hold the pushbutton.
9 Modify the program so that the LED flashes half as fast when you press and hold
the pushbutton.
ACTIVITY #4: TWO PUSHBUTTONS CONTROLLING TWO LED CIRCUITS
Let’s add a second pushbutton to the project and see how it works. To make things a
little more interesting, let’s also add a second LED circuit and use the second pushbutton
to control it.
Pushbutton and LED Circuit Parts
(2) Pushbuttons – normally open
(2) Resistors – 10 kΩ (brown-black-orange)
(2) Resistors – 470 Ω (yellow-violet-brown)
(2) Resistors – 220 Ω (red-red-brown)
(2) LEDs – any color
(3) Jumper wires
Page 74 · What’s a Microcontroller?
Adding a Pushbutton and LED Circuit
Figure 3-12 shows a second LED and pushbutton circuit added to the circuit you tested in
the previous activity.
9 Build the circuit shown in Figure 3-12. If you need help building the circuit
shown in the schematic, use the wiring diagram in Figure 3-13 as a guide.
9 Modify ReadPushbuttonState.bs2 so that it reads IN4 instead of IN3, and use it
to test your second pushbutton circuit.
P15
470 Ω
P14
470 Ω
LED
Vss
LED
Vss
Vdd
Vdd
Figure 3-12
Schematic for Two
Pushbuttons and LEDs
P4
220 Ω
P3
220 Ω
10 kΩ
Vss
10 kΩ
Vss
Dots indicate connections: There are three places where lines intersect in Figure 3-12,
but only two of those intersections have dots. When two lines intersect with a dot, it means
they are electrically connected. For example, the 10 kΩ resistor on the lower-right side of
Figure 3-12 has one of its terminals connected to one of the P3 circuit’s pushbutton
terminals and to one of its 220 Ω resistor terminals. When one line crosses another, but
there is no dot, it means the two wires DO NOT electrically connect. For example, the line
that connects the P4 pushbutton to the 10 kΩ resistor does not connect to the P3
pushbutton circuit because there is no dot at that intersection.
Digital Input – Pushbuttons · Page 75
Vdd
X3
Vin
Vss
++
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Figure 3-13
Wiring Diagram for Two Pushbuttons
and LEDs
Programming Pushbutton Control
In the previous activity, you experimented with making decisions using an
IF...THEN...ELSE statement. There is also such a thing as an IF...ELSEIF...ELSE
statement. It works great for deciding which LED to flash on and off. The next example
program shows how it works.
Example Program: PushbuttonControlOfTwoLeds.bs2
9 Enter and run PushbuttonControlOfTwoLeds.bs2 in the BASIC Stamp Editor.
9 Verify that the LED in the circuit connected to P14 flashes on and off while the
pushbutton in the circuit connected to P3 is held down.
9 Also check to make sure the LED in the circuit connected to P15 flashes while
the pushbutton in the circuit connected to P4 is held down
' What's a Microcontroller - PushbuttonControlOfTwoLeds.bs2
' Blink P14 LED if P3 pushbutton is pressed, and blink P15 LED if
' P4 pushbutton is pressed.
' {$STAMP BS2}
' {$PBASIC 2.5}
PAUSE 1000
Page 76 · What’s a Microcontroller?
DO
DEBUG HOME
DEBUG ? IN4
DEBUG ? IN3
IF (IN3 = 1) THEN
HIGH 14
PAUSE 50
ELSEIF (IN4 = 1) THEN
HIGH 15
PAUSE 50
ELSE
PAUSE 50
ENDIF
LOW 14
LOW 15
PAUSE 50
LOOP
How PushbuttonControlOfTwoLeds.bs2 Works
If the display of IN3 and IN4 scrolled down the Debug Terminal as they did in the
previous example, it would be difficult to read. One way to fix this is to always send the
cursor to the top-left position in the Debug Terminal using the HOME control character:
DEBUG HOME
By sending the cursor to the home position each time through the DO...LOOP, the
commands:
DEBUG ? IN4
DEBUG ? IN3
...display the values of IN4 and IN3 in the same part of the Debug Terminal each time.
The DO keyword begins the loop in this program:
DO
These commands in the IF statement are the same as the ones in the example program
from the previous activity:
Digital Input – Pushbuttons · Page 77
IF (IN3 = 1) THEN
HIGH 14
PAUSE 50
This is where the ELSEIF keyword helps. If IN3 is not 1, but IN4 is 1, we want to turn
the LED connected to P15 on instead of the one connected to P14.
ELSEIF (IN4 = 1) THEN
HIGH 15
PAUSE 50
If neither statement is true, we still want to pause for 50 ms without changing the state of
any LED circuits.
ELSE
PAUSE 50
When you’re finished with all the decisions, don’t forget the ENDIF.
ENDIF
It’s time to turn the LEDs off and pause again. You could try to decide which LED you
turned on and turn it back off. PBASIC commands execute pretty quickly, so why not
just turn them both off and forget about more decision making?
LOW 14
LOW 15
PAUSE 50
The LOOP statement sends the program back up to the DO statement, and the process of
checking the pushbuttons and changing the states of the LEDs starts all over again.
LOOP
Your Turn – What about Pressing Both Pushbuttons?
The example program has a flaw. Try pressing both pushbuttons at once, and you’ll see
the flaw. You would expect both LEDs to flash on and off, but they don’t because only
one code block in an IF...ELSEIF...ELSE statement gets executed before it skips to the
ENDIF. Here is how you can fix this problem:
Page 78 · What’s a Microcontroller?
9 Save PushbuttonControlOfTwoLeds.bs2 under a new name.
9 Replace this IF statement and code block:
IF (IN3 = 1) THEN
HIGH 14
PAUSE 50
...with this IF...ELSEIF statement:
IF (IN3 = 1) AND (IN4 = 1) THEN
HIGH 14
HIGH 15
PAUSE 50
ELSEIF (IN3 = 1) THEN
HIGH 14
PAUSE 50
A code block is a group of commands. The IF statement above has a code block with
three commands (HIGH, HIGH, and PAUSE). The ELSEIF statement has a code block with
two commands (HIGH, PAUSE).
9 Run your modified program and see if it handles both pushbutton and LED
circuits as you would expect.
The AND keyword can be used in an IF...THEN statement to check if more than one
condition is true. All conditions with AND have to be true for the IF statement to be true.
The OR keyword can also be used to check if at least one of the conditions are true.
You can also modify the program so that the LED that’s flashing stays on for different
amounts of time. For example, you can reduce the Duration of the PAUSE for both
pushbuttons to 10, increase the PAUSE for the P14 LED to 100, and increase the PAUSE
for the P15 LED to 200.
9 Modify the PAUSE commands in the IF and the two ELSEIF statements as
discussed.
9 Run the modified program.
9 Observe the difference in the behavior of each light.
Digital Input – Pushbuttons · Page 79
ACTIVITY #5: REACTION TIMER TEST
You are the embedded systems engineer at a video game company. The marketing
department recommends that a circuit to test the player’s reaction time be added to the
next hand-held game controller. Your next task is to develop a proof of concept for the
reaction timer test.
The solution you will build and test in this activity is an example of how to solve this
problem, but it’s definitely not the only solution. Before continuing, take a moment to
think about how you would design this reaction timer.
Reaction Timer Game Parts
(1) LED – bicolor
(1) Resistor – 470 Ω (yellow-violet-brown)
(1) Pushbutton – normally open
(1) Resistor – 10 kΩ (brown-black-orange)
(1) Resistor – 220 Ω (red-red-brown)
(2) Jumper wires
Building the Reaction Timer Circuit
Figure 3-14 shows a schematic and wiring diagram for a circuit that can be used with the
BASIC Stamp to make a reaction timer game.
9 Build the circuit shown in Figure 3-14 on page 80.
9 Run TestBiColorLED.bs2 from Chapter 2, Activity #5 to test the bicolor LED
circuit and make sure your wiring is correct.
9 If you just re-built the pushbutton circuit for this activity, run
ReadPushbuttonState.bs2 from Activity #2 in this chapter to make sure your
pushbutton is working properly.
Page 80 · What’s a Microcontroller?
P15
1
1
2
Vdd
P14
470 Ω
Vdd
P3
220 Ω
Vin
Vss
X3
2
10 kΩ
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Figure 3-14
Reaction
Timer Game
Circuit
Vss
Programming the Reaction Timer
This next example program will leave the bicolor LED off until the game player presses
and holds the pushbutton. When the pushbutton is held down, the LED will turn red for a
short period of time. When it turns green, the player has to let go of the pushbutton as
fast as he or she can. The program then measures time it takes the player to release the
pushbutton in reaction to the light turning green.
The example program also demonstrates how polling and counting work. Polling is the
process of checking something over and over again very quickly to see if it has changed.
Counting is the process of adding a number to a variable each time something does (or
does not) happen. In this program, the BASIC Stamp will poll from the time the bicolor
LED turns green until the pushbutton is released. It will wait 1/1000 of a second by using
the command PAUSE 1. Each time it polls and the pushbutton is not yet released, it will
add 1 to the counting variable named timeCounter. When the pushbutton is released,
the program stops polling and sends a message to the Debug Terminal that displays the
value of the timeCounter variable.
Example Program: ReactionTimer.bs2
9 Enter and run ReactionTimer.bs2.
9 Follow the prompts on the Debug Terminal (see Figure 3-15).
Digital Input – Pushbuttons · Page 81
Figure 3-15
Reaction Timer Game Instructions in
the Debug Terminal
' What's a Microcontroller - ReactionTimer.bs2
' Test reaction time with a pushbutton and a bicolor LED.
' {$STAMP BS2}
' {$PBASIC 2.5}
PAUSE 1000
timeCounter
' Wait 1 sec before 1st message.
VAR
Word
' Declare variable to store time.
DEBUG "Press and hold pushbutton", CR,
"to make light turn red.", CR, CR,
"When light turns green, let", CR,
"go as fast as you can.", CR, CR
' Display reaction instructions.
DO
' Begin main loop.
DO
LOOP UNTIL IN3 = 1
' Nested loop repeats...
' until pushbutton press.
HIGH 14
LOW 15
' Bicolor LED red.
PAUSE 1000
' Delay 1 second.
LOW 14
HIGH 15
' Bicolor LED green.
timeCounter = 0
' Set timeCounter to zero.
Page 82 · What’s a Microcontroller?
DO
' Nested loop, count time...
PAUSE 1
timeCounter = timeCounter + 1
LOOP UNTIL IN3 = 0
' until pushbutton is released.
LOW 15
' Bicolor LED off.
DEBUG "Your time was ", DEC timeCounter,
" ms.", CR, CR,
"To play again, hold the ", CR,
"button down again.", CR, CR
' Display time measurement.
LOOP
' Play again instructions.
' Back to "Begin main loop".
How ReactionTimer.bs2 Works
Since the program will have to keep track of the number of times the pushbutton was
polled, a variable called timeCounter is declared.
timeCounter VAR Word
' Declare variable to store time.
Variables initialize to zero: When a variable is declared in PBASIC, its value is
automatically zero until a command sets it to a new value.
The DEBUG commands contain instructions for the player of the game.
DEBUG "Press and hold pushbutton", CR,
"to make light turn red.", CR, CR,
"When light turns green, let", CR,
"go as fast as you can.", CR, CR
DO...LOOP statements can be nested. In other words, you can put one DO...LOOP inside
another.
DO
' Begin main loop.
DO
' Nested loop repeats...
LOOP UNTIL IN3 = 1
' until pushbutton press.
' Rest of program was here.
LOOP
' Back to "Begin main loop".
Digital Input – Pushbuttons · Page 83
The inner DO...LOOP deserves a closer look. A DO...LOOP can use a condition to decide
whether or not to break out of the loop and move on to more commands that come
afterwards. This DO...LOOP will repeat itself as long as the button is not pressed
(IN3 = 0). The DO...LOOP will execute over and over again, until IN3 = 1. Then, the
program moves on to the next command after the LOOP UNTIL statement. This is an
example of polling. The DO...LOOP UNTIL polls until the pushbutton is pressed.
DO
LOOP UNTIL IN3 = 1
' Nested loop repeats...
' until pushbutton press.
The commands that come immediately after the LOOP UNTIL statement turn the bicolor
LED red, delay for one second, then turn it green.
HIGH 14
LOW 15
' Bicolor LED red.
PAUSE 1000
' Delay 1 second.
LOW 14
HIGH 15
' Bicolor LED green.
As soon as the bicolor LED turns green, it’s time to start counting to track how long until
the player releases the button. The timeCounter variable is set to zero, then another
DO...LOOP with an UNTIL condition starts repeating itself. It repeats itself until the
player releases the button (IN3 = 0). Each time through the loop, the BASIC Stamp
delays for 1 ms using PAUSE 1, and it also adds 1 to the value of the timeCounter
variable.
timeCounter = 0
' Set timeCounter to zero.
DO
' Nested loop, count time...
PAUSE 1
timeCounter = timeCounter + 1
LOOP UNTIL IN3 = 0
' until pushbutton is released.
After the pushbutton is released, the bicolor LED is turned off.
LOW 15
The results are displayed in the Debug Terminal.
Page 84 · What’s a Microcontroller?
DEBUG "Your time was ", DEC timeCounter,
" ms.", CR, CR,
"To play again, hold the ", CR,
"button down again.", CR, CR
The last statement in the program is LOOP, which sends the program back to the very first
DO statement.
Your Turn – Revising the Design (Advanced Topics)
The marketing department gave your prototype to some game testers. When the game
testers were done, the marketing department came back to you with an itemized list of
three problems that have to be fixed before your prototype can be built into the game
controller.
9 Save ReactionTimer.bs2 under a new name (like ReactionTimerYourTurn.bs2).
The “itemized list” of problems and their solutions are discussed below.
Item 1: When a player holds the button for 30 seconds, his score is actually around
14,000 ms, a measurement of 14 seconds. This has to be fixed!
It turns out that executing the loop itself along with adding one to the timeCounter
variable takes about 1 ms without the PAUSE 1 command. This is called code overhead,
and it’s the amount of time it takes for the BASIC Stamp to execute the commands. A
quick fix that will improve the accuracy is to simply comment out the PAUSE 1 command
by adding an apostrophe to the left of it.
' PAUSE 1
9 Try commenting PAUSE 1 and test to see how accurate the program is.
Instead of commenting the delay, another way you can fix the program is to multiply
your result by two. For example, just before the DEBUG command that displays the
number of ms, you can insert a command that multiplies the result by two:
timeCounter = timeCounter * 2
' <- Add this
DEBUG "Your time was ", DEC timeCounter, " ms.", CR, CR
Digital Input – Pushbuttons · Page 85
9 Uncomment the PAUSE command by deleting the apostrophe, and try the
multiply-by-two solution instead.
For precision, you can use the */ operator to multiply by a value with a fraction. The */
operator is not hard to use; here’s how:
1)
Place the value or variable you want to multiply by a fractional value before the
*/ operator.
2)
Take the fractional value that you want to use and multiply it by 256.
3)
Round off to get rid of anything to the right of the decimal point.
4)
Place that value after the */ operator.
Example: Let’s say you want to multiply the timeCounter variable by 3.69.
1)
Start by placing timeCounter to the left of the */ operator:
timeCounter = timeCounter */
2)
Multiply your fractional value by 256: 3.69 x 256 = 944.64.
3)
Round off: 944.64 ≈ 945.
4)
Place that value to the right of the */ operator:
timeCounter = timeCounter */ 945
' multiply by 3.69
Multiplying by 2 will scale a result of 14,000 to 28,000, which isn’t quite 30,000.
30,000 ÷ 14,000 ≈ 2.14. To multiply by 2.14 with the */ operator for increased precision,
we need to figure out how many 256ths are in 2.14. So, 2.14 × 256 = 547.84 ≈ 548. You
can use this value and the */ operator to replace timecounter = timeCounter * 2.
9 Replace
timecounter = timeCounter *
timeCounter */ 548 and retest your program.
2 with timecounter
=
Your 30-second test with the original, unmodified program may yield a value that’s
slightly different from 14,000. If so, you can use the same procedure with your test
results to calculate a value for the */ operator to make your results even more precise.
9 Try it!
Page 86 · What’s a Microcontroller?
Item 2: Players soon figure out that the delay from red to green is 1 second. After
playing it several times, they get better at predicting when to let go, and their score
no longer reflects their true reaction time.
The BASIC Stamp has a RANDOM command. Here is how to modify your code for a
random number:
9 At the beginning of your code, add a declaration for a new variable called value,
and set it to 23. The value 23 is called the seed because it starts the pseudo
random number sequence.
timeCounter VAR Word
value VAR Byte
value = 23
' <- Add this
' <- Add this
9 Just before the PAUSE 1000 command inside the DO...LOOP, use the RANDOM
command to give value a new “random” value from the pseudo random
sequence that started with 23.
RANDOM value
DEBUG "Delay time ", ? 1000 + value, CR
' <- Add this
' <- Add this
9 Modify that PAUSE 1000 command so that the “random” value is added to its
Duration argument.
PAUSE 1000 + value
' <- Modify this
LOW 14
HIGH 15
9 Since the largest value a byte can store is 255, the PAUSE command only varies
by ¼ second. You can multiply the value variable by 4 to make the red light
delay vary from 1 to just over 2 seconds.
DEBUG "Delay time ", ? 1000 + (value*4), CR ' <- Modify
PAUSE 1000 + (value * 4)
' <- Modify this again
Digital Input – Pushbuttons · Page 87
What’s an algorithm? An algorithm is a sequence of mathematical operations.
What’s pseudo random? Pseudo random means that it seems random, but it isn’t really.
Each time you start the program over again, you will get the same sequence of values.
What’s a seed? A seed is a value that is used to start the pseudo random sequence. If you
use a different value for the seed (change value from 23 to some other number), it will
result in a different pseudo random sequence.
Item 3: A player that lets go of the button before the light turns green gets an
unreasonably good score (1 ms). Your microcontroller needs to figure out if a
player is cheating.
Pseudo code was introduced near the end of Activity #3 in this chapter. Here is some
pseudo code to help you apply an IF...THEN...ELSE statement to solve the problem.
Assuming you have made the other changes in items 1 and 2, timeCounter will now be
2 instead of 1 if the player releases the button before the light turns green. The changes
below will work if timeCounter is either 1 or 2.
•
•
•
•
If the value of timeCounter is less than or equal to 2 (timeCounter <= 2 )
o Display a message telling the player he or she has to wait until after the
light turns green to let go of the button.
Else, (if the value of timeCounter is greater than 1)
o Display the value of timeCounter (just like in ReactionTimer.bs2) time
in ms.
End If
Display a “To play again...” message.
9 Modify your program by implementing this pseudo code in PBASIC to fix the
cheating player problem.
SUMMARY
This chapter introduced the pushbutton and some common pushbutton circuits. This
chapter also introduced how to build and test a pushbutton circuit and how to use the
BASIC Stamp to read the state of one or more pushbuttons. The BASIC Stamp was
programmed to make decisions based on the state(s) of the pushbutton(s) and this
information was used to control LED(s). A reaction timer game was built using these
concepts. In addition to controlling LEDs, the BASIC Stamp was programmed to poll a
pushbutton and take time measurements.
Page 88 · What’s a Microcontroller?
Several programming concepts were introduced, including counting, pseudo code for
planning program flow, code overhead in timing-sensitive applications, and seed values
for pseudo random events.
Reading individual pushbutton circuits using the special I/O variables built into the
BASIC Stamp (IN3, IN4, etc.) was introduced. Making decisions based on these values
using IF...THEN...ELSE statements, IF...ELSEIF...ELSE statements, and code
blocks were also introduced. For evaluating more than one condition, the AND and OR
operators were introduced. Adding a condition to a DO...LOOP using the UNTIL keyword
was introduced along with nesting DO...LOOP code blocks. The RANDOM command was
introduced to add an element of unpredictability to an application, the Reaction Timer
game.
Questions
1. What is the difference between sending and receiving HIGH and LOW signals
using the BASIC Stamp?
2. What does “normally open” mean in regards to a pushbutton?
3. What happens between the terminals of a normally open pushbutton when you
press it?
4. What is the value of IN3 when a pushbutton connects it to Vdd? What is the
value of IN3 when a pushbutton connects it to Vss?
5. What does the command DEBUG ? IN3 do?
6. What kind of code blocks can be used for making decisions based on the value
of one or more pushbuttons?
7. What does the HOME control character do in the statement DEBUG HOME?
Exercises
1. Explain how to modify ReadPushbuttonState.bs2 on page 68 so that it reads the
pushbutton every second instead of every ¼ second.
2. Explain how to modify ReadPushbuttonState.bs2 so that it reads a normally open
pushbutton circuit with a pull-up resistor connected to I/O pin P6.
Project
1. Modify ReactionTimer.bs2 so that it is a two-player game. Add a second button
wired to P4 for the second player.
Digital Input – Pushbuttons · Page 89
Solutions
Q1. Sending uses the BASIC Stamp I/O pin as an output, whereas receiving uses the
I/O pin as an input.
Q2. Normally open means the pushbutton's normal state (not pressed) forms an open
circuit.
Q3. When pressed, the gap between the terminals is bridged by a conductive metal.
Current can then flow through the pushbutton.
Q4. IN3 = 1 when pushbutton connects it to Vdd. IN3 = 0 when pushbutton
connects it to Vss.
Q5. DEBUG ? IN3 displays the text “IN3 = ” followed by the value stored in IN3
(either a 0 or 1 depending on the state of I/O pin P3), followed by a carriage
return.
Q6. IF...THEN...ELSE and IF...ELSEIF...ELSE.
Q7. The HOME control character sends the cursor to the top left position in the Debug
Terminal.
E1. The DO...LOOP in the program repeats every ¼ second because of the command
PAUSE 250. To repeat every second, change the PAUSE 250 (250 ms = 0.25 s
= ¼ s), to PAUSE 1000 (1000ms = 1 s).
DO
DEBUG ? IN3
PAUSE 1000
LOOP
E2. Replace IN3 with IN6, to read I/O pin P6. The program only displays the
pushbutton state, and does not use the value to make decisions; it does not matter
whether the resistor is a pull-up or a pull-down. The DEBUG command will
display the button state either way.
DO
DEBUG ? IN6
PAUSE 250
LOOP
Page 90 · What’s a Microcontroller?
P1. First, a button was added for the second player, wired to BASIC Stamp I/O
pin P4. The schematic is based on Figure 3-14 on page 80.
Vdd
P15
Vdd
1
P4
P3
220 Ω
220 Ω
10 kΩ
2
P14
10 kΩ
470 Ω
Vss
Vss
Snippets from the solution program are included below, but keep in mind
solutions may be coded a variety of different ways. However, most solutions
will include the following modifications:
Use two variables to keep track of two player's times:
timeCounterA VAR
timeCounterB VAR
Word
Word
' Time score of player A
' Time score of player B
Change instructions to reflect two pushbuttons:
DEBUG "Press and hold pushbuttons", CR,
DEBUG "buttons down again.", CR, CR
Wait for both buttons to be pressed before turning LED red, by using the AND
operator:
LOOP UNTIL (IN3 = 1) AND (IN4 = 1)
Wait for both buttons to be released to end timing, again using the AND operator:
LOOP UNTIL (IN3 = 0) AND (IN4 = 0)
Add logic to decide which player's time is incremented:
IF (IN3 = 1) THEN
timeCounterA = timeCounterA + 1
ENDIF
IF (IN4 = 1) THEN
timeCounterB = timeCounterB + 1
ENDIF
Digital Input – Pushbuttons · Page 91
Change time display to show times of both players:
DEBUG "Player A Time:
DEBUG "Player B Time:
", DEC timeCounterA, " ms. ", CR
", DEC timeCounterB, " ms. ", CR, CR
Add logic to show which player had the faster reaction time:
IF (timeCounterA < timeCounterB) THEN
DEBUG "Player A is the winner!", CR
ELSEIF (timeCounterB < timeCounterA) THEN
DEBUG "Player B is the winner!", CR
ELSE
DEBUG "It's a tie!", CR
ENDIF
The complete solution is shown below.
'
'
'
'
'
'
What's a Microcontroller - Ch03Prj01_TwoPlayerReactionTimer.bs2
Test reaction time with a pushbutton and a bicolor LED.
Add a second player with a second pushbutton. Both players
play at once using the same LED. Quickest to release wins.
Pin P3: Player A Pushbutton, Active High
Pin P4: Player B Pushbutton, Active High
' {$STAMP BS2}
' {$PBASIC 2.5}
timeCounterA VAR
timeCounterB VAR
PAUSE 1000
Word
Word
DEBUG "Press and hold pushbuttons", CR,
"to make light turn red.", CR, CR,
"When light turns green, let", CR,
"go as fast as you can.", CR, CR
DO
' Time score of player A
' Time score of player B
' 1 s before 1st message
' Display reaction
' instructions.
' Begin main loop.
DO
' Nothing
LOOP UNTIL (IN3 = 1) AND (IN4 = 1)
' Loop until both press
HIGH 14
LOW 15
' Bicolor LED red.
PAUSE 1000
' Delay 1 second.
LOW 14
HIGH 15
' Bicolor LED green.
Page 92 · What’s a Microcontroller?
timeCounterA = 0
timeCounterB = 0
' Set timeCounters to zero
DO
PAUSE 1
IF (IN3 = 1) THEN
timeCounterA = timeCounterA + 1
ENDIF
IF (IN4 = 1) THEN
timeCounterB = timeCounterB + 1
ENDIF
' If button is still down,
' increment counter
LOOP UNTIL (IN3 = 0) AND (IN4 = 0)
' Loop until both buttons
' released.
LOW 15
' Bicolor LED off.
DEBUG "Player A Time: ", DEC timeCounterA, " ms. ", CR
DEBUG "Player B Time: ", DEC timeCounterB, " ms. ", CR, CR
IF (timeCounterA < timeCounterB) THEN
DEBUG "Player A is the winner!", CR
ELSEIF (timeCounterB < timeCounterA) THEN
DEBUG "Player B is the winner!", CR
ELSE
' A & B times are equal
DEBUG "It's a tie!", CR
ENDIF
DEBUG CR
DEBUG "To play again, hold the ", CR
DEBUG "buttons down again.", CR, CR
LOOP
' Play again instructions.
' Back to Begin main loop.
Controlling Motion · Page 93
Chapter 4: Controlling Motion
MICROCONTROLLED MOTION
Microcontrollers make sure things move to the right place all around you every day. If
you have an inkjet printer, the print head that goes back and forth across the page as it
prints is moved by a stepper motor that is controlled by a microcontroller. The automatic
grocery store doors that you walk through are controlled by microcontrollers, and the
automatic eject feature in your DVD player is also controlled by a microcontroller.
ON/OFF SIGNALS AND MOTOR MOTION
Just about all microcontrolled motors receive sequences of high and low signals similar
to the ones you’ve been sending to LEDs. The difference is that the microcontroller has
to send these signals at rates that are usually much faster than the blinking LED examples
from Chapter 2. If you were to use an LED circuit to monitor control signals, some
would make the LED flicker on/off so rapidly that the human eye could not detect the
switching. The LED would only appear to glow faintly. Others would appear as a rapid
flicker, and others would be more easily discernible.
Some motors require lots of circuitry to help the microcontroller make them work. Other
motors require extra mechanical parts to make them work right in machines. Of all the
different types of motors to start with, the hobby servo that you will experiment with in
this chapter is probably the simplest. As you will soon see, it is easy to control with the
BASIC Stamp, requires little or no additional circuitry, and has a mechanical output that
is easy to connect to things to make them move.
INTRODUCING THE SERVO
A hobby servo is a device that controls position, and you can find them in just about any
radio controlled (RC) car, boat or plane. In RC cars, the servo holds the steering to
control how sharply the car turns. In an RC boat, it holds the rudder in position for turns.
RC planes typically have several servos that position the different flaps to control the
plane’s motion. In RC vehicles with gas powered engines, another servo moves the
engine’s throttle lever to control how fast the engine runs. An example of an RC airplane
and its radio controller are shown in Figure 4-1. The hobbyist “flies” the airplane by
manipulating thumb joysticks on the radio controller, which causes the servos on the
plane to control the positions of the RC plane’s elevator flaps and rudder.
Page 94 · What’s a Microcontroller?
Figure 4-1
Model Airplane and
Radio Controller
So, how does holding the radio controller’s joystick in a certain position cause a flap on
the RC plane to hold a certain position? The radio controller converts the position of the
joysticks into pulses of radio activity that last certain amounts of time. The time each
pulse lasts indicates the position of one of the joysticks. On the RC plane, a radio
receiver converts these radio activity pulses to digital pulses (high/low signals) and sends
them to the plane’s servos. Each servo has circuitry inside it that converts these digital
pulses to a position that the servo maintains. The amount of time each pulse lasts is what
tells the servo what position to maintain. These control pulses only last a few
thousandths of a second, and repeat around 40 to 50 times per second to make the servo
maintain the position it holds.
Figure 4-2 shows a drawing of a Parallax Standard Servo. The plug (1) is used to connect
the servo to a power source (Vdd and Vss) and a signal source (a BASIC Stamp I/O pin).
The cable (2) has three wires, and it conducts Vdd, Vss and the signal line from the plug
into the servo. The horn (3) is the part of the servo that looks like a four-pointed star.
When the servo is running, the horn is the moving part that the servo holds in different
positions. The Phillips screw (4) holds the horn to the servo’s output shaft. The case (5)
contains the servo’s position sensing and control circuits, a DC motor, and gears. These
parts work together to take high/low signals from the BASIC Stamp and translate them
into positions held by the servo horn.
Controlling Motion · Page 95
2
Figure 4-2
The Parallax Standard
Servo
3
1
(1) Plug
(2) Cable
(3) Horn
(4) Screw that attaches
the horn to the servo’s
output shaft
(5) Case
4
5
In this chapter, you will program the BASIC Stamp to send signals to a servo that control
the servo horn’s position. By making the BASIC Stamp send signals that tell the servo to
hold different positions, your programs can also orchestrate the servo’s motion. Your
programs can even monitor pushbuttons and use information about whether the buttons
are pressed to adjust the position a servo holds (pushbutton servo position control). The
BASIC Stamp can also be programmed to receive messages that you type into the Debug
Terminal, and use those messages to control the servo’s position (terminal servo position
control).
ACTIVITY #1: CONNECTING AND TESTING THE SERVO
In this activity, you will follow instructions for connecting a servo to your particular
board’s power supply and BASIC Stamp.
Page 96 · What’s a Microcontroller?
Servo and LED Circuit Parts
(1) Parallax Standard Servo
(1) Resistor – 470 Ω (yellow-violet-brown)
(1) LED – any color
The LED circuit will be used to monitor the control signal the BASIC Stamp sends to the
servo. Keep in mind that the LED circuit is not required to help the servo operate. It is
just there to help “see” the control signals.
CAUTION: use only a Parallax Standard Servo for the activities in this text! Other
servos may be designed to different specifications that might not be compatible with these
activities.
Building the Servo and LED Circuits
In Chapter 1, you identified your board and revision using the BASIC Stamp Editor Help.
You will need to know which board and revision you have here so that you can find the
servo circuit building instructions for your board.
9 If you do not already know which board and revision you have, open the BASIC
Stamp Editor Help and click on the Getting Started with Stamps in Class link on
the home page. Then, follow the directions to determine which board you have.
9 If you have a Board of Education USB (any Rev) or Serial (Rev C or newer), go
to the Board of Education Servo Circuit section below.
9 If you have a BASIC Stamp HomeWork Board (Rev C or newer), go the BASIC
Stamp HomeWork Board Servo Circuit section on page 99.
9 If your board is not listed above, go to www.parallax.com/Go/WAM → Servo
Circuit Connections to find circuit instructions for your board. When you are
done with the servo circuit instructions for your board, go on to Activity #2:
Servo Control Test Program on page 102.
Board of Education Servo Circuit
These instructions are for all USB Board of Education Revisions as well as for the Serial
Board of Education Rev C or newer.
9 Turn off the power as shown in Figure 4-3.
Controlling Motion · Page 97
Figure 4-3
Disconnect Power
Reset
0
1
2
Set 3-position switch to 0
Figure 4-4 shows the servo header on the Board of Education. This is where you will
plug in your servo. This board features a jumper that you can use to connect the servo’s
power supply to either Vin or Vdd. The jumper is the removable black rectangular piece
indicated by the arrow between the two servo headers.
9 Verify that the jumper is set to Vdd as shown in Figure 4-4. If it is instead set to
Vin, lift the rectangular jumper up off of the pins it is currently on, and then
press it on the two pins closest to the Vdd label.
15 14 Vdd 13 12
Red
Black
X4
Figure 4-4
Servo Header Jumper Set to Vdd
X5
Vin
The jumper allows you to choose the power supply (Vin or Vdd) for the Parallax
Standard Servo.
ƒ
If you are using a 9 V battery, set it to Vdd. DO NOT USE Wall-mount 9 V Battery
“replacers.”
ƒ
If you are using a 4 AA cell, 6 V battery pack, either setting will work.
ƒ
If you are using a wall-mount DC power supply, use only Vdd. Before connecting
a wall-mount DC supply to the Board of Education, make sure to check the
specifications for acceptable DC supplies listed in the BASIC Stamp Editor Help.
Figure 4-5 shows the schematic of the circuit you will build on your Board of Education.
9 Build the circuit shown in Figure 4-5 and Figure 4-6.
9 Make sure you did not plug the servo in upside-down. The white, red and black
wires should line up as shown in Figure 4-6.
Page 98 · What’s a Microcontroller?
P14
470 Ω
LED
Figure 4-5
Servo and LED
Indicator Schematic
for Board of
Education
Vss
Vdd
White
P14
Red
Servo
Black
For Serial Board of
Education Rev C or
newer, or any USB
Board of Education
Vss
15 14 Vdd 13 12
White
Red
Black
Red
Black
X4
Vdd
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
X5
Vin
Vss
+
Figure 4-6
Servo and LED
Indicator on Board
of Education
standard servo
www.parallax.com
Controlling Motion · Page 99
Up until now, you have been using the 3-position switch in position 1. Now, you will
move it to position 2 to turn on the power to the servo header.
9 Supply power to the servo header by adjusting the 3-position switch as shown in
Figure 4-7. Your servo may move a bit when you connect the power.
Figure 4-7
Power turned on to Board of
Education and Servo Header
Reset
0
1
2
If you see instructions in this chapter that read “Connect power to your board” move the
3-position switch to position-2. Likewise, if you see instructions in this chapter that read
“Disconnect power from your board” move the 3-position switch to position-0.
9 Disconnect power from your board.
9 Go on to Activity #2 on page 102.
BASIC Stamp HomeWork Board Servo Circuit
If you are connecting your servo to a BASIC Stamp HomeWork Board (Rev C or newer),
you will need these extra parts from your kit:
(1) 3-pin male/male header (shown in Figure 4-8).
(4) Jumper wires
Figure 4-8
Extra Part for BASIC Stamp
HomeWork Board Servo Circuit
3-pin male/male header
Figure 4-9 shows the schematic of the servo and LED indicator circuits on the BASIC
Stamp HomeWork Board. The instructions that come after this figure will show you how
to safely build this circuit.
Page 100 · What’s a Microcontroller?
9 Disconnect your 9 V battery from your HomeWork Board.
9 Build the LED indicator and servo header circuit shown by the schematic in
Figure 4-9 and wiring diagram in Figure 4-10.
P14
470 Ω
LED
Vss
Figure 4-9
Schematic for Servo and
LED Indicator on
HomeWork Board
Vdd
P14
White
Red
Servo
Black
Vss
Figure 4-10
LED Indicator and Servo
Header Circuits on
HomeWork Board
Controlling Motion · Page 101
9 Connect the servo to the servo header as shown in Figure 4-11.
9 Make sure that the colors on the servo’s cable align properly with the colors
labeled in the picture.
9 Double-check your wiring.
WARNING
Use only a 9 V battery when your Parallax Standard Servo is connected to the BASIC
Stamp HomeWork Board. Do not use any kind of DC supply or “battery replacer” that
plugs into an AC outlet.
For best results, make sure your battery is new. If you are using a rechargeable battery,
make sure it is freshly recharged. It should also be rated for 100 mAh (milliamp hours) or
more.
9 Reconnect your 9 V battery to your HomeWork Board. The servo may twitch
slightly when you make the connection.
Figure 4-11
Servo Connected to
HomeWork Board
Page 102 · What’s a Microcontroller?
ACTIVITY #2: SERVO CONTROL TEST PROGRAM
A degree is an angle measurement denoted by the ° symbol. Example degree angle
measurements are shown in Figure 4-12, including 30°, 45°, 90°, 135°, and 180°. Each
degree of angle measurement represents 1/360th of a circle, so the 90° measurement is ¼
of a circle since 90 ÷ 360 = ¼. Likewise, 180° is ½ of a circle since 180 ÷ 360 = ½, and
you can calculate similar fractions for the other degree measurements in the figure.
Figure 4-12
Examples of
Degree Angle
Measurements
The Parallax Standard Servo can make its horn hold positions anywhere within a 180°
range, so degree measurements can be useful for describing the positions the servo holds.
Figure 4-13 shows examples of a servo with a loop of wire that has been threaded
through two of the holes in its horn and then twist-tied. The direction the twist tie points
indicates the angle of the servo’s horn, and the figure shows examples of 0°, 45°, 90°,
135°, and 180°.
Figure 4-13: Servo Horn Position Examples
Your servo horn’s range of motion and mechanical limits will probably be different from what’s
shown here. Instructions on how to adjust it to match this figure come after the first example
program.
Factory servo horn mounting is random, so your servo horn positions will probably be
different from the ones in Figure 4-13. In fact, compared to Figure 4-13, your servo’s
horn could be mounted anywhere in a +/- 45° range. The servo in Figure 4-14 shows an
example of a servo whose horn was mounted 20° clockwise from the one in Figure 4-13.
After you find the center of the servo horn’s range of motion, you can either use it as a
90° reference or mechanically adjust the servo’s horn so that it matches Figure 4-13 by
following instructions later in this activity.
Controlling Motion · Page 103
Figure 4-14: Servo Horn Position Examples before Mechanical Adjustment
This is an example of a horn that’s mounted on the servo’s output shaft about 20°
counterclockwise of how it was set in Figure 4-13.
You can find the center of the servo’s range of motion by gently rotating the horn to find
its clockwise and counterclockwise mechanical limits. The half way position between
these two limits is the center or 90° position. The servo’s center position could fall
anywhere in the region shown in Figure 4-15.
The center of your servo horn’s range of motion should
fall somewhere in this region
Figure 4-15
Range of Possible
Center Positions
Page 104 · What’s a Microcontroller?
In these next steps, twist the servo horn slowly and do not force it! The servo has
built-in mechanical limits to prevent the horn from rotating outside its 180° range of motion.
Twist the horn gently, and you’ll be able to feel the when it reaches one of its mechanical
limits. Don’t try to force it beyond those limits because it could strip the gears inside the
servo.
9 Verify that the power to your board is still disconnected.
9 Gently rotate the servo horn to find the servo’s clockwise and counterclockwise
mechanical limits. The servos horn will turn with very little twisting force until
you reach these limits. DO NOT TRY TO TWIST THE HORN PAST THESE
LIMITS; only twist it far enough to find them.
9 Rotate the servo’s horn so that it is half way between the two limits. This is
approximately the servo’s “center” position.
9 With the servo horn in its center position, thread a jumper wire through the horn
and twist tie it so that it points upward into the region shown in Figure 4-15.
Keep in mind the direction the twist tie is pointing in the figure is just an example; your
twist tie might point anywhere in the region. Wherever it points when it’s in the center of
its range of motion should be pretty close to the servo’s 90° position. Again, this position
can vary from one servo to the next because of the way the horn gets attached to the
servo.
Programming Servo Positions
The graph in Figure 4-16 is called a timing diagram, and it shows examples of the
high/low signals the BASIC Stamp has to send a servo to make it hold its 90° position.
Figure 4-16
Servo Signal Timing
Diagram
1.5 ms pulses make
the servo hold a 90°
“center” position.
The timing diagram shows high signals that last for 1.5 ms, separated by low signals that
last 20 ms. The ... to the right of the signal is a way of indicating that the 1.5 ms high
and 20 ms low signal has to be repeated over and over again to make the servo hold the
Controlling Motion · Page 105
position. The “~” symbol in “~20 ms” indicates that the low time can be approximate,
and it can actually vary a few milliseconds above or below 20 ms with next to no effect
on the where the servo positions its horn. That’s because amount of time the high signal
lasts is what tells the servo what position to hold, so it has to be precise.
There’s a special command called PULSOUT that gives your program precise control over
the durations of those very brief high signals, which are commonly referred to as pulses.
Here is the command syntax for PULSOUT:
PULSOUT Pin, Duration
With the PULSOUT command, you can write PBASIC code to make the BASIC Stamp set
the servo’s position to 90° using the Figure 4-16 timing diagram as a guide. The
PULSOUT command’s Pin argument has to be a number that tells the BASIC Stamp which
I/O pin should transmit the pulse. The PULSOUT command’s Duration argument is the
number of 2-millionths-of-a-second time increments the pulse should last. 2 millionths
of a second is equal to 2 microseconds, which is abbreviated 2 μs.
A millionth of a second is called a microsecond. The Greek letter μ is used in place of the
word micro and the letter s is used in place of second. This is handy for writing and taking
notes, because instead of writing 2 microseconds, you can write 2 μs.
Reminder: one thousandth of a second is called a millisecond, and is abbreviated ms.
Fact: 1 ms = 1000 μs. In other words, you can fit one thousand millionths of a second into
one thousandth of a second.
Now that we know how to use the PULSOUT command, ServoCenter.bs2 sends control
pulses repeatedly to make the servo hold its 90° position. The command PULSOUT 14,
750 will send a 1.5 ms pulse to the servo. That’s because the PULSOUT command’s
Duration argument specifies the number of 2 μs units the pulse should last. Since the
Duration argument is 750, the PULSOUT command will make the pulse last for 750 × 2 μs
= 1500 μs, which is 1.5 ms since there are 1000 μs in 1 ms. After the high pulse is done,
the PULSOUT command leaves the I/O pin sending a low signal. So, a PAUSE 20
command after PULSOUT makes the BASIC Stamp send a low signal for 20 ms. With
both of those commands inside a DO...LOOP, the 1.5 ms high followed by the 20 ms low
will repeat over and over again to make the servo hold its position.
Page 106 · What’s a Microcontroller?
Example Program: ServoCenter.bs2
' What's a Microcontroller - ServoCenter.bs2
' Hold the servo in its 90 degree center position.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!", CR
DO
PULSOUT 14, 750
PAUSE 20
LOOP
Test the Servo’s 90° “Center” Position
The servo’s 90° position is called its center position because the 90° point is in the
“center” of the servo’s 180° range of motion. The 1.5 ms pulses make the servo hold its
horn in this center position, which should be close to the half way point you determined
by finding the servo’s mechanical limits. You can either use whatever center position the
servo holds as your reference for 90°, or use a screwdriver to remove and reposition the
horn so that 90° makes the jumper wire twist tie point straight up. Instructions for this
are coming up in the section titled: Optional – Adjust Servo Horn to 90° Center. If you
use the center position as a reference without adjusting it, any other position the servo
holds will be relative that 90° position. For instance, the 45° position would be 1/8 of a
turn clockwise from it, and the 135° position would be 1/8 of a turn counterclockwise.
Examples of this were shown in Figure 4-14 on page 103.
Let’s first find what your servo’s actual center position is:
9 Gently turn the servo’s horn to one of its mechanical limits.
9 Reconnect power to your board. If you have a Board of Education, make sure to
slide the 3-position all the way to the right (to position-2).
9 Run ServoCenter.bs2.
As soon as the program loads, the servo’s horn should rotate to its center position and
stay there. The servo “holds” this position, because standard servos are designed to resist
external forces that push against it. That’s how the servo holds the RC car steering, boat
rudder, or airplane control flap in place.
9 Make a note of your servo’s center position.
Controlling Motion · Page 107
9 Apply gentle twisting pressure to the horn like you did while rotating the servo
to find its mechanical limits. The servo should resist and hold its horn in its
center position.
If you disconnect power, you can rotate the servo away from its center position. When
you reconnect power, the program will restart, and servo will immediately move the horn
back to its center position and hold it there.
9 Try it!
Optional – Adjust Servo Horn to 90° Center
You can optionally adjust your servo’s horn so that it makes the jumper wire twist tie
point straight up when ServoCenter.bs2 is running, like it does in the right side of Figure
4-17. If you make this mechanical adjustment, it’ll simplify tracking the servo’s angles
because each angle will resemble the ones in Figure 4-13 on page 102.
You will need a #2 Phillips screwdriver for this optional adjustment.
Output
shaft
Phillips
Screw
Figure 4-17
Mechanical Servo
Centering
You can remove
and reposition the
servo horn on the
output shaft with a
small screwdriver.
Horn
9 Disconnect power from your board.
Page 108 · What’s a Microcontroller?
9 Remove the screw that attaches the servo’s horn to its output shaft, and then
gently pull the horn away from case to free it. Your parts should resemble the
left side of Figure 4-17.
9 Reconnect power to your board. The program should make the servo hold its
output shaft in the center position.
9 Slip the horn back onto the servo’s output shaft so that it makes the twist tied
wire point straight up like it does on the right side of Figure 4-17.
Alignment Offset: It might not be possible to get it to line up perfectly because of the way
the horn fits onto the output shaft, but it should be close. You can then adjust the wire loop
to compensate for this small offset and make the twist tie point straight up.
9 Disconnect power from your board.
9 Retighten the Phillips screw.
9 Reconnect power so that the program makes the servo hold its center position
again. The twist tie should now point straight up (or almost straight up)
indicating the 90° position.
Your Turn – Programs to Point the Servo in Different Directions
Figure 4-18 shows a few PULSOUT commands that tell the servo to hold certain major
positions, like 0°, 45°, 90°, 135°, and 180°. These PULSOUT commands are approximate,
and you may have to adjust the values slightly to get more precise angular positions. You
can modify the PULSOUT command’s Duration argument to hold any position in this
range. For example, if you want the servo to hold the 30° position, your PULSOUT
command’s Duration argument would have to be 417, which is 2/3 of the way between
Duration arguments of 250 (0°) to 500 (45°).
The pulse durations in Figure 4-18 will get the servo horn close to the angles shown,
but they are not necessarily exact. You can experiment with different PULSOUT
Duration values for more precise positioning.
9 Save a copy of ServoCenter.bs2 as TestServoPositions.bs2
9 Change the program’s PULSOUT Duration argument from 750 to 500, and run the
modified program to verify that it makes the servo hold its 45° position.
9 Repeat this test of PULSOUT Duration arguments with 1000 (135°), and 417 (30°).
Controlling Motion · Page 109
9 Try predicting a PULSOUT Duration you would need for a position that’s not listed
in Figure 4-18, and test to make sure the servo turns the horn to and holds the
position you want. Example positions could include 60°, 120°, etc.
Keep your program’s PULSOUT Duration arguments in the 350 to 1150 range. The 250
to 1250 range is “in theory” but in practice the servo might try to push against its mechanical
limits. This can reduce the servo’s useful life. If you want to maximize your servo’s range of
motion, carefully test values that get gradually closer to the mechanical limits. So long as
you use PULSOUT Duration values that cause the servo to position its horn just inside its
mechanical limits, wear and tear will be normal instead of excessive.
Figure 4-18: Servo Horn Positions, PULSOUT Commands, and ms Pulse Durations
Page 110 · What’s a Microcontroller?
Do the Math
Along with each PULSOUT command in Figure 4-18, there’s a corresponding number of
milliseconds that each pulse lasts. For example, the pulse that PULSOUT 14, 417 sends
lasts 0.834 ms, and the pulse that PULSOUT 14, 500 sends lasts 1.0 ms. If you have a
BASIC Stamp 2 and want to convert time from milliseconds to a Duration for your
PULSOUT command, use this equation:
Duration = number of ms × 500
For example, if you didn’t already know that the PULSOUT Duration argument for 1.5 ms
is 750, here is how you could calculate it:
Duration = 1 .5 × 500
= 750
The reason we have to multiply the number of milliseconds in a pulse by 500 to get a
PULSOUT Duration argument is because Duration is in terms of 2 μs units for a BS2. How
many 2 μs units are in 1 ms? Just divide 2-one-millionths into 1-one-thousandth to find
out.
1
2
÷
= 500
1,000 1,000 ,000
If your command is PULSOUT 14, 500, the pulse will last for 500 × 2 μs = 1000 μs = 1.0
ms. (Remember, 1000 μs = 1 ms.)
You can also figure out the Duration of a mystery PULSOUT command using this equation:
number of ms =
Duration
500
ms
For example, if you see the command PULSOUT 14, 850, how long does that pulse
really last?
850
ms
500
= 1 .7 ms
number of ms =
Controlling Motion · Page 111
Write Code from Timing Diagrams
Figure 4-19 shows a timing diagram of the signal the BASIC Stamp can send to a servo
so its horn will hold a 135° position. Since this timing diagram features repeated pulses
separated by 20 ms low signals, the DO...LOOP from ServoCenter.bs2 provides a good
starting point, and all that needs to be adjusted is the high pulse duration. To calculate
the PULSOUT command’s Duration argument for the 2 ms pulses in the timing diagram,
you can use the Duration equation in the Do the Math section:
Duration = number of ms × 500
= 2.0 × 500
= 1000
When 1000 gets substituted into the PULSOUT command’s Duration argument, the servo
control loop should look like this:
DO
PULSOUT 14, 1000
PAUSE 20
LOOP
9 Test this DO...LOOP in a copy of ServoCenter.bs2 and verify that it positions the
servo’s horn at approximately 135°.
9 Repeat this exercise for the timing diagram in Figure 4-20.
Figure 4-19
Timing Diagram for
135° Position
2 ms pulses
separated by 20
ms
Page 112 · What’s a Microcontroller?
Figure 4-20
Timing Diagram for
45° Position
1 ms pulses
separated by 20 ms
ACTIVITY #3: CONTROL SERVO HOLD TIME
Animatronics uses electronics to animate props and special effects, and servos are a
common tool in this field. Figure 4-21 shows an example of a robotic hand animatronics
project, with servos controlling each finger. The PBASIC program that controls the hand
gestures has to make the servos hold positions for certain amounts of time for each
gesture. In the previous activity, our programs made the servo hold certain positions
indefinitely. This activity introduces how write code that makes the servo hold certain
positions for certain amounts of time.
Figure 4-21
Animatronic Hand
There are five servos in the lower
right of the figure that that pull bicycle
break cables that are threaded
through the fingers and thumb to
make them flex. This gives the
BASIC Stamp control over each
finger.
Controlling Motion · Page 113
FOR...NEXT Loops to Control the Time a Servo Holds a Position
If you write code to make an LED blink once every second, you can nest the code in a
FOR...NEXT loop that repeats three times to make the light blink for three seconds. If
your LED blinks five times per second, you’d have to make the FOR...NEXT loop repeat
fifteen times to get the LED to blink for three seconds. Since the PULSOUT and PAUSE
commands that control your servo are responsible for sending high/low signals, they also
make the LED blink. The signals we sent to the servo in the previous activity made the
LED glow faintly, maybe with some apparent flicker, because the on/off signals are so
rapid, and the high times are so brief. Let’s slow the signals down to 1/10th speed for
visible LED indicator light blinking.
Example Program: SlowServoSignalForLed.bs2
Compared to the servo center signal, this example program increases the PULSOUT and
PAUSE durations by a factor of ten so that we can see them as LED indicator light
blinking. The program’s FOR...NEXT loop repeats at almost 5 times per second, so 15
repetitions results in making the light blink for three seconds.
9 Disconnect Power to your servo:
o If you have a Board of Education, set the 3-position switch to postion-1
to disconnect power from the servo. Position-1 will still supply power
to the rest of the system.
o If you have a BASIC Stamp Homework Board, temporarily unplug the
end of the wire that’s plugged into Vdd and leave it floating. This will
disconnect power from your servo.
9 Enter and run SlowServoSignalsForLed.bs2.
9 Verify that the LED blinks rapidly for about three seconds.
9 Change the FOR...NEXT loop’s EndValue from 15 to 30 and re-run the program.
Since the loop repeated twice as many times, the light should blink for twice as
long – six seconds.
9 Reconnect power to your servo:
o If you have a Board of Education, set the 3-position switch back to
postion-2 to reconnect power to the servo.
o If you have a BASIC Stamp Homework Board, plug the end of the wire
you disconnected back into the Vdd socket.
' What's a Microcontroller – SlowServoSignalsForLed.bs2
' Slow down the servo signals to 1/10 speed so that they are we can
' see the LED indicator blink on/off.
Page 114 · What’s a Microcontroller?
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!", CR
counter
VAR
Word
FOR counter = 1 to 15
PULSOUT 14, 7500
PAUSE 200
NEXT
Example Program: ThreeServoPositions.bs2
If you change PULSOUT 14, 7500 to PULSOUT 14, 750 and PAUSE 200 to PAUSE 20,
you will have a FOR...NEXT loop that briefly sends the center position signal to the
servo.
Since the signals now last 1/10th of their durations in
SlowServoSignalsForLed.bs2, the entire FOR...NEXT loop will take 1/10th the time to
execute. If the goal is to make the servo hold a particular position for three seconds,
simply deliver ten times as many pulses by increasing the FOR...NEXT loop’s EndValue
argument from 15 to 150.
FOR counter = 1 to 150
PULSOUT 14, 750
PAUSE 20
LOOP
' Center for about 3 sec.
The ThreeServoPositions.bs2 example program makes the servo hold the three different
positions shown in Figure 4-22, each for about 3 seconds.
Figure 4-22
ThreeServoPositions.bs2
The program makes the
servo hold each position
for about three seconds.
9 Enter and run ThreeServoPositions.bs2.
9 Verify that the servo holds each position in the Figure 4-22 sequence for about
three seconds.
Controlling Motion · Page 115
The last position the servo will hold for 3 seconds is 135° and then the program stops.
The servo horn will stay in the same position even though the BASIC Stamp has stopped
sending control pulses. The difference is that during the three seconds that the BASIC
Stamp holds the 135° position, the servo resists any forces that try to push the horn away
from that position. After the 3 seconds is up, the servo’s horn can be turned by hand.
One way you can tell if the servo is receiving control signals is by watching the indicator
LED that is connected to P14. While the indicator LED glows, it means the servo is
receiving control signals and is holding its position. When the signal stops you’ll see the
glow in the indicator LED stop.
9 Re-run the program (or just press and release your board’s Reset Button).
9 As soon as the servo gets to the 135° position, keep an eye on the signal
indicator LED as you apply light twisting force to the horn.
You should be able to feel the servo resisting while the LED glows faintly indicating the
servo is still receiving a control signal. As soon as the LED turns off indicating that the
control signal has stopped, the servo will stop holding its position, and you will be able to
rotate the horn.
9 When the 135° signal stops, verify that the LED indicates the signal has stopped
and that the servo allows you to twist the horn away from the 135° position.
' What's a Microcontroller – ThreeServoPositions.bs2
' Servo holds the 45, 90, and 135 degree positions for about 3 seconds each.
' {$STAMP BS2}
' {$PBASIC 2.5}
counter
VAR
Word
PAUSE 1000
DEBUG "Position = 45 degrees...", CR
FOR counter = 1 TO 150
PULSOUT 14, 500
PAUSE 20
NEXT
DEBUG "Position = 90 degrees...", CR
' 45 degrees for about 3 sec.
Page 116 · What’s a Microcontroller?
FOR counter = 1 TO 150
PULSOUT 14, 750
PAUSE 20
NEXT
' 90 degrees for about 3 sec.
DEBUG "Position = 135 degrees...", CR
FOR counter = 1 TO 150
PULSOUT 14, 1000
PAUSE 20
NEXT
' 135 degrees for about 3 sec.
DEBUG "All done.", CR, CR
END
Your Turn – Adjusting Position vs. Adjusting Hold Time
ThreeServoPositions.bs2 assumes that executing 50 servo pulses in a FOR...NEXT loop
takes about 1 second. You can also use this to adjust a hold time by adjusting the
FOR...NEXT loop’s EndValue argument. For example, if you want the servo to only hold
its position for two about seconds, change the EndValue argument from 150 to 100. For
five seconds, change it from 150 to 250, and so on…
9 Save a copy of ThreeServoPositions.bs2.
9 Modify each FOR...NEXT loop’s EndValue argument and experiment with
different values for different hold times.
9 Optional: Customize the hold positions by adjusting each PULSOUT command’s
Duration argument.
FOR...NEXT Loop Repetition Time – It’s really 1/44th of a Second, not 1/50th
1/50th of a second is a rough approximation of the loop repetition. 1/44th of a second is a
much closer approximation. Consider how much time each element of the FOR...NEXT
loop takes to execute. The command PULSOUT 14, 750 is in the middle of the range of
possible pulse durations, so it can be the benchmark for average pulse duration. It sends
a pulse that lasts 750 × 2 μs = 1500 μs = 1.5 ms. The PAUSE 20 command makes the
program delay for 20 ms. A FOR...NEXT loop with a PULSOUT and PAUSE command
takes about 1.3 ms to process all the numbers and commands. Although this means that
the low signal between pulses really lasts for 21.3 ms instead of 20 ms, this does not
affect the servo’s performance. The low times can be a few ms off, it’s just the high
pulse durations that have to be precise, and the PULSOUT command is very precise.
Controlling Motion · Page 117
So, the total time the FOR...NEXT loop takes to repeat is 1.5 ms + 20 ms + 1.3 ms = 22.8
ms, which is 22.8 thousandths of a second. So, how many 22.8-thousandths-of-a-second
fit into 1-second? Let’s divide 0.0228 into 1 and find out:
1 second ÷ 0.0228 seconds/repetition ≈ 43.86 repetition s
≈ 44 repetitions
So that’s why the loop repeats at a rate of about 44 repetitions per second. The number
of repetitions in 1 second is called a hertz, abbreviated Hz. So, we can say that the servo
signal repeats or cycles at about 44 Hz.
Cycles and hertz (Hz): When a signal repeats itself a certain number of times, each
repetition is called a cycle. The number of cycles in a second is measured in hertz. Hertz is
abbreviated Hz.
Longer or shorter PULSOUT Duration values cause the FOR...NEXT loop to take a little
more or less time to repeat. The PULSOUT Duration of 750 is right in the middle of the
range of servo control pulse durations shown back in Figure 4-18 on page 109. So, you
can use 44 Hz as a benchmark for the number of servo pulses in a second for your code.
If you need to be more precise, just repeat the math for the PULSOUT command you are
using. For example, if the loop has a PULSOUT command with a Duration of 1000 instead
of 750, it takes 2 ms for the pulse instead of 1.5 ms. The loop still has a pause of 20 ms
and 1.3 ms of processing time. So that adds up to 2 + 20 + 1.3 ms = 23.3 ms. Divide that
in to 1 second to find out the FOR...NEXT loop’s rate, and we get 1 ÷ 0.0233 ≈ 42.9 ≈
43 Hz.
FOR...NEXT Loop Servo Control Summary
Figure 4-23 shows the part each number in a FOR...NEXT loop plays in servo control.
The FOR...NEXT loop’s EndValue determines the number of 44ths of a second the servo
holds a position. The PULSOUT command’s Duration argument tells the servo what
position to hold. The value 750 sends a 1.5 ms pulse, which instructs the servo to hold a
90° position according to Figure 4-18 on page 108. The PULSOUT command’s Pin
argument chooses the I/O pin for sending servo control signals. So, 14 makes the
PULSOUT command send its brief high signal (pulse) to the servo connected to I/O pin
P14. When the pulse ends, it leaves the I/O pin sending a low signal. Then, the
PAUSE 20 command ensures that the low signal lasts for approximately 20 ms before the
next pulse.
Page 118 · What’s a Microcontroller?
Servo I/O pin
Number of 44ths of a
second to hold the
position
FOR counter = 1 TO 132
PULSOUT 14, 750
PAUSE 20
NEXT
Position to hold
Figure 4-23
Servo Control
For…Next Loop
Required 20 ms
between each pulse
On the average, a FOR...NEXT loop that sends a single PULSOUT command to a servo,
followed by PAUSE 20, repeats at about 44 times per second. Since this loop repeats 132
times, it makes the servo hold the 135° position for about 3 seconds. That’s because:
132 repetition s ÷ 44 repetition s/second = 3 seconds
If your application or project needs to make the BASIC Stamp send a servo signal for a
certain number of seconds, just multiply the number of seconds by 44, and use the result
in your FOR...NEXT loop’s EndValue argument. For example, if your signal needs to last
five seconds:
5 seconds × 44 repetitions/second = 220 repetition s
ACTIVITY #4: CONTROLLING POSITION WITH YOUR COMPUTER
Factory automation often involves microcontrollers communicating with larger
computers. The microcontrollers read sensors and transmit that data to the main
computer. The main computer interprets and analyzes the sensor data, and then sends
position information back to the microcontroller. The microcontroller might then update
a conveyer belt’s speed, or a sorter’s position, or some other mechanical, motor
controlled task.
You can use the Debug Terminal to send messages from your computer to the BASIC
Stamp as shown in Figure 4-24. The BASIC Stamp has to be programmed to listen for
the messages you send using the Debug Terminal, and it also has to store the data you
send in one or more variables.
Controlling Motion · Page 119
Figure 4-24
Sending Messages to the
BASIC Stamp
Click the white field above
the message display pane
and type your message. A
copy of the message you
entered appears in the lower
windowpane. This copy is
called an echo.
In this activity, you will program the BASIC Stamp to receive two values from the Debug
Terminal, and then use these values to control the servo:
1. The number of pulses to send to the servo
2. The Duration value used by the PULSOUT command
You will also program the BASIC Stamp to use these values to control the servo.
Parts and Circuit
Same as Activity #2
Programming the BASIC Stamp to Receive Messages from Debug
Programming the BASIC Stamp to send messages to the Debug Terminal is done using
the DEBUG command. Programming the BASIC Stamp to receive messages from the
Debug Terminal is done using the DEBUGIN command. When using DEBUGIN, you also
have to declare one or more variables for the BASIC Stamp to store the information it
receives.
Page 120 · What’s a Microcontroller?
Here is an example of a variable you can declare for the BASIC Stamp to store a value:
pulses VAR Word
Later in the program, you can use this variable to store a number received by the
DEBUGIN command:
DEBUGIN DEC pulses
When the BASIC Stamp receives a numeric value from the Debug Terminal, it will store
it in the pulses variable. The DEC formatter tells the DEBUGIN command that the
characters you are sending will be digits that form a decimal number. As soon as you hit
the Enter key, the BASIC Stamp will store the digits it received in the pulses variable as
a decimal number, then move on.
Although it is not included in the example program, you can add a line to verify that the
message was processed by the BASIC Stamp.
DEBUG CR, "You sent the value: ", DEC pulses
Example Program: ServoControlWithDebug.bs2
Figure 4-25 shows the locations of the Debug Terminal’s Transmit windowpane along
with its Receive windowpane. The Receive windowpane is the one we’ve been using all
along to display messages that the Debug Terminal “receives” from the BASIC Stamp.
The Transmit windowpane allows you to type in characters and numbers and “transmit”
them to the BASIC Stamp.
Figure 4-25
Debug Terminal’s Windowpanes
← Transmit windowpane
← Receive windowpane
In Figure 4-25, the number 264 is typed into the Debug Terminal’s Transmit
windowpane. Below, in the Receive windowpane, a copy of the 264 value is shown next
Controlling Motion · Page 121
to the “Enter Run time…” message. This copy is called an echo, and it only displays in
the Receive windowpane if the Echo Off checkbox is left unchecked.
Echo is when you send a message through the Debug Terminal’s Transmit windowpane,
and a copy of that message appears in the Debug Terminal’s Receive windowpane. There
is an Echo Off checkbox in the lower right corner of the Debug Terminal, and you can click it
to toggle the checkmark. For this activity, we want to display the echoes in the Receive
windowpane, so the Echo Off checkbox should be unchecked.
9 Enter ServoControlWithDebug.bs2 into the BASIC Stamp Editor and run it.
9 If the Transmit windowpane is too small, resize it using your mouse to click,
hold, and drag the separator downward. The separator is shown just above the
text message “Enter run time as a” in Figure 4-25.
9 Make sure the Echo Off checkbox in the lower-right corner is unchecked.
9 Click the upper, Transmit windowpane to place your cursor there for typing
messages.
9 When the Debug Terminal prompts you to, “Enter run time as a number of
pulses:” type the number 132, then press your computer keyboard’s Enter key.
9 When the Debug Terminal prompts you to “Enter position as a PULSOUT
duration:” type the number 1000, then press Enter.
The PULSOUT Duration should be a number between 350 and 1150. If you enter
numbers outside that range, the program will change it to the closest number within that
range, either 350 or 1150. If the program did not have this safety feature, certain numbers
could be entered that would make the servo try to rotate to a position beyond its own
mechanical limits. Although it will not break the servo, it could shorten the device’s lifespan.
The BASIC Stamp will display the message “Servo is running…” while it is sending
pulses to the servo. When it is done sending pulses to the servo, it will display the
message “DONE” for one second. Then, it will prompt you to enter the number of pulses
again. Have fun with it, but make sure to follow the directive in the caution box about
staying between 350 and 1150 for your PULSOUT value.
9 Experiment with entering other values between 350 and 1150 for the PULSOUT
Duration and values between 1 and 65534 for the number of pulses.
Page 122 · What’s a Microcontroller?
It takes about 44 pulses to make the servo hold a position for 1 second. So, to make the
servo hold a position for about 5 minutes, you could enter 13200 at the “number of pulses”
prompt. That’s 44 pulses/second × 60 seconds/minute × 5 minutes = 13,200 pulses.
Why use values from 1 to 64434? If you really want to know, read all the way through the
FOR...NEXT section in the BASIC Stamp Manual to learn about the 16-bit rollover, or
variable range, error. It can cause a bug when you are making your own programs!
' What's a Microcontroller - ServoControlWithDebug.bs2
' Send messages to the BASIC Stamp to control a servo using
' the Debug Terminal.
' {$STAMP BS2}
' {$PBASIC 2.5}
counter
pulses
duration
VAR
VAR
VAR
PAUSE 1000
DEBUG CLS, "Servo
" ~44
"Servo
" 350
Word
Word
Word
Run Time:", CR,
pulses in 1 second", CR,
Position:", CR,
<= PULSOUT Duration <= 1150", CR, CR
DO
DEBUG "Enter run time as a ", CR,
"number of pulses: "
DEBUGIN DEC pulses
DEBUG "Enter position as a", CR,
"PULSOUT Duration: "
DEBUGIN DEC duration
duration = duration MIN 350 MAX 1150
DEBUG "Servo is running...", CR
FOR counter = 1 TO pulses
PULSOUT 14, duration
PAUSE 20
NEXT
DEBUG "DONE", CR, CR
PAUSE 1000
LOOP
Controlling Motion · Page 123
How ServoControlWithDebug.bs2 Works
Three Word variables are declared in this program:
counter
pulses
duration
Var
Var
Var
WORD
WORD
WORD
The counter variable is declared for use by a FOR...NEXT loop. (See Chapter 2,
Activity #3 for details.) The pulses and duration variables are used a couple of
different ways. They are both used to receive and store values sent from the Debug
Terminal. The pulses variable is also used to set the number of repetitions in the
FOR...NEXT loop that delivers pulses to the servo, and the duration variable is used to
set the duration of each pulse for the PULSOUT command.
A DEBUG command provides a reminder that there are about 44 pulses in 1 second in the
FOR...NEXT loop, and that the PULSOUT Duration argument that controls servo position
can be a value between 350 and 1150.
DEBUG CLS, "Servo
" ~44
"Servo
" 350
Run Time:", CR,
pulses in 1 second", CR,
Position:", CR,
<= PULSOUT Duration <= 1150", CR, CR
The rest of the program is nested inside a DO...LOOP without a WHILE or UNTIL
Condition argument so that the commands execute over and over again.
DO
' Rest of program not shown.
LOOP
The DEBUG command is used to send you (the “user” of the software) a message to enter
the number of pulses. Then, the DEBUGIN command waits for you to enter digits that
make up the number and press the Enter key on your keyboard. The digits that you enter
are converted to a value that is stored in the pulses variable. This process is repeated
with a second DEBUG and DEBUGIN command that loads another value you enter into the
duration variable too.
DEBUG "Enter run time as a ", CR,
"number of pulses: "
DEBUGIN DEC pulses
Page 124 · What’s a Microcontroller?
DEBUG "Enter position as a", CR,
"PULSOUT Duration: "
DEBUGIN DEC duration
After you enter the second value, it’s useful to display a message while the servo is
running so that you don’t try to enter a second value during that time:
DEBUG "Servo is running...", CR
While the servo is running, you can try to gently move the servo horn away from the
position it is holding. The servo resists light pressure applied to the horn.
FOR Counter = StartValue TO EndValue {STEP StepValue}...NEXT
This is the FOR...NEXT loop syntax from the BASIC Stamp Manual. It shows that you
need a Counter, StartValue and EndValue to control how many times the loop repeats
itself. There is also an optional StepValue if you want to add a number other than 1 to the
value of Counter each time through the loop.
As in previous examples, the counter variable was used to keep track of the
FOR...NEXT loop’s repetitions. The counter variable aside, this FOR...NEXT loop
introduces some new techniques for using variables to define how the program (and the
servo) behaves. Up until this example, the FOR...NEXT loops have used constants such
as 10 or 132 in the loop’s EndValue argument. In this FOR...NEXT loop, the value of the
pulses variable is used to control the FOR...NEXT loop’s EndValue. So, you set the
value of pulses by entering a number into the Debug Terminal, and it controls the
number of repetitions the FOR...NEXT loop makes, which in turn controls the time the
servo holds a given position.
FOR counter = 1 to pulses
PULSOUT 14, duration
PAUSE 20
NEXT
Also, in previous examples, constant values such as 500, 750, and 1000 were used for the
PULSOUT command’s Duration argument. In this loop, a variable named duration,
which you set by entering values into the Debug Terminal’s Transmit windowpane, now
defines the PULSOUT command’s pulse duration, which in turn controls the position the
servo holds.
Controlling Motion · Page 125
Take some time to understand the FOR…NEXT loop in ServoControlWithDebug.bs2.
It is one of the first examples of the amazing things you can do with variables in PBASIC
command arguments and loops, and it also highlights how useful a programmable
microcontroller module like the BASIC Stamp can be.
Your Turn – Setting Limits in Software
Let’s imagine that this computer servo control system is one that has been developed for
remote-control. Perhaps a security guard will use this to open a shipping door that he or
she watches on a remote camera. Maybe a college student will use it to control doors in a
maze that mice navigate in search of food. Maybe a military gunner will use it to point
the cannon at a particular target. If you are designing the product for somebody else to
use, the last thing you want is to give the user (security guard, college student, military
gunner) the ability to enter the wrong number that could damage the equipment.
While running ServoControlWithDebug.bs2, it is possible to make a mistake while
typing the Duration value into the Debug Terminal. Let’s say you accidentally typed 100
instead of 1000 and pressed Enter. The value 100 would cause the servo to try to turn to a
position beyond its mechanical limits. Although it won’t instantly break the servo, it’s
certainly not good for the servo or its useful lifespan. So the program has a line that
prevents this mistake from doing any damage:
duration = duration MIN 350 MAX 1150
This command would correct the 100 accident by changing the duration variable to
350. Likewise, if you accidentally typed 10000, it would reduce the duration variable
to 1150. You could do something equivalent with a couple of IF...THEN statements:
IF duration < 350 THEN duration = 350
IF duration > 1150 THEN duration = 1150
There are some machines where even automatically correcting to the nearest value could
have undesirable results. For example, if you are a computer controlling a machine that
cuts some sort of expensive material, you wouldn’t necessarily want the machine to just
assume you meant 350 when you tried to type 1000, but accidentally typed 100. If it just
cut the material at the 350 setting, it could turn out to be an expensive mistake. So,
another approach your program can take is to simply tell you that your value was out of
range, and to try again. Here is an example of how you can modify the code to do this:
Page 126 · What’s a Microcontroller?
9 Save the example program ServoControlWithDebug.bs2 under the new name
ServoControlWithDebugYourTurn.bs2.
9 Replace these two commands:
DEBUG "Enter position as a", CR,
"PULSOUT Duration: "
DEBUGIN DEC duration
…with this code block:
DO
DEBUG "Enter position as a", CR,
"PULSOUT Duration: "
DEBUGIN DEC duration
IF duration < 350 THEN
DEBUG "Value of duration must be at least 350", CR
PAUSE 1000
ENDIF
IF duration > 1150 THEN
DEBUG "Value of duration cannot be more than 1150", CR
PAUSE 1000
ENDIF
LOOP UNTIL duration >= 350 AND duration <= 1150
9 Save the program.
9 Run the program and verify that it repeats until you enter a value in the correct
350 to 1150 range.
ACTIVITY #5: CONVERTING POSITION TO MOTION
In this activity, you will program the servo to change position at different rates. By
changing position at different rates, you will cause your servo horn to rotate at different
speeds. You can use this technique to make the servo control motion instead of position.
Programming a Rate of Change for Position
You can use a FOR...NEXT loop to make a servo sweep through a range of motion like
this:
FOR counter = 500 TO 1000
PULSOUT 14, counter
PAUSE 20
NEXT
Controlling Motion · Page 127
The FOR...NEXT loop causes the servo’s horn to start at around 45° and then rotate
slowly counterclockwise until it gets to 135°. Because counter is the index of the
FOR...NEXT loop, it increases by one each time through. The value of counter is also
used in the PULSOUT command’s Duration argument, which means the duration of each
pulse gets a little longer each time through the loop. Since the counter variable
changes, so does the position of the servo’s horn.
FOR...NEXT loops have an optional STEP StepValue argument. The StepValue argument
can be used to make the servo rotate faster. For example, you can use the StepValue
argument to add 8 to counter each time through the loop (instead of 1) by modifying the
FOR statement like this:
FOR counter = 500 TO 1000 STEP 8
You can also make the servo turn the opposite direction by counting down instead of
counting up. In PBASIC, FOR...NEXT loops will count backwards if the StartValue
argument is larger than the EndValue argument. Here is an example of how to make a
FOR...NEXT loop count from 1000 down to 500:
FOR counter = 1000 TO 500
You can combine counting down with a StepValue argument to get the servo to rotate
more quickly in the clockwise direction like this:
FOR counter = 1000 TO 500 STEP 20
The trick to getting the servo to turn at different rates is to use these FOR...NEXT loops to
count up and down with different step sizes. The next example program uses these
techniques to make the servo’s horn rotate back and forth at different rates.
Example Program: ServoVelocities.bs2
9 Enter and run ServoVelocities.bs2.
9 As the program runs, watch how the value of counter changes in the Debug
Terminal.
9 Also, watch how the servo behaves differently through the two different
FOR...NEXT loops. Both the servo horn’s direction and speed change.
Page 128 · What’s a Microcontroller?
' What's a Microcontroller - ServoVelocities.bs2
' Rotate the servo counterclockwise slowly, then clockwise rapidly.
' {$STAMP BS2}
' {$PBASIC 2.5}
counter
VAR
Word
PAUSE 1000
DO
DEBUG "Pulse width increment by 8", CR
FOR counter = 500 TO 1000 STEP 8
PULSOUT 14, counter
PAUSE 7
DEBUG DEC5 counter, CR, CRSRUP
NEXT
DEBUG CR, "Pulse width decrement by 20", CR
FOR counter = 1000 TO 500 STEP 20
PULSOUT 14, counter
PAUSE 7
DEBUG DEC5 counter, CR, CRSRUP
NEXT
DEBUG CR, "Repeat", CR
LOOP
How ServoVelocities.bs2 Works
The first FOR...NEXT loop counts upwards from 500 to 1000 in steps of 8. Since the
counter variable is used as the PULSOUT command’s Duration argument, the servo horn’s
position rotates counterclockwise by steps that are eight times the smallest possible step.
FOR counter = 500 TO 1000 STEP 8
PULSOUT 14, counter
PAUSE 7
DEBUG DEC5 counter, CR, CRSRUP
NEXT
Why PAUSE 7 instead of PAUSE 20? The command DEBUG DEC5 counter, CR,
CRSRUP takes about 8 ms to execute. This means that PAUSE 12 would maintain the 20
ms delay between pulses. A few trial and error experiments showed that PAUSE 7 gave the
servo the smoothest motion. Since the 20 ms low time between servo pulses doesn’t need
to be precise, it’s okay to tune and adjust it.
Controlling Motion · Page 129
More DEBUG formatters and control characters are featured in the DEBUG command that
displays the value of the counter variable. This value is printed using the 5-digit decimal
format (DEC5). After the value is printed, there is a carriage return (CR). After that, the
control character CRSRUP (cursor up) sends the cursor back up to the previous line. This
causes the new value of counter to be printed over the old value each time through the
loop.
The second FOR...NEXT loop counts downwards from 1000 back to 500 in steps of 20.
The counter variable is also used as an argument for the PULSOUT command in this
example, so the servo horn rotates clockwise.
FOR counter = 1000 TO 500 STEP 20
PULSOUT 14, counter
PAUSE 7
DEBUG DEC5 counter, CR, CRSRUP
NEXT
Your Turn – Adjusting the Velocities
9
9
9
9
Try different STEP values to make the servo turn at different rates.
Re-run the program after each modification.
Observe the effect of each new StepValue value on how fast the servo horn turns.
Experiment with different PAUSE command Duration values (between 3 and 12)
to find the value that gives the servo the smoothest motion for each new
StepValue value.
ACTIVITY #6: PUSHBUTTON-CONTROLLED SERVO
In this chapter, you have written programs that make the servo go through a pre-recorded
set of motions, and you have controlled the servo using the Debug Terminal. You can
also program the BASIC Stamp to control the servo based on pushbutton inputs. In this
activity you will:
•
•
Build a circuit for a pushbutton servo control.
Program the BASIC Stamp to control the servo based on the pushbutton inputs.
When you are done, you will be able to press and hold one button to get the BASIC
Stamp to rotate the servo in one direction, and press and hold the other button to get the
servo to rotate in the other direction. When no buttons are pressed, the servo will hold
whatever position it moved to last.
Page 130 · What’s a Microcontroller?
Extra Parts for Pushbutton Servo Control
The same parts from the previous activities in this chapter are still used. In addition, you
will need to gather the following parts for the pushbutton circuits:
(2) Pushbuttons – normally open
(2) Resistors – 10 kΩ (brown-black-orange)
(2) Resistors – 220 Ω (red-red-brown)
(3) Jumper wires
Adding the Pushbutton Control Circuit
Figure 4-26 shows the pushbutton circuits that you will use to control the servo.
9 Add this circuit to the servo+LED circuit that you have been using up to this
point. When you are done your circuit should resemble:
o Figure 4-27 if you are using a Board of Education USB (any Rev) or
Serial (Rev C or newer).
o Figure 4-28 if you are using a BASIC Stamp HomeWork Board (Rev C
or newer).
9 If your board is not listed above, refer to the Servo Circuit Connections
download at go to www.parallax.com/Go/WAM to find circuit instructions for
your board.
Vdd
Vdd
P4
Figure 4-26
Pushbutton
Circuits for Servo
Control
220 Ω
P3
220 Ω
10 kΩ
Vss
10 kΩ
Vss
Controlling Motion · Page 131
15 14 Vdd 13 12
White
Red
Black
Red
Black
X4
Vdd
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Figure 4-27
Board of
Education Servo
Circuit with
Pushbutton
Circuits Added
X5
Vin
Vss
+
standard servo
For the Board of
Education Serial
Rev C or higher,
or USB of any
revision
www.parallax.com
Figure 4-28
HomeWork
Board Servo
Circuit with
Pushbutton
Circuits Added
For the
HomeWork
Board Rev C or
higher
Page 132 · What’s a Microcontroller?
9 Test the pushbutton connected to P3 using the original version of
ReadPushbuttonState.bs2. The section that has this program and the instructions
on how to use it begins on page 67.
9 Modify the program so that it reads P4.
9 Run the modified program to test the pushbutton connected to P4.
Programming Pushbutton Servo Control
IF...THEN code blocks can be used to check pushbutton states and either add to or
subtract from a variable named duration. This variable is used in the PULSOUT
command’s Duration argument. If one of the pushbuttons is pressed, the value of
duration increases. If the other pushbutton is pressed, the value of duration decreases.
A nested IF...THEN statement is used to decide if the duration variable is too large
(greater than 1000) or too small (smaller than 500).
Example Program: ServoControlWithPushbuttons.bs2
This example program makes the servo’s horn rotate counterclockwise when the
pushbutton connected to P4 is pressed. The servo’s horn will keep rotating so long as the
pushbutton is held down and the value of duration is smaller than 1000. When the
pushbutton connected to P3 is pressed, the servo horn rotates clockwise. The servo also
is limited in its clockwise motion because the duration variable is not allowed to go
below 500. The Debug Terminal displays the value of duration while the program is
running.
9 Enter the ServoControlWithPushbuttons.bs2 program into the BASIC Stamp
Editor and run it.
9 Verify that the servo turns counterclockwise when you press and hold the
pushbutton connected to P4.
9 Verify that as soon as the limit of duration > 1000 is reached or exceeded that
the servo stops turning any further in the counterclockwise direction.
9 Verify that the servo turns clockwise when you press and hold the pushbutton
connected to P3.
9 Verify that as soon as the limit of duration < 500 is reached or exceeded that
the servo stops turning any further in the clockwise direction.
Controlling Motion · Page 133
' What's a Microcontroller - ServoControlWithPushbuttons.bs2
' Press and hold P4 pushbutton to rotate the servo counterclockwise,
' or press the pushbutton connected to P3 to rotate the servo clockwise.
' {$STAMP BS2}
' {$PBASIC 2.5}
duration
VAR
duration = 750
PAUSE 1000
Word
DO
IF IN3 = 1 THEN
IF duration > 500 THEN
duration = duration - 25
ENDIF
ENDIF
IF IN4 = 1 THEN
IF duration < 1000 THEN
duration = duration + 25
ENDIF
ENDIF
PULSOUT 14, duration
PAUSE 10
DEBUG HOME, DEC4 duration, " = duration"
LOOP
Your Turn – Mechanical Limits vs. Software Limits
The servo’s mechanical stoppers prevent the servo from turning beyond about 0° and
180°, which corresponds to PULSOUT Duration arguments in the 250 and 1250
neighborhoods. ServoControlWithPushbuttons.bs2 also has software limits, imposed by
IF...THEN statements that prevent you from using a pushbutton to turn the servo beyond
a certain point. In contrast to the mechanical limits, the software limits are very easy to
adjust. For example, you can give your pushbutton controlled servo a wider range of
motion by simply replacing every instance of 500 with 350, and every instance of 1000
with 1150. Or, you could give your servo a narrower range of motion by replacing
instances of 500 with 650 and instances of 1000 with 850. The software limits don’t
even need to be symmetrical. For example, you could change the software limits from
the 500–1000 range to the 350–750 range.
9 Experiment with different software servo limits, including 350 to 1150, 650 to
850, and 350 to 750.
Page 134 · What’s a Microcontroller?
9 Test each set of software limits to make sure they perform as expected.
You can also change how quickly the servo turns as you hold a button down. For
example, if you change the two 25 values in the program to 50, the servo will respond
twice as quickly. Alternately, you could change them to 30 to make the servo respond
just a little faster, or to 20 to make them respond a little slower, or to 10 to make it
respond a lot slower.
9 Try it!
SUMMARY
This chapter introduced microcontrolled motion using a Parallax Standard Servo. A
servo is a device that moves to and holds a particular position based on electronic signals
it receives. These signals take the form of pulses that last anywhere between 0.5 and 2.5
ms, and they have to be delivered roughly every 20 ms for the servo to maintain its
position.
A programmer can use the PULSOUT command to make the BASIC Stamp send these
signals. Since pulses have to be delivered every 20 ms for the servo to hold its position,
the PULSOUT and PAUSE commands are usually placed in some kind of loop. Variables or
constants can be used to determine both the number of loop repetitions and the PULSOUT
command’s Duration argument.
In this chapter, several ways to get values into the variables were presented. The variable
can receive the value from your Debug Terminal using the DEBUGIN command. The
value of the variable can pass through a sequence of values if it is used as the Counter
argument of a FOR...NEXT loop. This technique can be used to cause the servo to make
sweeping motions. IF...THEN statements can be used to monitor pushbuttons and add
or subtract from the variable used in the PULSOUT command’s Duration argument when a
certain button is pressed. This allows both position control and sweeping motions
depending on how the program is constructed and how the pushbuttons are operated.
Controlling Motion · Page 135
Questions
1. What are the five external parts on a servo? What are they used for?
2. Is an LED circuit required to make a servo work?
3. What command controls the low time in the signal sent to a servo? What
command controls the high time?
4. What programming element can you use to control the amount of time that a
servo holds a particular position?
5. How do you use the Debug Terminal to send messages to the BASIC Stamp?
What programming command is used to make the BASIC Stamp receive
messages from the Debug Terminal?
6. What type of code block can you write to limit the servo’s range of motion?
Exercises
1. Write a code block that sweeps the value of PULSOUT controlling a servo from a
Duration of 700 to 800, then back to 700, in increments of (a) 1, (b) 4.
2. Add a nested FOR...NEXT loop to your answer to exercise 1b so that it delivers
ten pulses before incrementing the PULSOUT Duration argument by 4.
Project
1. Modify ServoControlWithDebug.bs2 so that it monitors a kill switch. If the kill
switch (P3 pushbutton) is pressed, the Debug Terminal should not accept any
commands, and it should display: “Press Start switch to start machinery.” When
the start switch (P4 pushbutton) is pressed, the program should function
normally. If power is disconnected and reconnected, the program should behave
as though the kill switch has been pressed.
Solutions
Q1. 1) Plug – connects servo to power and signal sources; 2) Cable – conducts power
and signals from plug into the servo; 3) Horn – the moving part of the servo; 4)
Screw – attaches servo’s horn to the output shaft; 5) Case – contains DC motor,
gears, and control circuits.
Q2. No, the LED just helps us see what's going on with the control signals.
Q3. The low time is controlled with the PAUSE command. The high time is
controlled with the PULSOUT command.
Q4. A FOR...NEXT loop.
Page 136 · What’s a Microcontroller?
Q5. Type messages into the Debug Terminal’s Transmit windowpane. Use the
DEBUGIN command and a variable to make the BASIC Stamp receive the
characters.
Q6. Either a nested IF...THEN statement or a command that uses the MAX and MIN
operators to keep the variable in certain ranges.
E1.
a) Increments of 1
b) Add STEP 4 to both FOR...NEXT loops.
FOR counter =
PULSOUT 14,
PAUSE 20
NEXT
FOR counter =
PULSOUT 14,
PAUSE 20
NEXT
700 TO 800
counter
800 TO 700
counter
FOR counter = 700 TO 800 STEP 4
PULSOUT 14, counter
PAUSE 20
NEXT
FOR counter = 800 TO 700 STEP 4
PULSOUT 14, counter
PAUSE 20
NEXT
E2. Assume a variable named pulses has been declared:
FOR counter = 700 TO 800 STEP 4
FOR pulses = 1 TO 10
PULSOUT 14, counter
PAUSE 20
NEXT
NEXT
FOR counter = 800 TO 700 STEP 4
FOR pulses = 1 TO 10
PULSOUT 14, counter
PAUSE 20
NEXT
NEXT
P1. There are many possible solutions; two are given here.
' What's a Microcontroller - Ch04Prj01Soln1__KillSwitch.bs2
' Send messages to the BASIC Stamp to control a servo using
' the Debug Terminal as long as kill switch is not being pressed.
' Contributed by: Professor Clark J. Radcliffe, Department
' of Mechanical Engineering, Michigan State University
' {$STAMP BS2}
' {$PBASIC 2.5}
Controlling Motion · Page 137
counter VAR Word
pulses
VAR Word
duration VAR Word
DO
PAUSE 2000
IF (IN3 = 1) AND (IN4 = 0) THEN
DEBUG "Press Start switch to start machinery.
ELSEIF (IN3 = 0) AND (IN4 = 1) THEN
DEBUG CLS, "Enter number of pulses:", CR
DEBUGIN DEC pulses
", CR ,CRSRUP
DEBUG "Enter PULSOUT duration:", CR
DEBUGIN DEC duration
DEBUG "Servo is running...", CR
FOR counter = 1 TO pulses
PULSOUT 14, duration
PAUSE 20
NEXT
DEBUG "DONE"
PAUSE 2000
ENDIF
LOOP
Below is a version that can even detect button presses while it’s sending a signal
to the servo.
This is important for machinery that needs to STOP
IMMEDIATELY when the kill switch is pressed. It utilizes the waiting
technique that was introduced in the Reaction Timer game in Chapter 3, Activity
#5 in three different places in the program. You can verify that the program
stops sending a control signal to the servo by monitoring the LED signal
indicator light connected to P14.
' What's a Microcontroller - Ch04Prj01Soln2__KillSwitch.bs2
' Send messages to the BASIC Stamp to control a servo using
' the Debug Terminal as long as kill switch is not being pressed.
' {$STAMP BS2}
' {$PBASIC 2.5}
counter
pulses
duration
VAR
VAR
VAR
Word
Word
Word
Page 138 · What’s a Microcontroller?
PAUSE 1000
DEBUG "Press Start switch (P4) to start machinery.", CR
DO:LOOP UNTIL IN4 = 1
DEBUG "Press Kill switch (P3) to stop machinery.", CR
DEBUG CR, CR, "Servo
" ~44
"Servo
" 350
Run Time:", CR,
pulses in 1 second", CR,
Position:", CR,
<= PULSOUT Duration <= 1150", CR, CR
DO
IF IN3 = 1 THEN
DEBUG "Press Start switch (P4) to start machinery.", CR
DO:LOOP UNTIL IN4 = 1
DEBUG "Press Kill switch (P3) to stop machinery.", CR
ENDIF
DEBUG "Enter run time as a ", CR,
"number of pulses: "
DEBUGIN DEC pulses
DEBUG "Enter position as a", CR,
"PULSOUT Duration: "
DEBUGIN DEC duration
duration = duration MIN 350 MAX 1150
DEBUG "Servo is running...", CR
FOR counter = 1 TO pulses
PULSOUT 14, duration
PAUSE 20
IF IN3 = 1 THEN
DEBUG "Press Start switch (P4) to start machinery.", CR
DO:LOOP UNTIL IN4 = 1
DEBUG "Press Kill switch (P3) to stop machinery.", CR
ENDIF
NEXT
DEBUG "DONE", CR, CR
PAUSE 1000
LOOP
Measuring Rotation · Page 139
Chapter 5: Measuring Rotation
ADJUSTING DIALS AND MONITORING MACHINES
Many households have dials to control the lighting in a room. Twist the dial one
direction, and the lights get brighter; twist the dial in the other direction, and the lights get
dimmer. Model trains use dials to control motor speed and direction. Many machines
have dials or cranks used to fine tune the position of cutting blades and guiding surfaces.
Dials can also be found in audio equipment, where they are used to adjust how music and
voices sound. Figure 5-1 shows a simple example of a dial with a knob that is turned to
adjust the speaker’s volume. By turning the knob, a circuit inside the speaker changes,
and the volume of the music the speaker plays changes. Similar circuits can also be
found inside joysticks, and even inside the servo used in Chapter 4: Controlling Motion.
Figure 5-1
Volume Adjustment on a
Speaker
THE VARIABLE RESISTOR UNDER THE DIAL – A POTENTIOMETER
The device inside many sound system dials, joysticks and servos is called a
potentiometer, often abbreviated as a “pot.” Figure 5-2 shows a picture of some common
potentiometers. Notice that they all have three pins.
Page 140 · What’s a Microcontroller?
Figure 5-2
A Few Potentiometer
Examples
Figure 5-3 shows the schematic symbol and part drawing of the potentiometer you will
use in this chapter. Terminals A and B are connected to a 10 kΩ resistive element.
Terminal W is called the wiper terminal, and it is connected to a wire that touches the
resistive element somewhere between its ends.
Figure 5-3
Potentiometer Schematic Symbol
and Part Drawing
Figure 5-4 shows how the wiper on a potentiometer works. As you adjust the knob on
top of the potentiometer, the wiper terminal contacts the resistive element at different
places. As you turn the knob clockwise, the wiper gets closer to the A terminal, and as
you turn the knob counterclockwise, the wiper gets closer to the B terminal.
Figure 5-4
Adjusting the Potentiometer’s Wiper
Terminal
Measuring Rotation · Page 141
ACTIVITY #1: BUILDING AND TESTING THE POTENTIOMETER CIRCUIT
Placing different size resistors in series with an LED causes different amounts of current
to flow through the circuit. Large resistance in the LED circuit causes small amounts of
current to flow through the circuit, and the LED glows dimly. Small resistances in the
LED circuit causes more current to flow through the circuit, and the LED glows more
brightly. By connecting the W and A terminals of the potentiometer, in series with an
LED circuit, you can use it to adjust the resistance in the circuit. This in turn adjusts the
brightness of the LED. In this activity, you will use the potentiometer as a variable
resistor and use it to change the brightness of the LED.
Dial Circuit Parts
(1) Potentiometer – 10 kΩ
(1) Resistor – 220 Ω (red-red-brown)
(1) LED – red
(1) Jumper wire
Building the Potentiometer Test Circuit
Figure 5-5 shows a circuit that can be used for adjusting the LED’s brightness with a
potentiometer.
9 Build the circuit shown in Figure 5-5.
Tip: If you have trouble keeping the potentiometer seated in the breadboard sockets, check
its legs. If each one has a small bend, use a needle-nose pliers to straighten them out and
then try plugging the pot into the breadboard again. When the pot’s legs are straight, they
may maintain better contact with the breadboard sockets.
Page 142 · What’s a Microcontroller?
Figure 5-5
Potentiometer-LED
Test Circuit
Testing the Potentiometer Circuit
9 Turn the potentiometer clockwise until it reaches its mechanical limit shown in
Figure 5-6 (a).
Press the pot against the breadboard a little as you turn its knob. For these activities,
the potentiometer needs to be firmly seated in the breadboard sockets. If you’re not careful
when you turn the knob, the pot can become disconnected from the breadboard sockets,
and that can lead to incorrect measurements. So, apply a little downward pressure as you
turn the potentiometer’s knob to keep it seated in the breadboard.
Handle with care: If your potentiometer will not turn this far, do not try to force it. Just turn
it until it reaches its mechanical limit; otherwise, it might break.
9 Gradually rotate the potentiometer counterclockwise to the positions shown in
Figure 5-6 (b), (c), (d), (e), and (f) noting the how brightly the LED glows at
each position.
Measuring Rotation · Page 143
(a)
(c)
(e)
(b)
(d)
(f)
Figure 5-6
Potentiometer Knob
(a) through (f) show the
potentiometer’s wiper
terminal set to different
positions.
How the Potentiometer Circuit Works
The total resistance in your test circuit is 220 Ω plus the resistance between the A and W
terminals of the potentiometer. The resistance between the A and W terminals increases
as the knob is adjusted further clockwise, which in turn reduces the current through the
LED, making it dimmer.
ACTIVITY #2: MEASURING RESISTANCE BY MEASURING TIME
This activity introduces a new part called a capacitor. A capacitor behaves like a
rechargeable battery that only holds its charge for short durations of time. This activity
also introduces RC-time, which is an abbreviation for resistor-capacitor time. RC-time is
a measurement of how long it takes for a capacitor to lose a certain amount of its stored
charge as it supplies current to a resistor. By measuring the time it takes for the capacitor
to discharge with different size resistors and capacitors, you will become more familiar
with RC-time. In this activity, you will program the BASIC Stamp to charge a capacitor
and then measure the time it takes the capacitor to discharge through a resistor.
Introducing the Capacitor
Figure 5-7 shows the schematic symbol and part drawing for the type of capacitor used in
this activity. Capacitance value is measured in microfarads (µF), and the measurement is
typically printed on the capacitors.
The cylindrical case of this particular capacitor is called a canister. This type of
capacitor, called an electrolytic capacitor, must be handled carefully.
9 Read the CAUTION box on the next page.
Page 144 · What’s a Microcontroller?
CAUTION: This capacitor has a positive (+) and a negative (-) terminal. The negative
terminal is the lead that comes out of the metal canister closest to the stripe with a negative
(–) sign. Always make sure to connect these terminals as shown in the circuit diagrams.
Connecting one of these capacitors incorrectly can damage it. In some circuits, connecting
this type of capacitor incorrectly and then connecting power can cause it to rupture or even
explode.
CAUTION: Do not apply more voltage to an electrolytic capacitor than it is rated to
handle. The voltage rating is printed on the side of the canister.
CAUTION: Safety goggles or safety glasses are recommended.
3300 µF
3300 µF
+
Figure 5-7
3300 µF Capacitor Schematic
Symbol and Part Drawing
-
Pay careful attention to the leads and
how they connect to the Positive and
Negative Terminals.
Resistance and Time Circuit Parts
(1) Capacitor – 3300 µF
(1) Capacitor – 1000 µF
(1) Resistor – 220 Ω (red-red-brown)
(1) Resistor – 470 Ω (yellow-violet-brown)
(1) Resistor – 1 kΩ (brown-black-red)
(1) Resistor – 2 kΩ (red-black-red)
(1) Resistor – 10 kΩ (brown-black-orange)
Building and Testing the Resistor-Capacitor (RC) Time Circuit
Figure 5-8 shows the circuit schematic and Figure 5-9 shows the wiring diagram for this
activity. You will be taking time measurements using different resistor values in place of
the resistor labeled Ri.
Measuring Rotation · Page 145
9 Read the SAFETY box carefully.
SAFETY
Always observe polarity when connecting the 3300 or 1000 μF capacitor. Remember,
the negative terminal is the lead that comes out of the metal canister closest to the stripe
with a negative (–) sign. Use Figure 5-7 to identify the (+) and (-) terminals.
Your 3300 μF capacitor will work fine in this experiment so long as you make sure that the
positive (+) and negative (-) terminals are connected EXACTLY as shown in Figure 5-8 and
Figure 5-9.
Never reverse the supply polarity on the 3300 μF or any other polar capacitor. The
voltage at the capacitor’s (+) terminal must always be higher than the voltage at its (-)
terminal. Vss is the lowest voltage (0 V) on the Board of Education and BASIC Stamp
HomeWork Board. By connecting the capacitor’s negative terminal to Vss, you ensure that
the polarity across the capacitor’s terminals will always be correct.
Never apply voltage to the capacitor that exceeds the voltage rating on the canister.
Wear safety goggles or safety glasses during this activity.
Always disconnect power before you build or modify circuits.
Keep your hands and face away from this capacitor when power is connected.
9 With power disconnected, build the circuit as shown starting with a 470 Ω
resistor in place of the resistor labeled Ri.
P7
220 Ω
Ri
3300 µF
R1 = 470 Ω
R2 = 1 kΩ
R3 = 2 kΩ
R4 = 10 kΩ
Figure 5-8
Schematic for Testing
RC-time Voltage Decay
The four different
resistors will be used one
at a time as Ri in the
schematic.
Vss
Four different resistors will be used as Ri shown in the schematic. First, the schematic
will be built and tested with Ri = 470 Ω, and then Ri = 1 kΩ, etc. will be used later.
Page 146 · What’s a Microcontroller?
R3
Vdd
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
R2
R1
Vin
Vss
-
33
0
F
0µ
+
R4
+
Figure 5-9
Wiring Diagram for
Viewing RC-time Voltage
Decay
Make sure that the
negative lead of the
capacitor is connected on
your board the same way
it is shown in this figure,
with the negative lead
connected to Vss.
9 Make sure that the negative lead of the capacitor is connected on your board the
same way it is shown in this figure, with the negative lead connected to Vss.
Polling the RC-Time Circuit with the BASIC Stamp
Although a stopwatch can be used to record how long it takes the capacitor’s charge to
drop to a certain level, the BASIC Stamp can also be programmed to monitor the circuit
and give you a more consistent time measurement.
Example Program: PolledRcTimer.bs2
9 Enter and run PolledRcTimer.bs2.
9 Observe how the BASIC Stamp charges the capacitor and then measures the
discharge time.
9 Record the measured time (the capacitor’s discharge time) in the 470 Ω row of
Table 5-1.
9 Disconnect power from your Board of Education or BASIC Stamp HomeWork
Board.
9 Remove the 470 Ω resistor labeled Ri in Figure 5-8 and Figure 5-9 on page 146,
and replace it with the 1 kΩ resistor.
9 Reconnect power to your board.
9 Record your next time measurement (for the 1 kΩ resistor).
Measuring Rotation · Page 147
9 Repeat these steps for each resistor value in Table 5-1.
Table 5-1: Resistance and RC-time for C = 3300 μF
Resistance (Ω)
Measured Time (s)
470
1k
2k
10 k
' What's a Microcontroller - PolledRcTimer.bs2
' Reaction timer program modified to track an RC-time voltage decay.
' {$STAMP BS2}
' {$PBASIC 2.5}
timeCounter
counter
PAUSE 1000
VAR
VAR
Word
Nib
DEBUG CLS
HIGH 7
DEBUG "Capacitor Charging...", CR
FOR counter = 5 TO 0
PAUSE 1000
DEBUG DEC2 counter, CR, CRSRUP
NEXT
DEBUG CR, CR, "Measure decay time now!", CR, CR
INPUT 7
DO
PAUSE 100
timeCounter = timeCounter + 1
DEBUG ? IN7
DEBUG DEC5 timeCounter, CR, CRSRUP, CRSRUP
LOOP UNTIL IN7 = 0
DEBUG CR, CR, CR, "The RC decay time was ",
DEC timeCounter, CR,
"tenths of a second.", CR, CR
END
Page 148 · What’s a Microcontroller?
How PolledRcTimer.bs2 Works
Two variables are declared. The timeCounter variable is used to track how long it takes
the capacitor to discharge through Ri. The counter variable is used to count down
while the capacitor is charging.
timeCounter
counter
VAR
VAR
Word
Nib
The command DEBUG CLS clears the Debug Terminal so that it doesn’t get cluttered with
successive measurements. HIGH 7 sets P7 high and starts charging the capacitor, then a
“Capacitor charging…” message is displayed. After that, a FOR...NEXT loop counts
down while the capacitor is charging. As the capacitor charges, the voltage across its
terminals increases toward anywhere between 3.4 and 4.9 V (depending on the value
of Ri).
DEBUG CLS
HIGH 7
DEBUG "Capacitor Charging...", CR
FOR counter = 5 TO 0
PAUSE 1000
DEBUG DEC2 counter, CR, CRSRUP
NEXT
A message announces when the decay starts getting polled.
DEBUG CR, CR, "Measure decay time now!", CR, CR
In order to let the capacitor discharge itself through the Ri resistor, the I/O pin is changed
from HIGH to INPUT. As an input, the I/O pin, has no effect on the circuit, but it can
sense high or low signals. As soon as the I/O pin releases the circuit, the capacitor
discharges as it feeds current through the resistor. As the capacitor discharges, the
voltage across its terminals gets lower and lower (decays).
INPUT 7
Back in the pushbutton chapter, you used the BASIC Stamp to detect a high or low signal
using the variables IN3 and IN4. At that time, a high signal was considered Vdd, and a
low signal was considered Vss. To the BASIC Stamp, actually a high signal is any
voltage above about 1.4 V. Of course, it could be up to 5 V. Likewise, a low signal is
Measuring Rotation · Page 149
anything between 1.4 V and 0 V. This DO...LOOP checks P7 every 100 ms until the
value of IN7 changes from 1 to 0, which indicates that the capacitor voltage decayed to
1.4 V.
DO
PAUSE 100
timeCounter = timeCounter + 1
DEBUG ? IN7
DEBUG DEC5 timeCounter, CR, CRSRUP, CRSRUP
LOOP UNTIL IN7 = 0
The result is then displayed and the program ends.
DEBUG CR, CR, CR, "The RC decay time was ",
DEC timeCounter, CR,
"tenths of a second.", CR, CR
END
Your Turn – A Faster Circuit
By using a capacitor that has roughly 1/3 the capacity to hold charge, the time
measurement for each resistor value that is used in the circuit will be reduced by 1/3.
Later on in the next activity, you will use a capacitor that has 1/33,000 the capacity! The
BASIC Stamp will still take the time measurements for you, using a command called
RCTIME.
9 Disconnect power to your Board of Education or HomeWork Board.
9 Replace the 3300 µF capacitor with a 1000 µF capacitor.
9 Confirm that the polarity of your capacitor is correct. The negative terminal
should be connected to Vss.
9 Reconnect power.
9 Repeat the steps in the Example Program: PolledRcTimer.bs2 section, and
record your time measurements in Table 5-2.
9 Compare your time measurements to the ones you took earlier in Table 5-1.
How close are they to 1/3 the value of the measurements taken with the 3300 µF
capacitor?
Page 150 · What’s a Microcontroller?
Table 5-2: Resistance and RC-time for C = 1000 μF
Resistance (Ω)
Measured Time (s)
470
1k
2k
10 k
ACTIVITY #3: READING THE DIAL WITH THE BASIC STAMP
In Activity #1, a potentiometer was used as a variable resistor. The resistance in the
circuit varied depending on the position of the potentiometer’s adjusting knob. In
Activity #2, an RC-time circuit was used to measure different resistances. In this
activity, you will build an RC-time circuit to read the potentiometer, and use the BASIC
Stamp to take the time measurements. The capacitor you use will be very small, and the
time measurements will be in the microseconds range. Even though the measurements
take very short durations of time, the BASIC Stamp will give you an excellent indication
of the resistance between the potentiometer’s A and W terminals which in turn indicates
the knob’s position.
Parts for Reading RC-Time with the BASIC Stamp
(1) Potentiometer – 10 kΩ
(1) Resistor – 220 Ω (red-red-brown)
(2) Jumper wires
(1) Capacitor – 0.1 µF
(1) Capacitor – 0.01 µF
(2) Jumper wires
These capacitors do not have + and – terminals. They are non-polar. So, you can safely
connect these capacitors to a circuit without worrying about positive and negative terminals.
104
0.1 µF
0.01 µF
103
Figure 5-10
Ceramic Capacitors
0.1 µF capacitor (left)
0.01 µF capacitor (right)
Measuring Rotation · Page 151
Building an RC Time Circuit for the BASIC Stamp
Figure 5-11 shows a schematic and wiring diagram for the fast RC-time circuit. This is
the circuit that you will use to monitor the position of the potentiometer’s knob with the
help of the BASIC Stamp and a PBASIC program.
9 Build the circuit shown in Figure 5-11.
Figure 5-11
Schematic and wiring
diagram for BASIC
Stamp RCTIME Circuit
with Potentiometer
Programming RC-Time Measurements
The example program in Activity #2 measured RC decay time by checking whether
IN7 = 0 every 100 ms, and it kept track of how many times it had to check. When IN7
changed from 1 to 0, it indicated that the capacitor’s voltage decayed to 1.4 V. The result
when the program was done polling was that the timeCounter variable stored the
number of tenths of a second it took for the capacitor’s voltage to decay to 1.4 V.
This next example program uses a PBASIC command called RCTIME that makes the
BASIC Stamp measure RC decay in terms of 2 μs units. So, instead of tenths of a
Page 152 · What’s a Microcontroller?
second, the result RCTIME 7, 1, time stores in the time variable is the number of twomillionths of a second units that it takes for the capacitor’s voltage to decay below 1.4 V.
Since the RCTIME command has such fine measurement units, you can reduce the
capacitor size from 3300 μF to 0.1 or even 0.01 μF, and still get time measurements that
indicate the resistor’s value. Since the resistance between the potentiometer’s A and W
terminals changes as you turn the knob, the RCTIME measurement will give you a time
measurement, which corresponds to the position of the potentiometer’s knob.
Example Program: ReadPotWithRcTime.bs2
9 Enter and run ReadPotWithRcTime.bs2
9 Try rotating the potentiometer’s knob while monitoring the value of the time
variable using the Debug Terminal.
Remember to apply a little downward pressure to keep the potentiometer seated on the
breadboard as you twist its knob. If your servo starts twitching back and forth unexpectedly
instead of holding its position, an un-seated pot may be the culprit.
' What's a Microcontroller - ReadPotWithRcTime.bs2
' Read potentiometer in RC-time circuit using RCTIME command.
' {$STAMP BS2}
' {$PBASIC 2.5}
time VAR Word
PAUSE 1000
DO
HIGH 7
PAUSE 100
RCTIME 7, 1, time
DEBUG HOME, "time = ", DEC5 time
LOOP
Your Turn – Changing Time by Changing the Capacitor
9 Replace the 0.1 µF capacitor with a 0.01 µF capacitor.
9 Try the same positions on the potentiometer that you did in the main activity and
compare the value displayed in the Debug Terminal with the values obtained for
the 0.1 µF capacitor. Are the RCTIME measurements about one tenth the value
for a given potentiometer position?
Measuring Rotation · Page 153
9 Go back to the 0.1 µF capacitor.
9 With the 0.1 µF capacitor back in the circuit and the 0.01 µF capacitor removed,
turn the pot’s knob to its limit in both directions and make notes of the highest
and lowest values for the next activity. Highest:_________ Lowest:_________
How ReadPotWithRcTime.bs2 Works
Figure 5-12 shows how the ReadPotWithRcTime.bs2’s HIGH, PAUSE and RCTIME
commands interact with the circuit in Figure 5-11.
Figure 5-12: Voltage at P7 through HIGH, PAUSE, and RCTIME
On the left, the HIGH 7 command causes the BASIC Stamp to internally connect its I/O
pin P7 to the 5 V supply (Vdd). Current from the supply flows through the
potentiometer’s resistor and also charges the capacitor. The closer the capacitor gets to
its final charge (almost 5 V), the less current flows into it. The PAUSE 100 command is
primarily to make the Debug Terminal display update at about 10 times per second;
PAUSE 1 is usually sufficient to charge the capacitor. On the right, the RCTIME 7, 1,
time command changes the I/O pin direction from output to input and starts counting
time in 2 μs increments. As an input the I/O pin no longer supplies the circuit with 5 V.
Page 154 · What’s a Microcontroller?
In fact, as an input, it’s pretty much invisible to the RC circuit. So, the capacitor starts
losing its charge through the potentiometer. As the capacitor loses its charge, its voltage
decays. The RCTIME command keeps counting time until P7 senses a low signal,
meaning the voltage across the capacitor has decayed to 1.4 V, at which point it stores its
measurement in the time variable.
Figure 5-12 also shows a graph of the voltage across the capacitor during the HIGH,
PAUSE, and RCTIME commands. In response to the HIGH 7 command, which connects the
circuit to 5 V, the capacitor quickly charges. Then, it remains level at its final voltage
during most of the PAUSE 100 command. When the program gets to the RCTIME 7, 1,
time command, it changes the I/O pin direction to input, so the capacitor starts to
discharge through the potentiometer. As the capacitor discharges, the voltage at P7
decays. When the voltage decays to 1.4 V (at the 150 μs mark in this example), the
RCTIME command stops counting time and stores the measurement result in the time
variable. Since the RCTIME command counts time in 2 μs units, the result for 150 μs that
gets stored in the time variable is 75.
I/O Pin Logic Threshold: 1.4 V is a BASIC Stamp 2 I/O pin’s logic threshold. When the I/O
pin is set to input, it stores a 1 in its input register if the voltage applied is above 1.4 V or a 0
if the input voltage is 1.4 V or below. The first pushbutton example back in Chapter 3,
Activity #2 applied either 5 V or 0 V to P3. Since 5 V is above 1.4 V, IN3 stored a 1, and
since 0 V is below 1.4 V, IN3 stored a 0.
RCTIME State Argument: In ReadPotWithRcTime.bs2, the voltage across the capacitor
decays from almost 5 V, and when it gets to 1.4 V, the value in the IN7 register changes
from 1 to 0. At that point, the RCTIME command stores its measurement in its Duration,
which is the time variable in the example program. The RCTIME command’s State
argument is 1 in RCTIME 7, 1, time, which tells the RCTIME command that the IN7
register will store a 1 when the measurement starts. The RCTIME command measures how
long it takes for the IN7 register to change to the opposite state, which happens when the
voltage decays below the I/O pin’s 1.4 V logic threshold.
For more information: Look up the RCTIME command in either the BASIC Stamp Manual
or the BASIC Stamp Editor’s Help.
Figure 5-13 shows how the decay time changes with the potentiometer’s resistance for
the circuit in Figure 5-11. Each position of the potentiometer’s knob sets it at a certain
resistance. Turn it further one direction, and the resistance increases, and in the other
direction, the resistance decreases. When the resistance is larger, the decay takes a longer
time, and the RCTIME command stores a larger value in the time variable. When the
resistance is smaller, the decay takes a shorter time, and the RCTIME command stores a
Measuring Rotation · Page 155
smaller value in the time variable. The DEBUG command in ReadPotWithRcTime.bs2
displays this time measurement in the Debug Terminal, and since the decay time changes
with the potentiometer’s resistance, which in turn changes with the potentiometer knob’s
position, the number in the Debug Terminal indicates the knob’s position.
Figure 5-13
How Potentiometer Resistance
Affects Decay Time
Why does the capacitor charge to a lower voltage when the potentiometer has less
resistance?
Take a look at the schematic in the upper-left corner of Figure 5-12 on page 153. Without
the 220 Ω resistor, the I/O pin would be able to charge the capacitor to 5 V, but the 220 Ω
resistor is necessary to prevent possible I/O pin damage from a current inrush when it starts
charging the capacitor. It also prevents the potentiometer from drawing too much current if it
is turned to 0 Ω while the I/O pin sends its 5 V high signal.
With 5 V applied across the 220 Ω resistor in series with the potentiometer, the voltage
between them has to be some fraction of 5 V. When two resistors conducting current are
placed in series, which results in an intermediate voltage, the circuit is called a voltage
divider. So the 220 Ω resistor and potentiometer form a voltage divider circuit, and for any
given potentiometer resistance (Rpot), you can use this equation to calculate the voltage
across the potentiometer (Vpot):
Vpot = 5 V × Rpot ÷ (Rpot + 220 Ω)
The value of Vpot sets the ceiling on the capacitor’s voltage. In other words, whatever the
voltage across the potentiometer would be if the capacitor wasn’t connected, that’s the
voltage the capacitor can charge to, and no higher. For most of the potentiometer knob’s
range, the resistance values are in the kΩ, and when you calculate Vpot for kΩ Rpot values,
the results are pretty close to 5 V. The 220 Ω resistor doesn’t prevent Vpot from charging
above 1.4 V until the potentiometer’s value is down at 85.6 Ω, which is less than 1% of the
potentiometer’s range of motion. This 1% would have resulted in the lowest measurements
anyhow, so it’s difficult to tell that measurements of 1 in this range are anything out of the
ordinary. Even with the additional 220 Ω resistors built into BASIC Stamp HomeWork board
I/O pin connections, only the lowest 1.7% of the potentiometer’s range is affected, so it’s still
virtually unnoticeable.
So the 220 Ω resistor protects the I/O pin, with minimal impact on the RC decay
measurement’s ability to tell you where you positioned the potentiometer’s knob.
Page 156 · What’s a Microcontroller?
ACTIVITY #4: CONTROLLING A SERVO WITH A POTENTIOMETER
Thumb joysticks like the one in Figure 5-14 are commonly found in video game
controllers. Each joystick typically has two potentiometers that allow the electronics
inside the game controller to report the joystick’s position to the video game console.
One potentiometer rotates with the joystick’s horizontal motion (left/right), and the other
rotates with the joystick’s vertical motion (forward/backward).
Horizontal
potentiometer
Figure 5-14
Potentiometers Inside
the Parallax Thumb
Joystick Module
Vertical
potentiometer
Another thumb joystick application that uses potentiometers is the RC radio controller
and model airplane in Figure 4-1 on page 94. The controller has two joysticks, and each
has two potentiometers. Each potentiometer’s position is responsible for controlling a
different servo on the RC plane.
In this activity, you will use a potentiometer similar to the ones found in thumb joysticks
to control a servo’s position. As you turn the potentiometer’s knob, the servo’s horn will
mirror this motion. This activity utilizes two circuits, the potentiometer circuit from
Activity #3 in this chapter, and the servo circuit from Chapter 4, Activity #1. The
PBASIC program featured in this chapter repeatedly measures the potentiometer’s
position with an RCTIME command, and then uses the measurement and some math to
control the servo’s position with a PULSOUT command.
Measuring Rotation · Page 157
The BASIC Stamp can measure the joystick’s position. Since there are two
potentiometers in each thumb joystick, each of them can replace the stand alone
potentiometer in the circuits in Figure 5-11 on page 151. One RCTIME command can then
measure the vertical potentiometer’s position, and another can measure the horizontal
potentiometer.
Potentiometer Controlled Servo Parts
(1) Potentiometer – 10 kΩ
(1) Resistor – 220 Ω (red-red-brown)
(1) Resistor – 470 Ω (yellow-violet-brown)
(1) Capacitor – 0.1 µF
(1) Parallax Standard Servo
(1) LED – any color
(2) Jumper wires
HomeWork Board users will also need:
(1) 3-pin male-male header
(4) Jumper wires
Building the Dial and Servo Circuits
This activity will use two circuits that you have already built individually: the
potentiometer circuit from the activity you just finished and the servo circuit from the
previous chapter.
9 Leave your potentiometer RC-time circuit from Activity #3 on your prototyping
area. If you need to rebuild it, use Figure 5-11 on page 151. Make sure to use
the 0.1 µF capacitor, not the 0.01 µF capacitor.
9 Add your servo circuit from Chapter 4, Activity #1 to the project. Remember
that your servo circuit will be different depending on your carrier board. Below
are the pages for the sections that you will need to jump to:
o Page 96: Board of Education Servo Circuit
o Page 99: BASIC Stamp HomeWork Board Servo Circuit
Page 158 · What’s a Microcontroller?
Programming Potentiometer Control of the Servo
You will need the smallest and largest value of the time variable that you recorded from
your RC-time circuit while using a 0.1 µF capacitor.
9 If you have not already completed the Your Turn section of the previous activity,
go back and complete it now.
For this next example, here are the time values that were measured by a Parallax
technician; your values will probably be slightly different:
•
•
All the way clockwise:
All the way counterclockwise:
1
691
So how can these input values be adjusted so that they map to the 500–1000 range for
controlling the servo with the PULSOUT command? The answer is by using multiplication
and addition. First, multiply the input values by something to make the difference
between the clockwise (minimum) and counterclockwise (maximum) values 500 instead
of almost 700. Then, add a constant value to the result so that its range is from 500 to
1000 instead of 1 to 500. In electronics, these operations are called scaling and offset.
Here’s how the math works for the multiplication (scaling):
500
= 691× 0.724 = 500
691
500
time(minimum) = 1 ×
= 0.724
691
time(maximum) = 691 ×
After the values are scaled, here is the addition (offset) step.
time(maximum) = 500 + 500 = 1000
time(minimum) = 0.724 + 500 = 500
The */ operator that was introduced on page 85 is built into PBASIC for scaling by
fractional values, like 0.724. Here again are the steps for using */ applied to 0.724:
1. Place the value or variable you want to multiply by a fractional value before the
*/ operator.
Measuring Rotation · Page 159
time = time */
2. Take the fractional value that you want to use and multiply it by 256.
new fractional value = 0.724 × 256 = 185.344
3. Round off to get rid of anything to the right of the decimal point.
new fractional value = 185
4. Place that value after the */ operator.
time = time */ 185
That takes care of the scaling, now all we need to do is add the offset of 500. This can be
done with a second command that adds 500 to time:
time = time */ 185
time = time + 500
Now, time is ready to be recycled into the PULSOUT command’s Duration argument.
time = time */ 185
time = time + 500
PULSOUT 14, time
' Scale by 0.724.
' Offset by 500.
' Send pulse to servo.
Example Program: ControlServoWithPot.bs2
9 Enter and run this program, then twist the potentiometer’s knob and make sure
that the servo’s movements echo the potentiometer’s movements.
' What's a Microcontroller - ControlServoWithPot.bs2
' Read potentiometer in RC-time circuit using RCTIME command.
' Scale time by 0.724 and offset by 500 for the servo.
' {$STAMP BS2}
' {$PBASIC 2.5}
PAUSE 1000
DEBUG "Program Running!"
time
VAR
Word
Page 160 · What’s a Microcontroller?
DO
HIGH 7
PAUSE 10
RCTIME 7, 1, time
time = time */ 185
time = time + 500
PULSOUT 14, time
' Scale by 0.724 (X 256 for */).
' Offset by 500.
' Send pulse to servo.
LOOP
Your Turn – Scaling the Servo’s Relationship to the Dial
Your potentiometer and capacitor will probably give you time values that are somewhat
different from the ones discussed in this activity. These are the values you gathered in
the Your Turn section of the previous activity.
9 Repeat the math discussed in the Programming Potentiometer Control of the
Servo section on page 158 using your maximum and minimum values.
9 Substitute your scale and offset values in ControlServoWithPot.bs2.
9 Comment out DEBUG "Program Running!" with an apostrophe at the
beginning of that line.
9 Add this line of code between the PULSOUT and LOOP commands so that you can
view your results:
DEBUG HOME, DEC5 time
' Display adjusted time value.
9 Run the modified program and check your work. Because the values were
rounded off, the limits may not be exactly 500 and 1000, but they should be
pretty close.
Declaring Constants and Pin Directives
In larger programs, you may end up using the value of the scale factor (which was 185)
and the offset (which was 500) many times in the program. Numbers like 185 and 500 in
your program are called constants because unlike variables, their values cannot be
changed while the program is running. In other words, the value remains “constant.”
You can create names for these constants with CON directives:
ScaleFactor
Offset
delay
CON
CON
CON
185
500
10
Measuring Rotation · Page 161
These CON directives are just about always declared near the beginning of the
program so that they are easy to find.
Once your constant values have been given names with CON directives, you can use
ScaleFactor in place of 185 in your program and Offset in place of 500. For example:
time = time */ scaleFactor
time = time + offset
' Scale by 0.724.
' Offset by 500.
With the values we assigned to the constant names with CON directives, the commands
are really:
time = time */ 185
time = time + 500
' Scale by 0.724.
' Offset by 500.
One important advantage to using constants is that you can change one CON directive, and
it updates every instance of that constant name in your program. For example, if you
write a large program that uses the ScaleFactor constant in 11 different places, one
change to Scale Factor CON…, and all the instances of ScaleFactor in your program
will use that updated value for the next program download. So, if you changed
ScaleFactor CON 500 to ScaleFactor CON 510, every command with ScaleFactor
will use 510 instead of 500.
You can also give I/O pins names using PIN directives. For example, you can declare a
PIN directive for I/O pin P7 like this:
RcPin
PIN 7
There are two places in the previous example program where the number 7 is used to
refer to I/O pin P7. The first can now be written as:
HIGH RcPin
The second can be written as:
RCTIME RcPin, 1, time
If you later change your circuit to use different I/O pins, all you have to do is change the
value in your PIN directive, and both the HIGH and RCTIME commands will be
Page 162 · What’s a Microcontroller?
automatically updated. Likewise, if you have to recalibrate your scale factor or offset,
you can also just change the CON directives at the beginning of the program.
The PIN directive has an additional feature: The PBASIC compiler can detect whether
the pin name is used as an input or output, and it substitutes either the I/O pin number for
output, or the corresponding input register bit variable for input. For example, you could
declare two pin directives, like LedPin PIN 14 and ButtonPin PIN 3. Then, your code
can make a statement like IF ButtonPin = 1 THEN HIGH LedPin. The PBASIC
compiler converts this to IF IN3 = 1 THEN HIGH 14. The IF ButtonPin = 1… made
a comparison, and the PBASIC compiler knows that you are using ButtonPin as an input.
So it uses the input register bit IN3 instead of the number 3. Likewise, the PBASIC
compiler knows that HIGH LedPin uses the LedPin pin name as the constant value 14 for
an output operation, so it substitutes HIGH 14.
Example Program: ControlServoWithPotUsingDirectives.bs2
This program works just like ControlServoWithPot.bs2 but makes use of named
constants and I/O pins.
9 Enter and run ControlServoWithPotUsingDirectives.bs2.
9 Observe how the servo responds to the potentiometer and verify that it behaves
the same as ControlServoWithPot.bs2.
' What's a Microcontroller - ControlServoWithPotUsingDirectives.bs2
' Read potentiometer in RC-time circuit using RCTIME command.
' Apply scale factor and offset, then send value to servo.
' {$STAMP BS2}
' {$PBASIC 2.5}
rcPin
servoPin
PIN
PIN
7
14
' I/O Pin Definitions
scaleFactor
offset
delay
CON
CON
CON
185
500
10
' Constant Declarations
time
VAR
Word
' Variable Declaration
PAUSE 1000
' Initialization
Measuring Rotation · Page 163
DO
HIGH rcPin
PAUSE delay
RCTIME rcPin, 1, time
time = time */ scaleFactor
time = time + offset
PULSOUT servoPin, time
DEBUG HOME, DEC5 time
' Main Routine
' RC decay measurement
'
'
'
'
Scale scaleFactor.
Offset by offset.
Send pulse to servo.
Display adjusted time value.
LOOP
Your Turn – Updating a PIN Directive
As mentioned earlier, if you connect the RC circuit to a different I/O pin, you can simply
change the value of the RcPin PIN directive, and this change automatically reflects in
the HIGH RcPin and RCTIME RcPin, 1, time commands.
9 Save the example program under a new name.
9 Change scaleFactor and offset to the unique values for your RC circuit that
you determined in the previous Your Turn section.
9 Run the modified program and verify that it works correctly.
9 Modify your circuit by moving the RC-time circuit connection from I/O pin P7
to I/O pin P8.
9 Modify the rcPin declaration so that it reads:
rcPin
PIN 8
9 Re-run the program and verify that the HIGH and RCTIME commands are still
functioning properly on the different I/O pin with just one change to the RcPin
PIN directive.
Page 164 · What’s a Microcontroller?
SUMMARY
This chapter introduced the potentiometer, a part often found under various knobs and
dials. The potentiometer has a resistive element that typically connects its outer two
terminals and a wiper terminal that contacts a variable point on the resistive element. The
potentiometer can be used as a variable resistor if the wiper terminal and one of the two
outer terminals is used in a circuit.
The capacitor was also introduced in this chapter. A capacitor can be used to store and
release charge. The amount of charge a capacitor can store is related to its value, which
is measured in farads, (F). The symbol µ is engineering notation for micro, and it means
one-millionth. The capacitors used in this chapter’s activities ranged from 0.01 to
3300 µF.
A resistor and a capacitor can be connected together in a circuit that takes a certain
amount of time to charge and discharge. This circuit is commonly referred to as an RCtime circuit. The R and C in RC-time stand for resistor and capacitor. When one value
(C in this chapter’s activities) is held constant, the change in the time it takes for the
circuit to discharge is related to the value of R. When the value of R changes, the value
of the time it takes for the circuit to charge and discharge also changes. The overall time
it takes the RC-time circuit to discharge can be scaled by using a capacitor of a different
size.
Polling was used to monitor the discharge time of a capacitor in an RC circuit where the
value of C was very large. Several different resistors were used to show how the
discharge time changes as the value of the resistor in the circuit changes. The RCTIME
command was then used to monitor a potentiometer (a variable resistor) in an RC-time
circuit with smaller value capacitors. Although these capacitors cause the discharge
times to range from roughly 2 to 1500 µs (millionths of a second), the BASIC Stamp has
no problem tracking these time measurements with the RCTIME command. The I/O pin
must be set HIGH, and then the capacitor in the RC-time circuit must be allowed to charge
by using PAUSE before the RCTIME command can be used.
PBASIC programming can be used to measure a resistive sensor such as a potentiometer
and scale its value so that it is useful to another device, such as a servo. This involves
performing mathematical operations on the measured RC discharge time, which the
RCTIME command stores in a variable. This variable can be adjusted by adding a constant
value to it, which comes in handy for controlling a servo. In the Projects section, you
may find yourself using multiplication and division as well.
Measuring Rotation · Page 165
The CON directive can be used at the beginning of a program to substitute a name for a
constant value (a number). After a constant is named, the name can be used in place of
the number throughout the program. This can come in handy, especially if you need to
use the same number in 2, 3, or even 100 different places in the program. You can
change the number in the CON directive, and all 2, 3, or even 100 different instances of
that number are automatically updated next time you run the program. PIN directives
allow you to name I/O pins. The I/O pin name is context sensitive, so the PBASIC
compiler substitutes the corresponding I/O pin number for a pin name in commands like
HIGH, LOW, and RCTIME. If the pin name gets used in a conditional statement, it instead
substitutes the corresponding input register, like IN2, IN3, etc.
Questions
1. When you turn the dial or knob on a sound system, what component are you
most likely adjusting?
2. In a typical potentiometer, is the resistance between the two outer terminals
adjustable?
3. How is a capacitor like a rechargeable battery? How is it different?
4. What can you do with an RC-time circuit to give you an indication of the value
of a variable resistor?
5. What happens to the RC discharge time as the value of R (the resistor) gets
larger or smaller?
6. What does the CON directive do? Explain this in terms of a name and a number.
Exercise
1. Let’s say that you have a 0.5 µF capacitor in an RC timer circuit, and you want
the measurement to take 10 times as long. Calculate the value of the new
capacitor.
Projects
1. Add a bicolor LED circuit to Activity #4. Modify the example program so that
the bicolor LED is red when the servo is rotating counterclockwise, green when
the servo is rotating clockwise, and off when the servo holding its position.
2. Use IF...THEN to modify the first example program from Activity #4 so that the
servo only rotates between PULSOUT values of 650 and 850.
Page 166 · What’s a Microcontroller?
Solutions
Q1. A potentiometer.
Q2. No, it’s fixed. The variable resistance is between either outer terminal and the
wiper (middle) terminal.
Q3. A capacitor is like a rechargeable battery in that it can be charged up to hold
voltage. The difference is that it only holds a charge for a very small amount of
time.
Q4. You can measure the time it takes for the capacitor to discharge (or charge).
This time is related to the resistance and capacitance. If the capacitance is
known and the resistance is variable, then the discharge time gives an indication
of the resistance.
Q5. As R gets larger, the RC discharge time increases in direct proportion to the
increase in R. As R gets smaller, the RC discharge time decreases in direct
proportion to the decrease in R.
Q6. The CON directive substitutes a name for a number.
E1. New cap = (10 x old cap value) = (10 x 0.5µF) = 5 µF
P1. Activity #4 with bicolor LED added.
P13
1
2
P12
470 Ω
'
'
'
'
'
'
'
Potentiometer schematic from Figure 5-11
on page 151, servo from Chapter 4,
Activity #1, and bicolor LED from Figure
2-19 on page 53 with P15 and P14 changed
to P13 and P12 as shown.
What's a Microcontroller - Ch5Prj01_ControlServoWithPot.bs2
Read potentiometer in RC-time circuit using RCTIME command.
The time var ranges from 126 to 713, and an offset of 330 is needed.
Bicolor LED on P12, P13 tells direction of servo rotation:
green for CW, red for CCW, off when servo is holding position.
{$STAMP BS2}
{$PBASIC 2.5}
PAUSE 1000
DEBUG "Program Running!"
time
prevTime
VAR
VAR
Word
Word
' time reading from pot
' previous reading
Measuring Rotation · Page 167
DO
prevTime = time
HIGH 7
PAUSE 10
RCTIME 7, 1, time
time = time + 350
IF ( time > prevTime + 2) THEN
HIGH 13
LOW 12
ELSEIF ( time < prevTime - 2) THEN
LOW 13
HIGH 12
ELSE
LOW 13
LOW 12
ENDIF
' Store previous time reading
' Read pot using RCTIME
' Scale pot, match servo range
' increased, pot turned CCW
' Bicolor LED red
' value decreased, pot turned CW
' Bicolor LED green
' Servo holding position
' LED off
PULSOUT 14, time
LOOP
P2. The key is to add IF...THEN blocks; an example is shown below. CLREOL is a
handy DEBUG control character meaning “clear to end of line.”
'
'
'
'
'
What's a Microcontroller - Ch5Prj02_ControlServoWithPot.bs2
Read potentiometer in RC-time circuit using RCTIME command.
Modify with IF…THEN so the servo only rotates from 650 to 850.
The time variable ranges from 1 to 691, so an offset of at least
649 is needed.
' {$STAMP BS2}
' {$PBASIC 2.5}
PAUSE 1000
DEBUG "Program Running!"
time VAR Word
DO
HIGH 7
PAUSE 10
RCTIME 7, 1, time
time = time + 649
IF (time < 650) THEN
time = 650
ENDIF
' Read pot with RCTIME
' Scale time to servo range
' Constrain range from 650 to 850
Page 168 · What’s a Microcontroller?
IF (time > 850) THEN
time = 850
ENDIF
PULSOUT 14, time
DEBUG HOME, "time = ", DEC4 time, CLREOL
LOOP
Digital Display · Page 169
Chapter 6: Digital Display
THE EVERYDAY DIGITAL DISPLAY
Figure 6-1 shows a display on the front of an oven door. When the oven is not in use, it
displays the time. When the oven is in use, it displays the oven’s timer, cooking settings,
and it flashes on and off at the same time an alarm sounds to let you know the food is
done. A microcontroller inside the oven door monitors the pushbuttons and updates the
display. It also monitors sensors inside the oven and switches devices that turn the
heating elements on and off.
Figure 6-1
Digital Clock 7-Segment
Display on Oven Door
Each of the three digits in Figure 6-1 is called a 7-segment display. In this chapter, you
will program the BASIC Stamp to display numbers and letters on a 7-segment display.
WHAT’S A 7-SEGMENT DISPLAY?
A 7-segment display is rectangular block of 7 lines of equal length that can be lit
selectively with LEDS to display digits and some letters. Figure 6-2 shows a part drawing
of the 7-segment LED display you will use in this chapter’s activities. It also has a dot
that can be used as a decimal point. Each of the segments (A through G) and the dot
contain a separate LED, which can be controlled individually. Most of the pins have a
number along with a label that corresponds to one of the LED segments. Pin 5 is labeled
DP, which stands for decimal point. Pins 3 and 8 are labeled “common cathode” and
they will be explained when the schematic for this part is introduced.
Page 170 · What’s a Microcontroller?
Common
Cathode
10 9 8 7 6
G F
A B
A
F
B
G
C
E
Figure 6-2
7-Segment LED Display Part
Drawing and Pin Map
D
E D
C DP
1 2 3 4 5
Common
Cathode
Pin Map: Figure 6-2 is an example of a pin map. A pin map contains useful information that
helps you connect a part to other circuits. Pin maps usually show a number for each pin, a
name for each pin, and a reference.
Take a look at Figure 6-2. Each pin is numbered, and the name for each pin is the segment
letter next to the pin. The reference for this part is the decimal point. Orient the part so that
the decimal point is at the bottom-right. Then you can see from the pin map that Pin 1 is at
the bottom-left, and the pin numbers increase counterclockwise around the case.
Figure 6-3 shows a schematic of the LEDs inside the 7-segment LED display. Each LED
anode is connected to an individual pin. All the cathodes are connected together by wires
inside the part. Because all the cathodes share a common connection, the 7-segment LED
display can be called a common cathode display. By connecting either pin 3 or pin 8 of
the part to Vss, you will connect all the LED cathodes to Vss.
Digital Display · Page 171
1
4
6
7
9
10
5
E
C
B
A
F
G
DP
LED’s
3
Figure 6-3
7-Segment LED Display
Schematic
8
ACTIVITY #1: BUILDING AND TESTING THE 7-SEGMENT LED DISPLAY
In this activity, you will manually build circuits to test each segment in the display.
7-Segment LED Display Test Parts
(1) 7-segment LED display
(5) Resistors – 1 kΩ (brown-black-red)
(1) Jumper wire
7-Segment LED Display Test Circuits
9 With power disconnected from your Board of Education or HomeWork Board,
build the circuit shown in Figure 6-4 and Figure 6-5.
9 Reconnect power and verify that the A segment emits light.
What’s the x with the nc above it in the schematic? The nc stands for not connected or
no-connect. It indicates that a particular pin on the 7-segment LED display is not connected
to anything. The x at the end of the pin also means not connected. Schematics sometimes
use just the x or just the nc.
Page 172 · What’s a Microcontroller?
Vdd
nc
X
X
X
nc
nc
nc
X
nc
X
nc
X
nc
X
1 kΩ
1
4
6
7
9
10
5
E
C
B
A
F
G
DP
Figure 6-4
Test Circuit Schematic
for the “A” Segment
LED Display
LED’s
8
X
3
Vss
nc
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vdd
Vin
Vss
Figure 6-5
Test Circuit Wiring
Diagram for the “A”
Segment LED Display
9 Disconnect power, and modify the circuit by connecting the resistor to the B
LED input as shown in Figure 6-6 and Figure 6-7.
Digital Display · Page 173
Vdd
X
nc
nc
nc
X
X
nc
X
nc
X
nc
X
nc
X
1 kΩ
1
4
6
7
9
10
5
E
C
B
A
F
G
DP
Figure 6-6
Test Circuit Schematic
for the ”B” Segment
LED Display
LED’s
8
X
3
Vss
nc
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vdd
Vin
Vss
Figure 6-7
Test Circuit Wiring
Diagram for the “B”
Segment LED Display
9 Reconnect power and verify that the B segment emits light.
9 Using the pin map from Figure 6-2 as a guide, repeat these steps for segments C
through G.
Page 174 · What’s a Microcontroller?
Your Turn – The Number 3 and the Letter H
Figure 6-8 and Figure 6-9 show the digit “3” hardwired into the 7-segment LED display.
Vdd
Vdd
Vdd
Vdd
Vdd
1 kΩ (all)
X
nc
X
nc
X
nc
1
4
6
7
9
10
5
E
C
B
A
F
G
DP
Figure 6-8
Hardwired Digit “3”
LED’s
8
X
3
Vss
nc
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vdd
Vin
Figure 6-9
Wiring Diagram for
Figure 6-8
Vss
9 Build and test the circuit shown in Figure 6-8 and Figure 6-9, and verify that it
displays the number three.
9 Draw a schematic that will display the number 2 on the 7-segment LED.
9 Build and test the circuit to make sure it works. Trouble-shoot if necessary.
9 Determine the circuit needed for the letter “H” and then build and test it.
Digital Display · Page 175
ACTIVITY #2: CONTROLLING THE 7-SEGMENT LED DISPLAY
In this activity, you will connect the 7-segment LED display to the BASIC Stamp, and
then run a simple program to test and make sure each LED is properly connected.
7-Segment LED Display Parts
(1) 7-segment LED display
(8) Resistors – 1 kΩ (brown-black-red)
(5) Jumper wires
Connecting the 7-Segment LED Display to the BASIC Stamp
Figure 6-11 shows the schematic and Figure 6-12 shows the wiring diagram for this
BASIC Stamp controlled 7-segment LED display example.
9 Build the circuit shown in Figure 6-11 and Figure 6-12.
Schematic and pin map: If you are trying to build the circuit from the schematic in Figure
6-11 without relying on Figure 6-12, make sure to consult the 7-segment LED display’s pin
map, shown here again in Figure 6-10 for convenience.
Common
Cathode
10 9 8 7 6
G F
A B
A
F
B
G
C
E
D
E D
C DP
1 2 3 4 5
Common
Cathode
Figure 6-10
7-Segment LED Display Part
Drawing and Pin Map
Page 176 · What’s a Microcontroller?
1 kΩ
(All)
P15
P14
P13
P12
P11
P10
P9
P8
E
C
G
DP
F
A
B
Figure 6-11
BASIC Stamp
Controlled 7Segment
LED Display
Schematic
LED’s
common
Vss
Be careful with the resistors connected to P13 and P14. Look closely at the resistors
connected to P13 and P14 in Figure 6-12. There is gap between these two resistors. The
gap is shown because pin 8 on the 7-segment LED display is left unconnected. A resistor
connects I/O pin P13 to 7-segment LED display pin 9. Another resistor connects P14 to
7-segment LED display pin 7.
Digital Display · Page 177
DP
EDC GFAB
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vdd
Figure 6-12
Wiring Diagram for
Figure 6-11
Vin
Use the segment letters
above this diagram as a
reference.
Vss
Parallel Device: The 7-segment LED display is called a parallel device because the BASIC
Stamp has to use a group of I/O lines to send data (high and low information) to the device.
In the case of this 7-segment LED display, it takes 8 I/O pins to instruct the device what to
display.
Parallel Bus: The wires that transmit the HIGH/LOW signals from the BASIC Stamp to the
7-segment LED display are called a parallel bus. Note that these wires are drawn as
parallel lines in Figure 6-11. The term “parallel” kind of makes sense given the geometry of
the schematic.
Programming the 7-Segment LED Display Test
The HIGH and LOW commands will accept a variable as a Pin argument. To test each
segment, one at a time, simply place the HIGH and LOW commands in a FOR...NEXT loop,
and use the index to set the I/O pin high, then low again.
9 Enter and run SegmentTestWithHighLow.bs2.
9 Verify that every segment in the 7-segement LED display lights briefly, turning
on and then off again.
9 Record a list of which segment each I/O pin controls.
Page 178 · What’s a Microcontroller?
Example Program: SegmentTestWithHighLow.bs2
' What's a Microcontroller - SegmentTestWithHighLow.bs2
' Individually test each segment in a 7-Segment LED display.
'{$STAMP BS2}
'{$PBASIC 2.5}
pinCounter
VAR
Nib
PAUSE 1000
DEBUG "I/O Pin", CR,
"-------", CR
FOR pinCounter = 8 TO 15
DEBUG DEC2 pinCounter, CR
HIGH pinCounter
PAUSE 1000
LOW pinCounter
NEXT
Your Turn – A Different Pattern
Removing the command LOW pinCounter will have an interesting effect:
9 Comment the LOW pinCounter command by adding an apostrophe to the left of
it.
9 Run the modified program and observe the effect.
ACTIVITY #3: DISPLAYING DIGITS
If you include the decimal point there are eight different BASIC Stamp I/O pins that send
high/low signals to the 7-segment LED display. That’s eight different HIGH or LOW
commands just to display one number. If you want to count from zero to nine, that would
be a huge amount of programming. Fortunately, there are special variables you can use
to set the high and low values for groups of I/O pins.
In this activity, you will use 8-digit binary numbers instead of HIGH and LOW commands
to control the high/low signals sent by BASIC Stamp I/O pins. By setting special
variables called DIRH and OUTH equal to the binary numbers, you will be able to control
the high/low signals sent by all the I/O pins connected to the 7-segment LED display
circuit with a single PBASIC command.
Digital Display · Page 179
8 bits: A binary number that has 8 digits is said to have 8 bits. Each bit is a slot where you
can store either a 1 or a 0.
A byte is a variable that contains 8 bits. There are 256 different combinations of zeros and
ones that you can use to count from 0 to 255 with 8 bits. This is why a byte variable can
store a number between 0 and 255.
Parts and Circuit for Displaying Digits
Same as previous activity
Programming On/Off Patterns Using Binary Numbers
In this activity, you will experiment with the variables DIRH and OUTH. DIRH is a variable
that controls the direction (input or output) of I/O pins P8 through P15. OUTH controls the
high or low signals that each of these I/O pin sends. As you will soon see, OUTH is
especially useful because you can use it to set the high/low signals for eight different I/O
pins at once with just one command. Here is an example program that shows how these
two variables can be used to count from 0 to 9 on the 7-segment LED display without
using HIGH and LOW commands:
Example Program: DisplayDigits.bs2
This example program will cycle the 7-Segment LED display through the digits 0
through 9.
9 Enter and run DisplayDigits.bs2.
9 Verify that the digits 0 through 9 are displayed.
' What's a Microcontroller - DisplayDigits.bs2
' Display the digits 0 through 9 on a 7-segment LED display.
'{$STAMP BS2}
'{$PBASIC 2.5}
DEBUG "Program Running!"
OUTH = %00000000
DIRH = %11111111
'
BAFG.CDE
OUTH = %11100111
PAUSE 1000
OUTH = %10000100
PAUSE 1000
' OUTH initialized to low.
' Set P8-P15 to all output-low.
' Digit:
' 0
' 1
Page 180 · What’s a Microcontroller?
OUTH = %11010011
PAUSE 1000
OUTH = %11010110
PAUSE 1000
OUTH = %10110100
PAUSE 1000
OUTH = %01110110
PAUSE 1000
OUTH = %01110111
PAUSE 1000
OUTH = %11000100
PAUSE 1000
OUTH = %11110111
PAUSE 1000
OUTH = %11110110
PAUSE 1000
' 2
DIRH = %00000000
' I/O pins to input,
' segments off.
' 3
' 4
' 5
' 6
' 7
' 8
' 9
END
How DisplayDigits.bs2 Works
Vin
Vss
Figure 6-13 shows how you can use the DIRH and OUTH variables to control the direction
and state (high/low) of I/O pins P8 through P15.
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
X3
Vdd
Figure 6-13
Using DIRH and OUTH
to set all I/O Pins to
Output-Low
OUTH = %00000000
DIRH = %11111111
Digital Display · Page 181
The first command:
OUTH = %00000000
...gets all the I/O pins (P8 through P15) ready to send the low signals. If they all send
low signals, it will turn all the LEDs in the 7-segment LED display off. If you wanted all
the I/O pins to send a high signal, you could use OUTH = %11111111 instead.
What does % do? The % binary formatter is used to tell the BASIC Stamp Editor that the
number is a binary number. For example, the binary number %00001100 is the same as the
decimal number 12. As you will see in this activity, binary numbers can make many
programming tasks much easier.
The low signals will not actually be sent by the I/O pins until you use the DIRH variable
to change all the I/O pins from input to output. The command:
DIRH = %11111111
...sets I/O pins P8 through P15 to output. As soon as this command is executed, P8
through P15 all start sending low signals. This is because the command OUTH =
%00000000 was executed just before this DIRH command. As soon as the DIRH command
set all the I/O pins to output, they started sending their low signals. You can also use
DIRH = %00000000 to change all the I/O pins back to inputs.
Before I/O pins become outputs: Up until the I/O pins are changed from input to output,
they just listen for signals and update the INH variable. This is the variable that contains
IN8, IN9, up through IN15. These variables can be used the same way that IN3 and IN4
were used for reading pushbuttons in Chapter 3 Digital Input – Pushbuttons.
All BASIC Stamp I/O pins start out as inputs. This is called a default. You have to tell a
BASIC Stamp I/O pin to become an output before it starts sending a high or low signal. Both
the HIGH and LOW commands automatically change a BASIC Stamp I/O pin’s direction to
output. Placing a 1 in the DIRH variable also makes one of the I/O pins an output.
Always set values in a given OUT register before making them outputs with values in
the corresponding DIR register. This prevents briefly sending unintended signals. For
example, if DIR5 = 1 is followed by OUT5 = 1 at the beginning of a program, it will briefly
send an unintended low signal before switching to high because OUT5 stores 0 when the
program starts. (All PBASIC variables/registers initialize to 0.) If OUT5 = 1 is instead
followed by DIR5 = 1, the I/O pin will send a high signal as soon as it becomes an output.
Since the values stored by all variables default to 0 when the program starts, the
command OUTH = %00000000 is actually redundant.
Page 182 · What’s a Microcontroller?
Figure 6-14 shows how to use the OUTH variable to selectively send high and low signals
to P8 through P15. A binary-1 is used to send a high signal, and a binary-0 is used to
send a low signal. This example displays the number three on the 7-segment LED
display:
Vin
Vss
'
BAFG.CDE
OUTH = %11010110
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
X3
Vdd
Figure 6-14
Using OUTH to Control
the High/Low Signals of
P8 – P15
‘
BAFG.CDE
OUTH = %11010110
The display is turned so that the three on the display is upside-down because it more
clearly shows how the values in OUTH line up with the I/O pins. The command
OUTH = %11010110 uses binary zeros to set I/O pins P8, P11, and P13 low, and it uses
binary ones to set P9, P10, P12, P14 and P15 high. The line just before the command is a
comment that shows the segment labels line up with the binary value that turns that
segment on/off.
Inside the HIGH and LOW commands:
HIGH 15
...is really the same as:
OUT15 = 1
DIR15 = 1
...is the same as:
OUT15 = 0
DIR15 = 1
Likewise, the command :
LOW 15
If you want to change P15 back to an input, use DIR15 = 0. You can then use IN15 to
detect (instead of send) high/low signals.
Digital Display · Page 183
Your Turn – Displaying A through F
9 Figure out what bit patterns (combinations of zeros and ones) you will need to
display the letters A, b, C, d, E, and F.
9 Modify DisplayDigits.bs2 so that it displays A, b, C, d, E, F.
Decimal vs. Hexadecimal The basic digits in the decimal (base-10) number system are:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9
In the hexadecimal (base-16) number system the basic digits are:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, b, C, d, E, F.
Base-16 is used extensively in both computer and microcontroller programming. Once you
figure out how to display the characters A through F, you can further modify your program to
count in hexadecimal from 0 to F.
Keeping Lists of On/Off Patterns
The LOOKUP command makes writing code for 7-segment LED display patterns much
easier. The LOOKUP command lets you “look up” elements in a list. Here is a code
example that uses the LOOKUP command:
LOOKUP index, [7, 85, 19, 167, 28], value
There are two variables used in this command, index and value. If the index is 0,
value stores 7. If index is 1, value stores 85. In the next example program, index is
set to 2, so the LOOKUP command places 19 into value, and that’s what the Debug
Terminal displays.
Example Program: SimpleLookup.bs2
9
9
9
9
Enter and run SimpleLookup.bs2.
Run the program as-is, with the index variable set equal to 2.
Try setting the index variable equal to numbers between 0 and 4.
Re-run the program after each change to the index variable and note which
value from the list gets placed in the value variable.
9 Optional: Modify the program by placing the LOOKUP command in a
FOR...NEXT loop that counts from 0 to 4.
Page 184 · What’s a Microcontroller?
' What's a Microcontroller - SimpleLookup.bs2
' Debug a value using an index and a lookup table.
' {$STAMP BS2}
' {$PBASIC 2.5}
value
index
VAR
VAR
Byte
Nib
index = 2
PAUSE 1000
DEBUG ? index
LOOKUP index, [7, 85, 19, 167, 28], value
DEBUG ? value, CR
DEBUG "Change the index variable to a ", CR,
"different number(between 0 and 4).", CR, CR,
"Run the modified program and ", CR,
"check to see what number the", CR,
"LOOKUP command places in the", CR,
"value variable."
END
Example Program: DisplayDigitsWithLookup.bs2
This example program shows how the LOOKUP command can come in really handy for
storing the bit patterns used in the OUTH variable. Again, the index variable is used to
choose which binary value is placed into the OUTH variable. This example program
counts from 0 to 9 again. The difference between this program and DisplayDigits.bs2 is
that this program is much more versatile. It is much quicker and easier to adjust for
different number sequences using lookup tables.
9 Enter and run DisplayDigitsWithLookup.bs2.
9 Verify that it does the same thing as the previous program (with much less
work).
9 Take a look at the Debug Terminal while the program runs. It shows how the
value of index is used by the LOOKUP command to load the correct binary value
from the list into OUTH.
Digital Display · Page 185
' What's a Microcontroller - DisplayDigitsWithLookup.bs2
' Use a lookup table to store and display digits with a 7-segment LED display.
'{$STAMP BS2}
'{$PBASIC 2.5}
index
VAR
Nib
OUTH = %00000000
DIRH = %11111111
PAUSE 1000
DEBUG "index
"-----
OUTH
", CR,
--------", CR
FOR index = 0 TO 9
LOOKUP index, [ %11100111, %10000100, %11010011,
%11010110, %10110100, %01110110,
%01110111, %11000100, %11110111, %11110110 ], OUTH
DEBUG "
", DEC2 index, "
PAUSE 1000
NEXT
", BIN8 OUTH, CR
DIRH = %00000000
END
Your Turn – Displaying 0 through F Again
9 Modify DisplayDigitsWithLookup.bs2 so that it counts from 0 through F in
hexadecimal. Don’t forget to update the FOR...NEXT loop’s EndValue argument.
ACTIVITY #4: DISPLAYING THE POSITION OF A DIAL
In Chapter 5, Activity #4 you used the potentiometer to control the position of a servo. In
this activity, you will display the position of the potentiometer using the 7-segment LED
display.
Dial and Display Parts
(1) 7-segment LED display
(8) Resistors – 1 kΩ (brown-black-red)
(1) Potentiometer – 10 kΩ
(1) Resistor – 220 Ω (red-red-brown)
(1) Capacitor – 0.1 µF
(7) Jumper wires
Page 186 · What’s a Microcontroller?
Building the Dial and Display Circuits
Figure 6-15 shows a schematic of the potentiometer circuit that should be added to the
project. Figure 6-16 shows a wiring diagram of the circuit from Figure 6-15 combined
with the circuit from Figure 6-11 on page 176.
9 Add the potentiometer circuit to the 7-segment LED display circuit as shown in
Figure 6-16.
Figure 6-15
Schematic of
Potentiometer Circuit
Added to the Project
Figure 6-16
Wiring Diagram for
Figure 6-15
Programming the Dial and Display
There is a useful command called LOOKDOWN, and yes, it is the reverse of the LOOKUP
command. While the LOOKUP command gives you a number based on an index, the
LOOKDOWN command gives you an index based on a number.
Digital Display · Page 187
Example Program: SimpleLookdown.bs2
This example program demonstrates how the LOOKDOWN command works.
9 Enter and run SimpleLookdown.bs2.
9 Run the program as-is, with the value variable set equal to 167, and use the
Debug Terminal to observe the value of index.
9 Try setting the value variable equal to each of the other numbers listed by the
LOOKDOWN command: 7, 85, 19, 28.
9 Re-run the program after each change to the value variable and note which
value from the list gets placed in the index variable.
Trick question: What happens if your value is greater than 167? This little twist in the
LOOKDOWN command can cause problems because the LOOKDOWN command doesn’t make
any changes to the index.
' What's a Microcontroller - SimpleLookdown.bs2
' Debug an index using a value and a lookup table.
' {$STAMP BS2}
' {$PBASIC 2.5}
value
index
VAR
VAR
Byte
Nib
value = 167
PAUSE 1000
DEBUG ? value
LOOKDOWN value, [7, 85, 19, 167, 28], index
DEBUG ? index, CR
DEBUG "Change the value variable to a ", CR,
"different number in this list:", CR,
"7, 85, 19, 167, or 28.", CR, CR,
"Run the modified program and ", CR,
"check to see what number the ", CR,
"LOOKDOWN command places in the ", CR,
"index variable."
END
Page 188 · What’s a Microcontroller?
Unless you tell it to make a different kind of comparison, the LOOKDOWN command checks
to see if a value is equal to an entry in the list. You can also check to see if a value is
greater than, less than or equal to, etc. For example, to search for an entry that the value
variable is less than or equal to, use the <= operator just before the first bracket that starts
the list. In other words, the operator returns the index of the first value in the list that
makes the statement in the instruction true.
9 Modify SimpleLookdown.bs2 by substituting this value and LOOKDOWN
statement in place of the existing ones:
value = 35
LOOKDOWN value, <= [ 7, 19, 28, 85, 167 ], index
9 Modify the DEBUG command so that it reads:
DEBUG "Change the value variable to a ", CR,
"different number in this range:", CR,
"0 to 170.", CR, CR,
"Run the modified program and ", CR,
"check to see what number the ", CR,
"LOOKDOWN command places in the ", CR,
"index variable."
9 Experiment with different values and see if the index variable displays what you
would expect.
Example Program: DialDisplay.bs2
This example program mirrors the position of the potentiometer’s knob by lighting
segments around the outside of the 7-segment LED display as shown in Figure 6-17.
Figure 6-17
Displaying the Potentiometer’s
Position with the 7-Segment LED
Display
Digital Display · Page 189
9 Enter and run DialDisplay.bs2.
9 Twist the potentiometer’s knob and make sure it works. Remember to press
down to keep the pot seated in the breadboard.
9 When you run the example program, it may not be as precise as shown in Figure
6-17. Adjust the values in the LOOKDOWN table so that that the digital display
more accurately depicts the position of the potentiometer.
' What's a Microcontroller - DialDisplay.bs2
' Display POT position using 7-segment LED display.
'{$STAMP BS2}
'{$PBASIC 2.5}
PAUSE 1000
DEBUG "Program Running!"
index
time
VAR
VAR
Nib
Word
OUTH = %00000000
DIRH = %11111111
DO
HIGH 5
PAUSE 100
RCTIME 5, 1, time
LOOKDOWN time, <= [40, 150, 275, 400, 550, 800], index
LOOKUP index, [ %11100101, %11100001, %01100001,
%00100001, %00000001, %00000000 ], OUTH
LOOP
How DialDisplay.bs2 Works
This example program takes an RCTIME measurement of the potentiometer and stores it in
a variable named time.
HIGH 5
PAUSE 100
RCTIME 5, 1, time
Page 190 · What’s a Microcontroller?
The time variable is then used in a LOOKDOWN table. The LOOKDOWN table decides which
number in the list time is smaller than, and then loads the entry number (0 to 5 in this
case) into the index variable.
LOOKDOWN time, <= [40, 150, 275, 400, 550, 800], index
Next, the index variable is used in a LOOKUP table to choose the binary value to load into
the OUTH variable.
LOOKUP index, [ %11100101, %11100001, %01100001,
%00100001, %00000001, %00000000 ], OUTH
Your Turn – Adding a Segment
DialDisplay.bs2 only makes five of the six segments turn on when you turn the dial. The
sequence for turning the LEDs on in DialDisplay.bs2 is E, F, A, B, C, but not D.
9 Save DialDisplay.bs2 under the name DialDisplayYourTurn.bs2.
9 Modify DialDisplayYourTurn.bs2 so that it causes all six outer LEDs to turn on
in sequence as the potentiometer is turned. The sequence should be: E, F, A, B,
C, and D.
Tip: Leave your 7-segment LED circuit on your board. We’ll be using the 7-segment LED
again along with other circuits in Chapter 7, Activity #4.
Digital Display · Page 191
SUMMARY
This chapter introduced the 7-segment LED display, and how to read a pin map. This
chapter also introduced some techniques for devices and circuits that have parallel inputs.
The DIRH and OUTH variables were introduced as a means of controlling the values of
BASIC Stamp I/O pins P8 through P15. The LOOKUP and LOOKDOWN commands were
introduced as a means for referencing the lists of values used to display letters and
numbers.
Questions
1. In a 7-segment LED display, what is the active ingredient that makes the display
readable when a microcontroller sends a high or low signal?
2. What does common cathode mean? What do you think common anode means?
3. What is the group of wires that conduct signals to and from a parallel device
called?
4. What are the names of the commands in this chapter that are used to handle lists
of values?
Exercises
1. Write an OUTH command to set P8, P10, P12 high and P9, P11, P13 low.
Assuming all your I/O pins started as inputs, write the DIRH command that will
cause the I/O pins P8 through P13 to send high/low signals while leaving P14
and P15 configured as inputs.
2. Write the values of OUTH required to make the letters: a, C, d, F, H, I, n, P, S.
Project
1. Spell “FISH CHIPS And dIP” over and over again with your 7-segment LED
display. Make each letter last for 400 ms.
Solutions
Q1. The active ingredient is an LED.
Q2. Common cathode means that all the cathodes are connected together, i.e., they
share a common connection point. Common anode would mean that all the
anodes are connected together.
Q3. A parallel bus.
Q4. LOOKUP and LOOKDOWN handle lists of values.
Page 192 · What’s a Microcontroller?
E1. The first step for configuring OUTH is set to "1" in each bit position specified as
HIGH. So bits 8, 10, and 12 get set to "1". Then put a "0" for each LOW, so bits 9,
11, and 13 get a "0", as shown. To configure DIRH, the specified pins, 8, 10, 12,
9, 11, and 13 must be set as outputs by setting those bit to "1". 15 and 14 are
configured as inputs by placing zeroes in bits 15 and 14. The second step is to
translate this to a PBASIC statement.
Bit 15 14 13 12 11 10
OUTH 0 0 0 1 0 1
9
0
8
1
OUTH = %00010101
Bit 15 14 13 12 11 10
DIRH 0 0 1 1 1 1
9
1
8
1
DIRH = %00111111
E2. The key to solving this problem is to draw out each letter and note which
segments must be lit. Place a 1 in every segment that is to be lit. Translate that
to the binary OUTH value. The BAFG.CDE segment list for bits in OUTH came
from Figure 6-14 on page 182.
Letter
a
C
d
F
H
I
n
P
S
LED Segments
e, f, a, b, c, g
a, f, e, d
b, c, d, e, g
a, f, e, g
f, e, b, c, g
f, e
e, g, c
all but c and d
a, f, g, c, d
B A F G.C D E
11110101
01100011
10010111
01110001
10110101
00100001
00010101
11110001
01110110
From Figure 6-2 on page 170.
OUTH Value =
%11110101
%01100011
%10010111
%01110001
%10110101
%00100001
%00010101
%11110001
%01110110
Common
Cathode
10 9 8 7 6
G F
A B
A
F
B
G
C
E
D
E D
C DP
1 2 3 4 5
Common
Cathode
P1. Use the schematic from Figure 6-11 on page 176. To solve this problem, modify
DisplayDigitsWithLookup.bs2, using the letter patterns worked out in
Exercise 2. In the solution, the letters have been set up as constants to make the
program more intuitive. Using the binary values is fine too, but more prone to
errors.
Digital Display · Page 193
' What's a Microcontroller - Ch6Prj01_FishAndChips.bs2
' Use a lookup table to store and display digits with
' a 7-segment display. Spell out the message: FISH CHIPS And dIP
'{$STAMP BS2}
'{$PBASIC 2.5}
' Patterns of 7-Segment Display to create letters
A
CON
%11110101
C
CON
%01100011
d
CON
%10010111
F
CON
%01110001
H
CON
%10110101
I
CON
%00100001
n
CON
%00010101
P
CON
%11110001
S
CON
%01110110
space
CON
%00000000
index
VAR
Byte
' 19 chars in message
OUTH = %00000000
DIRH = %11111111
' All off to start
' All LEDs must be outputs
PAUSE 1000
' 1 sec. before 1st message
DO
DEBUG "index
"-----
OUTH
", CR,
--------", CR
FOR index = 0 TO 18
' 19 chars in message
LOOKUP index, [ F, I, S, H, space, C, H, I, P, S, space,
A, n, d, space, d, I, P, space ], OUTH
DEBUG "
PAUSE 400
NEXT
LOOP
", DEC2 index, "
", BIN8 OUTH, CR
' 400 ms between letters
Page 194 · What’s a Microcontroller?
Measuring Light · Page 195
Chapter 7: Measuring Light
DEVICES THAT CONTAIN LIGHT SENSORS
Earlier chapters introduced pushbuttons as contact/pressure sensors and potentiometers as
rotation/position sensors. Both of these sensors are common in electronic products—just
think of how many devices with buttons and dials you use on a daily basis. Another
common sensor found in many products is the light sensor. Here are a few examples of
devices that need light sensors to function properly:
•
•
•
•
•
•
Car headlights that automatically turn on when it’s dark
Streetlights that automatically turn on when it gets dark
Outdoor security lights that turn on when someone walks by (but only at night)
Laptop displays that get brighter in well lit areas and dimmer in poorly lit areas
Cameras with automatic exposure settings
The sensor inside TVs, DVD players and other entertainment system
components that detects the infrared light from a handheld remote
The first three examples in the list are automatic lighting, and they depend on ambient
light sensors to distinguish day from night. The electronics inside those devices only
needs to know whether it’s light or dark, so they can treat their light sensors as binary
outputs like pushbuttons. Laptop displays and camera auto exposures adjust to area
lighting conditions by getting information from their light sensors about how bright or
dark it is. They have to treat their light sensors as analog outputs that supply a number
that indicates how bright or dark it is, much like the Chapter 5 potentiometer examples
where numbers indicated the knob’s position.
The light sensors inside TVs and other entertainment system components detect infrared
(IR), which is a light that is not visible to the human eye, but can be detected by many
electronic devices. For example, if you look at the end of the remote that you point at a
TV or other entertainment devices, you will find a clear IR LED. When you press a
button on the remote, it sends coded signals to the entertainment system component by
flashing the IR LED on/off. Since we can’t see infrared light, it doesn’t look like the
remote’s LED does anything when you press a button. However, if you try this while
looking at the LED through the lens of a digital camera, the LED will look almost white.
White light contains all the colors in the spectrum. The red green and blue sensors inside
a camera chip all report that they detect light in response to white light. It so happens
Page 196 · What’s a Microcontroller?
that the red/green/blue sensors all detect the infrared light from the remote’s IR LED as
well. So the camera also interprets light from an infrared LED as white.
More about infrared LEDs and detectors:
Robotics with the Boe-Bot has examples where the BASIC Stamp controlled Boe-Bot robot
uses the IR LED found inside TV remotes and the IR receiver found inside TV sets for
detecting objects in front of it. The Boe-Bot uses the IR LED as a tiny flashlight and the IR
receiver found inside TVs to detect the IR flashlights’ reflections off objects in front of it. IR
Remote for the Boe-Bot explains how TV remotes code the messages they send to TVs and
has examples of how to program the BASIC Stamp microcontroller to decode messages
from the remote so that you can send messages to your Boe-Bot robot, and even drive it
around, all with a TV remote.
The type of light a given device senses depends on what it’s designed to do. For
example, light sensors for devices that adjust to ambient lighting conditions need to sense
visible light. The red, green and blue pixel sensors inside digital cameras are each
sensing the levels of specific colors for a digital image. The IR sensor inside a TV is
looking for infrared light that’s flashing on/off in the 40 kHz neighborhood. These are
just a few examples, and each application requires a different kind of light sensor.
Figure 7-1 shows a few examples of the many light sensors available for various lightsensing requirements. From left to right, it shows a phototransistor, cadmium sulfide
photoresistor, linear light sensor, blue enhanced photodiode, light to frequency converter,
infrared phototransistor, and an infrared remote receiver from a TV.
Figure 7-1 Examples of Light Sensors
Measuring Light · Page 197
About the Cadmium Sulfide (CdS) Cell or Photoresistor
The Cadmium Sulfide (CdS) cell or photoresistor was one of the most common ambient light
sensors in automatic lighting. With the advent of the European Union’s Restriction of use of
certain Hazardous Substances (RoHS) directive, cadmium sulfide photoresistors can no
longer be built into most devices imported into or manufactured in Europe. This has
increased the use of a number of photoresistor replacement products, including the
phototransistor and linear light sensor. As a result of these changes, this edition now
features a phototransistor for detecting light levels instead of a cadmium sulfide
photoresistor.
Documentation for each light sensor describes the type of light it detects in terms of
wavelength. Wavelength is the measure of distance between repeating shapes or cycles.
For example, picture a wave traveling through the ocean, bobbing up and down. The
wavelength of that wave would be the distance between each peak (or whitecap) of the
wave’s cycle. The wavelength of light can be measured in a similar way, instead we’re
measuring the distance between two peaks in the electromagnetic oscillations of light.
Each color of light has its own wavelength and is considered to be visible light, meaning
it can be detected by the human eye. Figure 7-2 shows wavelengths for visible light as
well as for some types of light the human eye cannot detect, including ultraviolet and
infrared. These wavelengths are measured in nanometers, abbreviated nm. One
nanometer is one billionth of a meter.
Figure 7-2 Wavelengths and their Corresponding Colors
Wavelength (nm) 10…380
Color
450
495
Violet
Ultraviolet
570 590 620
Green
Blue
Orange
Yellow
750…100,000
Infrared
Red
NOTE: If you are viewing this in the grayscale printed book, you may download a full-color PDF
from www.parallax.com/go/WAM.
Page 198 · What’s a Microcontroller?
INTRODUCING THE PHOTOTRANSISTOR
A transistor is like a valve that allows a certain amount of electric current to pass through
two of its terminals. The third terminal of a transistor controls just how much current
passes through the other two. Depending on the type of transistor, the current flow can
be controlled by voltage, current, or in the case of the phototransistor, by light. Figure
7-3 shows the schematic and part drawing for the phototransistor in your What’s a
Microcontroller kit. The more light that strikes the phototransistor’s base (B) terminal,
the more current it will conduct into its collector (C) terminal, which flows out of its
emitter (E) terminal. Conversely, less light shining on the base terminal results in less
current conducted.
B
C
Figure 7-3
Phototransistor Schematic Symbol
and Part Drawing
B
E
E
C
This phototransistor’s peak sensitivity is at 850 nm, which according to Figure 7-2, is in
the infrared range. It also responds to visible light, thought it’s somewhat less sensitive,
especially to wavelengths below 450 nm, which are left of blue in Figure 7-2. Light from
halogen and incandescent lamps, and especially sunlight, are much stronger sources of
infrared than fluorescent lamps. The infrared transistor responds well to all these sources
of light, but it is most sensitive to sunlight, then to halogen and incandescent lamps, and
somewhat less sensitive to fluorescent lamps.
Circuit designs using the transistor can be adjusted to work best in certain types of
lighting conditions, and the phototransistor circuits in this chapter are designed for indoor
use. There is one outdoor light sensing application, but it will use a different device that
might at first seem like an unlikely candidate for a light sensor: a light emitting diode.
Measuring Light · Page 199
ACTIVITY #1: BUILDING AND TESTING THE LIGHT METER
Chapter 5 introduced RC decay measurements with the RCTIME command for measuring
the time it took a capacitor to lose its charge through the variable resistor inside a
potentiometer. With larger resistance (to the flow of electric current), the potentiometer
slowed down the rate the capacitor lost its charge, and smaller resistances sped up that
rate. The decay time measurement gave an indication of the potentiometer’s resistance,
which in turn made it possible for the BASIC Stamp to know the position of the
potentiometer’s dial.
When placed in an RC decay circuit, a phototransistor, which conducts more or less
current when more or less light shines on it, behaves a lot like the potentiometer. When
more light shines on the phototransistor, it conducts more current, and the capacitor loses
its charge more quickly. With less light, the phototransistor conducts less current, and
the capacitor loses its charge less quickly. So the same RCTIME measurement that gave
us an indication of the dial’s position with a potentiometer in Chapter 5 can now be used
to measure light levels with a phototransistor.
In this activity, you will build and test an RC decay circuit that measures the time it takes
a capacitor’s charge to decay through a phototransistor. The RC decay measurement will
give you an idea of the light levels sensed by the phototransistor’s light-collecting
surface. As with the potentiometer tests, the time values measured by the RCTIME
command will be displayed in the Debug Terminal.
Light Detector Test Parts
(1) Phototransistor
(1) Resistor – 220 Ω (red-red-brown)
(2) Capacitors – 0.01 µF (labeled 103)
(1) Capacitor – 0.1 µF (labeled 104)
(1) Jumper wire
Building the RC Time Circuit with a Phototransistor
Figure 7-4 shows a schematic and wiring diagram of the RC-time circuit you will use in
this chapter. This circuit is different from the potentiometer circuit from Chapter 5,
Activity #3 in two ways. First, the I/O pin used to measure the decay time is different
(P2). Second, the potentiometer has been replaced with a phototransistor.
Page 200 · What’s a Microcontroller?
Tip: Leave your 7-segment LED connected, and add the phototransistor circuit to
your board. We’ll use the 7-segment LED with the phototransistor in Activity #4.
9 Build the circuit shown in Figure 7-4.
9 Make sure the phototransistor’s collector and emitter (C and E) terminals are
connected as shown in the wiring diagram.
Vdd
Vin
Figure 7-4
Phototransistor RC-time
Circuit Schematic and
Wiring Diagram
Vss
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Start with the 0.01 µF
capacitor, labeled 103.
Longer pin
(C) terminal
Flat spot
(E) terminal
Programming the Phototransistor Test
The first example program (TestPhototransistor.bs2) is really just a slightly revised
version of ReadPotWithRcTime.bs2 from Chapter 5, Activity #3. The potentiometer
circuit from Chapter 5 was connected to I/O pin P7. The circuit in this activity is
connected to P2. Because of this difference, the example program has to have two
commands updated to make it work. The HIGH 7 command from the previous example
program is now HIGH 2 since the phototransistor circuit is connected to P2 and not P7.
Measuring Light · Page 201
For the same reason, the command that was RCTIME 7, 1, time has been changed to
RCTIME 2, 1, time.
Example Program: TestPhototransistor.bs2
The phototransistor’s light collecting surface is at the top of its clear plastic dome, which
is the base (B) terminal shown in Figure 7-3. A small black square should be visible
through that dome. That black square is the actual phototransistor, a tiny piece of silicon.
The rest of the device is packaging, including plastic case, lead frame, and leads.
Instead of twisting the potentiometer’s knob like we did in Chapter 5, this circuit is tested
by exposing the phototransistor’s light collecting surface to different light levels. When
the example program is running, the Debug Terminal should display small values for
bright light conditions and large values for low light conditions.
Avoid direct sunlight! The circuit and program you are using is designed to detect
variations in indoor lighting and do not work in direct sunlight. Leave the indoor lights on,
but close the blinds if sun is streaming in through nearby windows.
9 Enter and run TestPhototransistor.bs2.
9 Make a note of the time variable on the Debug Terminal under normal lighting
conditions.
9 Cast a shadow over the circuit with your hand and check the time variable
again. It should be a larger number.
9 Cup your hand over the circuit to cast a darker shadow; the Debug Terminal
should display a significantly larger value for time.
' What's a Microcontroller - TestPhototransistor.bs2
' Read phototransistor in RC-time circuit using RCTIME command.
' {$STAMP BS2}
' {$PBASIC 2.5}
time
PAUSE 1000
VAR
Word
DO
HIGH 2
PAUSE 100
RCTIME 2, 1, time
DEBUG HOME, "time =
LOOP
", DEC5 time
Page 202 · What’s a Microcontroller?
Your Turn – Using a Different Capacitor for Different Light Conditions
The time measurements with a 0.1 μF capacitor will take ten times as long as those with
the 0.01 μF capacitor, which means the value of the time variable displayed by the
Debug Terminal should be ten times as large. Replacing the 0.01 μF capacitor with a 0.1
μF capacitor can be useful for more brightly lit rooms where you will typically see
smaller measurements with the 0.01 μF capacitor. For example, let’s say the lighting
conditions are very bright, and the measurements are only ranging from 1 to 13 with 0.01
μF capacitor. If you replace it with a 0.1 μF capacitor, your measurements will instead
range from 1 to 130, and your application will be more sensitive to light variations within
the room.
9 Modify the circuit by replacing the 0.01 μF capacitor with a 0.1 μF capacitor
(labeled 104).
9 Re-run TestPhototransistor.bs2 and verify that the RC-time measurements are
roughly ten times their former values.
The longest time interval the RCTIME command can measure is 65535 units of 2 µs each,
which corresponds to a decay time of: 65535 × 2 μs = 131 ms = 0.131 s. If the decay
time exceeds 0.131 seconds, the RCTIME command returns 0 to indicate that the
maximum measurement time was exceeded.
9 Can you cast a dark enough shadow over the phototransistor to exceed the
maximum 65535 measurement and make the RCTIME command return 0?
The next activity will rely on the smaller of the two capacitors.
9 Before you move on to the next activity, return the circuit to the original one
shown in Figure 7-4 by removing the 0.1 μF capacitor and replacing it with the
0.01 μF capacitor.
ACTIVITY #2: TRACKING LIGHT EVENTS
One of the more useful features of the BASIC Stamp module’s program memory is that
you can disconnect power to the board without losing your program. As soon as power is
reconnected, the program will start running again from the beginning. Since the code for
your application typically doesn’t fill up the BASIC Stamp module’s memory, any
portion that is not used for the program can be used to store data. This memory is
especially good for storing data that you do not want the BASIC Stamp to forget. While
Measuring Light · Page 203
the values stored by variables get erased when the power gets disconnected, the BASIC
Stamp will still remember all the values stored in its program memory when the power
gets reconnected.
What is datalogging? Datalogging is what a microcontroller does when it records and
stores periodic sensor measurements for a certain amount of time. Datalogging devices, or
dataloggers, are especially useful in scientific research. For example, instead of posting a
person in a remote location to take weather measurements, a datalogging weather station
can be deployed. It records periodic measurements, and scientists visit the station every so
often to collect the data, or in some cases, it uploads its measurements to a computer by
cell phone, radio, or satellite.
The chip on the BASIC Stamp that stores program memory and data is shown in Figure
7-5. This chip is called an EEPROM, which stands for Electrically Erasable
Programmable Read-Only Memory. That’s quite a mouthful, and pronouncing each of
the first letters in EEPROM is still a lot of work. So, when people talk about an
EEPROM, they usually pronounce it: “E-E-Prom”.
2 KB EEPROM
stores your
PBASIC source
code.
Figure 7-5
EEPROM Chip on BASIC
Stamp Module
This EEPROM stores
your program code and
any other data your
program places there,
even when power is
disconnected.
Figure 7-6 showss the BASIC Stamp Editor’s Memory Map window. You can view this
window by clicking the BASIC Stamp Editor’s Run menu and selecting Memory Map.
The Memory Map uses different colors to show how both the BASIC Stamp module’s
RAM (variables in random access memory) and EEPROM (program memory) are being
used. The red square in the scroll bar at the far left indicates what portion of the
EEPROM is visible in the EEPROM Map. You can click and drag this square up and
down to view various portions of the EEPROM memory. By dragging that red square
down to the bottom, you can see how much EEPROM memory space is used by
TestPhototransistor.bs2 from Activity #1. The bytes that contain program tokens are
Page 204 · What’s a Microcontroller?
highlighted in blue, and only 35 bytes out of the 2048 byte EEPROM are used for the
program. The remaining 2013 bytes are free to store data.
Figure 7-6
Memory Map
To view this
window, click
Run, and select
Memory Map.
The EEPROM Map shows the addresses as hexadecimal values, which were discussed
briefly in the Decimal vs. Hexadecimal box on page 183. The values along the left side
show the starting address of each row of bytes. The numbers along the top show the byte
number within that row, from 0 to F in hexadecimal, which is 0 to 15 in decimal. For
example, in Figure 7-6, the hexadecimal value C1 is stored at address 7E0. CC is stored
at address 7E1, 6D is stored at address 7E2, and so on, up through E8, which is stored at
address 7EF. If you scroll up and down with the scroll bar, you’ll see that the largest
memory addresses are at the bottom of the EEPROM Map, and the smallest addresses are
at the top, with the very top row starting at 000.
PBASIC programs are always stored at the largest addresses in EEPROM, which are
shown at the bottom of the EEPROM Map. So, if your program is going to store data in
EEPROM, it should start with the smallest addresses, starting with address 0. This helps
ensure that your stored data won’t overwrite your PBASIC program, which will usually
result in a program crash. In the case of the EEPROM Map shown in Figure 7-6, the
PBASIC program resides in addresses 7FF through 7DD, starting at the largest address
and building to smaller addresses. So your application can store data from address 000
through 7DC, building from the smallest to the largest. In decimal, that’s addresses 0
through 2012.
If you plan on storing data to EEPROM, it is important to be able convert from
hexadecimal to decimal in order to calculate the largest writable address. Below is the
Measuring Light · Page 205
math for converting the number 7DC from hexadecimal to decimal. Hexadecimal is a
numerical system with a base of 16, meaning it uses 16 different digits to represent its
values. The digits 0-9 represent the first 10 values, and the letters A-F represent values
10-15. When converting to from hexadecimal to decimal, each digit from the right
represents a larger power of sixteen. The rightmost digit is the number of ones, which is
the number of 160s. The next digit from the right is the number of 16s, which is the
number of 161s. The third digit is the number of 256s, which is the number of 162s.
Hexadecimal 7DC
=
=
=
=
=
(7 × 162) + (D × 161) + (C × 160)
(7 × 162) + (13 × 161) + (12 × 160)
(7 × 256) + (13 × 16) + (12 × 1)
1792 + 208 + 12
2012 (decimal value)
This conversion approach works the same in other bases, including base 10 decimal
values. For example:
2102 = (2 × 103) + (1 × 102) + (0 × 101) + (2 × 100)
= (2 × 1000) + (1 × 100) + (0 × 10) + (2 × 1)
2048 bytes = 2 KB.
Although both the upper case “K” and the lower-case “k”’ are called “kilo,” they are slightly
different. In electronics and computing, the upper-case K is used to indicate a binary
10
kilobyte, which is 1 × 2 = 1024. When referring to exactly 1000 bytes, use the lower-case
3
k, which stands for kilo and is 1 x 10 = 1000 in the metric system.
Also, the upper-case “B”’ stands for bytes, while the lower-case “b” stands for bits. This can
make a big difference because 2 Kb means 2048 bits, which is 2048 different numbers, but
each number is limited to a value of either 0 or 1. In contrast, 2 KB, is 2048 bytes, each of
which can store a value in the 0 to 255 range.
Using the EEPROM for data storage can be very useful for remote applications. One
example of a remote application would be a temperature monitor placed in a truck that
hauls frozen food. It could track the temperature during the entire trip to see if it always
stayed cool enough to make sure none of the shipment thawed. A second example is a
weather monitoring station. One of the pieces of data a weather station might store for
later retrieval is light levels. This can give an indication of cloud cover at times of day,
and some studies use it to monitor the effects of pollution and airplane condensation trails
(con trails) on light levels that reach the Earth’s surface.
Page 206 · What’s a Microcontroller?
With light level tracking in mind, this activity introduces a technique for storing
measured light levels to the EEPROM and then retrieving them again. In this activity,
you will run one PBASIC example program that stores a series of light measurements in
the BASIC Stamp module’s EEPROM. After that program is finished, you will run a
second program that retrieves the values from EEPROM and displays them in the Debug
Terminal.
Programming Long Term Data Storage
The WRITE command is used to store values in the EEPROM, and the READ command is
used to retrieve those values.
The syntax for the WRITE command is:
WRITE Location, {WORD} Value
For example, if you want to write the value 195 to address 7 in the EEPROM, you could
use the command:
WRITE 7, 195
Word values can be anywhere from 0 to 65565 while byte values can only contain
numbers from 0 to 255. A word value takes the space of two bytes. If you want to write
a word size value to EEPROM, you have to use the optional Word modifier. Be careful
though. Since a word takes two bytes, you have to skip one of the byte size addresses in
EEPROM before you can write another word. Let’s say you need to save two word
values to EEPROM: 659 and 50012. If you want to store the first value at address 8, you
will have to write the second value to address 10.
WRITE 8, Word 659
WRITE 10, Word 50012
Measuring Light · Page 207
Is it possible to write over your program? Yes, and if you do, the program is likely to
either start behaving strangely or stop running altogether. Since the PBASIC program
tokens reside in the largest EEPROM addresses, it’s best to use the smallest Location
values for storing numbers with the WRITE command.
How do I know if the Location I’m using is too large? You can use the Memory Map to
figure out the largest value not used by your PBASIC program. The explanation after Figure
7-6 on page 204 describes how to calculate how many memory addresses are available. As
a shortcut for converting hexadecimal to decimal, you can use the DEBUG command’s DEC
formatter and the $ hexadecimal formatter like this:
DEBUG DEC $7DC
Your program will display the decimal value of hexadecimal 7DC because the $
hexadecimal formatter tells the DEBUG command that 7DC is a hexadecimal number. Then,
the DEC formatter makes the DEBUG command display that value in decimal format.
Example Program: StoreLightMeasurementsInEeprom.bs2
This example program demonstrates how to use the WRITE command by taking light
measurements every 5 seconds for 2 ½ minutes and storing them in EEPROM. Like the
previous activity’s example program, it displays the measurements in the Debug
Terminal, but it also stores each measurement in EEPROM for later retrieval by a
different program that uses the READ command.
9 Enter and run StoreLightMeasurementsInEeprom.bs2.
9 Record the measurements displayed by the Debug Terminal so that you can
verify the measurements read back from the EEPROM.
9 Gradually increase the shade over the phototransistor during the 2 ½ minute test
period for meaningful data.
Especially if you have a USB board, reconnecting it to the computer could reset the
BASIC Stamp and restart the program, in which case, it would start taking a new set of
measurements.
9 After StoreLightMeasurementsInEeprom.bs2 has completed, disconnect power
from your board immediately and leave it disconnected until you are ready to run
the next example program: ReadLightMeasurementsFromEeprom.bs2.
Page 208 · What’s a Microcontroller?
You can change the pauses in the FOR…NEXT loop. This example program has 5 second
pauses, which emphasize the periodic measurements that datalogging devices take. They
might seem kind of long, so, you can reduce the PAUSE 5000 to PAUSE 500 to make the
program execute ten times more quickly for testing.
' What's a Microcontroller - StoreLightMeasurementsInEeprom.bs2
' Write light measurements to EEPROM.
' {$STAMP BS2}
' {$PBASIC 2.5}
time
eepromAddress
VAR
VAR
Word
Byte
PAUSE 1000
DEBUG "Starting measurements...", CR, CR
"Measurement
Value", CR,
"---------------", CR
FOR eepromAddress = 0 TO 58 STEP 2
HIGH 2
PAUSE 5000
RCTIME 2, 1, time
DEBUG DEC2 eepromAddress,
"
", DEC time, CR
WRITE eepromAddress, Word time
NEXT
DEBUG "All done. Now, run:", CR,
"ReadLightMeasurementsFromEeprom.bs2"
END
How StoreLightMeasurementsInEeprom.bs2 Works
The FOR...NEXT loop that measures the RC-time values and stores them to EEPROM
has to count in steps of 2 because word values are written into the EEPROM.
FOR eepromAddress = 0 to 58 STEP 2
The RCTIME command loads the decay time measurement into the word size time
variable.
RCTIME 2, 1, time
The value the time variable stores is copied to the EEPROM address given by the current
value of the eepromAddress variable each time through the loop. Remember, that the
address for a WRITE command is always in terms of bytes. So, the eepromAddress
Measuring Light · Page 209
variable is incremented by two each time through the loop because a Word variable takes
up two bytes.
WRITE eepromAddress, Word time
NEXT
Programming Data Retrieval
To retrieve the values you just recorded from EEPROM, you can use the READ command.
The syntax for the READ command is:
READ Location, {WORD} Variable
While the WRITE command can copy a value from either a constant or a variable to
EEPROM, the READ command has to copy the value stored at an address in EEPROM to
a variable, so as its name suggests, the Variable argument has to be a variable.
Keep in mind that variables are stored in the BASIC Stamp module’s RAM. Unlike
EEPROM, RAM values get erased whenever the power disconnected and also whenever
the Reset button on your board gets pressed.
The BASIC Stamp 2 has 26 bytes of RAM, shown on the right side of the Memory Map in
Figure 7-6 on page 204. If you declare a word variable, you are using up two bytes. A byte
variable declaration uses one byte, a nibble uses half a byte, and one bit uses 1/8 of a byte.
Let’s say that eepromValueA and eepromValueB are Word variables, and littleEE is a
Byte variable. These variables would have to be defined at the beginning of a program
with VAR variable declarations. Here are some commands to retrieve the values that were
stored at certain EEPROM addresses earlier with WRITE commands, maybe even in a
different program.
READ 7, littleEE
READ 8, Word eepromValueA
READ 10, Word eepromValueB
The first command retrieves a byte value from EEPROM address 7 and copies it to the
variable named littleEE. The next command copies the word occupying EEPROM
byte addresses 8 and 9 and stores it in the eepromValueA word variable. The last of the
three commands copies the word occupying EEPROM byte addresses 10 and 11 and
stores it in the eepromValueB variable.
Page 210 · What’s a Microcontroller?
Example Program: ReadLightMeasurementsFromEeprom.bs2
This example program demonstrates how to use the READ command to retrieve the light
measurements that were stored in EEPROM by StoreLightMeasurementsInEeprom.bs2.
9 Reconnect power to your board.
9 Enter ReadLightMeasurementsFromEeprom.bs2 into the BASIC Stamp Editor.
9 If you have disconnected power from your board, when you reconnect,
immediately click the BASIC Stamp Editor’s Run button to download the
program into the BASIC Stamp.
Don’t wait longer than 6 seconds between reconnecting power and loading
ReadLightMeasurementsFromEeprom.bs2 into the BASIC Stamp or the program that’s still
in the program memory (StoreLightMeasurementsInEeprom.bs2) will start recording over
previous measurements. Also, if you reduced the PAUSE command’s Duration from
5000 to 500, you will only have 1.5 seconds!
9 Compare the Debug Terminal table that is displayed by this program with the
one displayed by StoreLightMeasurementsInEeprom.bs2, and verify that the
values are the same.
' What's a Microcontroller - ReadLightMeasurementsFromEeprom.bs2
' Read light measurements from EEPROM.
' {$STAMP BS2}
' {$PBASIC 2.5}
time
eepromAddress
VAR
VAR
Word
Byte
PAUSE 1000
DEBUG "Retrieving measurements", CR, CR,
"Measurement
Value", CR,
"--------------", CR
FOR eepromAddress = 0 TO 58 STEP 2
READ eepromAddress, Word time
DEBUG DEC2 eepromAddress, "
NEXT
END
", DEC time, CR
Measuring Light · Page 211
How ReadLightMeasurementsFromEeprom.bs2 Works
As with the WRITE command, the READ command relies on byte addresses. Since we
want to read word values from EEPROM, the eepromAddress variable has to have 2
added to it each time through the FOR...NEXT loop.
FOR eepromAddress = 0 to 58 STEP 2
The READ command gets the word size value at eepromAddress, and the value gets
copied to the time variable.
READ eepromAddress, Word time
The values of the time and eepromAddress variables are displayed in adjacent columns
as a table in the Debug Terminal.
DEBUG DEC2 eepromAddress, "
NEXT
", DEC time, CR
Your Turn – More Measurements
9 Modify StoreLightMeasurementsInEeprom.bs2 so that it takes and records twice
as many measurements in the same amount of time.
9 Modify ReadLightMeasurementsFromEeprom.bs2 so that it displays all of the
measurements from the just-modified StoreLightMeasurementsInEeprom.bs2.
ACTIVITY #3: GRAPHING LIGHT MEASUREMENTS (OPTIONAL)
Lists of measurements like the ones in Activity #2 can be tedious to analyze. Imagine
reading through hundreds of those numbers looking for when the sun set. Or maybe
you’re looking for a particular event, like when the light sensor was briefly covered. You
might even be looking for a pattern in how frequently the light sensor was covered. This
information could be useful if the light sensor is placed in an area where a person or
animal walks over it, or an object passing over it on a conveyer belt needs to be recorded
and analyzed. Regardless of the application, if all you have to work with is a long list of
numbers, finding those events and patterns can be a difficult and time-consuming task.
If you instead make a graph of the list of measurements, it makes finding events and
patterns a lot easier. The person, animal or object passing over the light sensor will show
up as a high point or spike in the measurements. Figure 7-7 shows an example of a graph
that might indicate that rate at which objects on a conveyer belt are passing over the
sensor. The spikes in the graph occur when the measurements get large. In the case of a
Page 212 · What’s a Microcontroller?
conveyer belt, it would indicate that an object passed over the sensor casting a shadow.
This graph makes it easy to see at a glance that an object passes over the sensor roughly
every 7 seconds, but that the object we were expecting at 28 seconds was wasn’t there.
Figure 7-7 Graph of Phototransistor Light Measurements
Decay Time Vs. Time
for Phototransistor RC Circuit
9000
8000
Decay Time (2 us)
7000
6000
5000
4000
3000
2000
1000
0
0
"Decay Time"
10
20
30
40
50
60
Tim e (s)
The graph in Figure 7-7 was generated by copying and pasting values in the Debug
Terminal to a text file which was then imported into a Microsoft Excel spreadsheet.
Some graphing utilities can take the place of the Debug Terminal and plot the values
directly instead of displaying them as lists of numbers. Figure 7-8 shows an example of
one of these utilities, called StampPlot LITE.
Measuring Light · Page 213
Figure 7-8 StampPlot LITE
In this optional activity, you can go to www.parallax.com/go/WAM and then follow the
Data Plotting link to try a number of activities that demonstrate how to plot values using
various spreadsheets and graphing utility software packages.
Page 214 · What’s a Microcontroller?
ACTIVITY #4: SIMPLE LIGHT METER
Light sensor information can be communicated in a variety of ways. The light meter you
will work with in this activity changes the rate that the display flickers depending on the
light intensity it detects.
Light Meter Parts
(1) Phototransistor
(1) Resistor – 220 Ω (red-red-brown)
(2) Capacitors – 0.01 μF (labeled 103)
(1) Capacitor – 0.1 μF (labeled 104)
(1) 7-segment LED display
(8) Resistors – 1 kΩ (brown-black-red)
(6) Jumper wires
Building the Light Meter Circuit
Figure 7-9 shows the 7-segment LED display and phototransistor circuit schematics that
will be used to make the light meter, and Figure 7-10 shows a wiring diagram of the
circuits. The phototransistor circuit is the same one you have been using in the last two
activities, and the 7-segment LED display circuit is the same one from Figure 6-11 on
page 176.
9 Build the circuit shown in Figure 7-9 and Figure 7-10.
9 Test the 7-segment LED display to make sure it is connected properly, using the
program SegmentTestWithHighLow.bs2 from Chapter 6, Activity #2, which
starts on page 175.
Measuring Light · Page 215
Figure 7-9
Light Meter Circuit
Schematic
Figure 7-10
Wiring Diagram for
Figure 7-9
Page 216 · What’s a Microcontroller?
Using Subroutines
Most of the programs you have written so far operate inside a DO...LOOP. Since the
entire program’s main activity happens inside the DO...LOOP, it is usually called the
main routine. As you add more circuits and more useful functions to your program, it can
get kind of difficult to keep track of all the code in the main routine. Your programs will
be much easier to work with if you instead organize them into smaller segments of code
that do certain jobs. PBASIC has some commands that you can use to make the program
jump out of the main routine, do a job, and then return right back to the same spot in the
main routine. This will allow you to keep each segment of code that does a particular job
somewhere other than your main routine. Each time you need the program to do one of
those jobs, you can write a command inside the main routine that tells the program to
jump to that job, do it, and come back when the job is done. The jobs are called
subroutines and this process is calling a subroutine.
Figure 7-11 shows an example of a subroutine and how it’s used. The command GOSUB
Subroutine_Name causes the program to jump to the Subroutine_Name: label. When
the program gets to that label, it keeps running and executing commands until it gets to a
RETURN command. Then, the program goes back to command that comes after the
GOSUB command. In the case of the example in Figure 7-11, the next command is:
DEBUG "Next command".
DO
GOSUB Subroutine_Name
DEBUG "Next command"
LOOP
Subroutine_Name:
DEBUG "This is a subroutine..."
PAUSE 3000
RETURN
Figure 7-11
How Subroutines Work
Measuring Light · Page 217
What’s a label? A label is a name that can be used as a placeholder in your program.
GOSUB is one of the commands you can use to jump to a label. Some others are GOTO, ON
GOTO, and ON GOSUB. A label must end with a colon, and for the sake of style, separate
words with the underscore character so they are easy to recognize. When picking a name
for a label, make sure not to use a reserved word or a name that is already used in a
variable or constant. The rest of the rules for a label name are the same as the ones for
naming variables, which are listed in the information box on page 43.
Example Program: SimpleSubroutines.bs2
This example program shows how subroutines work by sending messages to the Debug
Terminal.
9 Examine SimpleSubroutines.bs2 and try to guess the order in which the DEBUG
commands will be executed.
9 Enter and run the program.
9 Compare the program’s actual behavior with your predictions.
' What's a Microcontroller - SimpleSubroutines.bs2
' Demonstrate how subroutines work.
' {$STAMP BS2}
' {$PBASIC 2.5}
PAUSE 1000
DO
DEBUG
PAUSE
GOSUB
DEBUG
PAUSE
GOSUB
DEBUG
PAUSE
CLS, "Start main routine.", CR
2000
First_Subroutine
"Back in main.", CR
2000
Second_Subroutine
"Repeat main...", CR
2000
LOOP
First_Subroutine:
DEBUG " Executing first "
DEBUG "subroutine.", CR
PAUSE 3000
RETURN
Page 218 · What’s a Microcontroller?
Second_Subroutine:
DEBUG " Executing second "
DEBUG "subroutine.", CR
PAUSE 3000
RETURN
How SimpleSubroutines.bs2 Works
Figure 7-12 shows how the First_Subroutine call in the main routine (the DO...LOOP)
works.
The command GOSUB First_Subroutine sends the program to the
First_Subroutine: label. Then, the three commands inside that subroutine are
executed. When the program gets to the RETURN command, it jumps back to the
command that comes right after GOSUB First_Subroutine, which is DEBUG "Back in
Main.", CR.
What’s a subroutine call? When you use the GOSUB command to make the program jump
to a subroutine, it is called a subroutine call.
PAUSE 2000
GOSUB First_Subroutine
DEBUG "Back in main.", CR
First_Subroutine:
DEBUG "
Executing first "
Figure 7-12
First Subroutine Call
DEBUG "subroutine.", CR
PAUSE 3000
RETURN
Figure 7-13 shows a second example of the same process with the second subroutine call
(GOSUB Second_Subroutine).
Measuring Light · Page 219
PAUSE 2000
GOSUB Second_Subroutine
DEBUG "Repeat main...", CR
Second_Subroutine:
DEBUG "
Executing second "
Figure 7-13
Second Subroutine Call
DEBUG "subroutine", CR
PAUSE 3000
RETURN
Your Turn – Adding and Nesting Subroutines
You can add subroutines after the two that are in the program and call them from within
the main routine.
9 Add the subroutine shown in Figure 7-11 on page 216 to SimpleSubroutines.bs2.
9 Make any necessary adjustments to the DEBUG commands so that the display
looks right with all three subroutines.
You can also call one subroutine from within another. This is called nesting subroutines.
9 Try moving the GOSUB command that calls Subroutine_Name into one of the
other subroutines, and see how it works.
When nesting subroutines the rule is no more than four deep. See the BASIC Stamp
Manual or the BASIC Stamp Editor’s Help for more information. Look up GOSUB and
RETURN.
Light Meter Using Subroutines
The next program, LightMeter.bs2 uses subroutines to control the display of the 7Segment LED depending on the level of light detected by the phototransistor. The
display’s segments cycle on and off in a circular pattern that gets faster when the light on
the phototransistor gets brighter. When the light gets dimmer, the circular pattern cycling
goes slower.
The LightMeter.bs2 example program uses a subroutine named
Update_Display to control the order in which the light meter segments advance.
Page 220 · What’s a Microcontroller?
The program that runs the light meter will deal with three different operations:
1. Read the phototransistor.
2. Calculate how long to wait before updating the 7-segment LED display.
3. Update the 7-segment LED display.
Each operation is contained within its own subroutine, and the main DO...LOOP routine
will call each one in sequence, over and over again.
Example Program: LightMeter.bs2
Controlled lighting conditions make a big difference! For best results, conduct this test
in a room lit by fluorescent lights with little or no direct sunlight (close the blinds). For
information on how to calibrate this meter to other lighting conditions, see the Your Turn
section.
9 Enter and run LightMeter.bs2.
9 Verify that the cycling speed of the circular pattern displayed by the 7-segment
LED is controlled by the lighting conditions the phototransistor is sensing. Do
this by casting a shadow over it with your hand or a piece of paper and verify
that the rate of the circular display pattern slows down.
' What's a Microcontroller - LightMeter.bs2
' Indicate light level using 7-segment display.
' {$STAMP BS2}
' {$PBASIC 2.5}
PAUSE 1000
DEBUG "Program Running!"
index
time
VAR
VAR
OUTH = %00000000
DIRH = %11111111
Nib
Word
' Variable declarations.
' Initialize 7-segment display.
Measuring Light · Page 221
DO
' Main routine.
GOSUB Get_Rc_Time
GOSUB Delay
GOSUB Update_Display
LOOP
' Subroutines
Get_Rc_Time:
' RC-time subroutine
HIGH 2
PAUSE 3
RCTIME 2, 1, time
RETURN
Delay:
' Delay subroutine.
PAUSE time / 3
RETURN
Update_Display:
' Display updating subroutine.
IF index = 6 THEN index = 0
'
BAFG.CDE
LOOKUP index, [ %01000000,
%10000000,
%00000100,
%00000010,
%00000001,
%00100000 ], OUTH
index = index + 1
RETURN
How LightMeter.bs2 Works
The first two lines of the program declare variables. It doesn’t matter whether these
variables are used in subroutines or the main routine, it’s always best to declare variables
(and constants) at the beginning of your program.
Since this is such a common practice, this section of code has a name, “Variable
declarations.” This name is shown in the comment to the right of the first variable
declaration.
index VAR Nib
time VAR Word
' Variable declarations.
Page 222 · What’s a Microcontroller?
Many programs also have things that need to get done once at the beginning of the
program. Setting all the 7-segment I/O pins low and then making them outputs is an
example. This section of a PBASIC program also has a name, “Initialization.”
OUTH = %00000000
DIRH = %11111111
' Initialize 7-segment display.
The next segment of code is called the main routine. The main routine calls the
Get_Rc_Time subroutine first. Then, it calls the Delay subroutine, and after that, it calls
the Update_Display subroutine. Keep in mind that the program goes through the three
subroutines as fast as it can, over and over again.
DO
GOSUB Get_Rc_Time
GOSUB Delay
GOSUB Update_Display
LOOP
' Main routine.
All subroutines are usually placed after the main routine. The first subroutine’s name is
Get_Rc_Time:, and it takes the RC-time measurement on the phototransistor circuit.
This subroutine has a PAUSE command that charges up the capacitor. The Duration of this
command is small because it only needs to pause long enough to make sure the capacitor
is charged. Note that the RCTIME command sets the value of the time variable. This
variable will be used by the second subroutine.
Get_Rc_Time:
HIGH 2
PAUSE 3
RCTIME 2, 1, time
RETURN
' Subroutines
' RC-time subroutine
The second subroutine’s name is Delay, and all it contains is PAUSE time / 3. The
PAUSE command allows the measured decay time (how bright the light is) to control the
delay between turning on each light segment in the 7-segment LED’s revolving circular
display. The value to the right of the divide / operator can be made larger for faster
rotation in lower light conditions or smaller to slow the display for brighter light
conditions. You could also use * to multiply the time variable by a value instead of
dividing to make the display go a lot slower, and for more precise control over the rate,
don’t forget about the */ operator. More on this operator in the Your Turn section.
Measuring Light · Page 223
Delay:
PAUSE time / 3
RETURN
The third subroutine is named Update_Display. The LOOKUP command in this
subroutine contains a table with six bit patterns that are used to create the circular pattern
around the outside of the 7-segment LED display. By adding 1 to the index variable
each time the subroutine is called, it causes the next bit pattern in the sequence to get
placed in OUTH. There are only six entries in the LOOKUP table for index values from 0
through 5. What happens when the value of index gets to 6? The LOOKUP command
doesn’t automatically know to go back to the first entry, but you can use an IF...THEN
statement to fix that problem. The command IF index = 6 THEN index = 0 resets
the value of index to 0 each time it gets to 6. It also causes the sequence of bit patterns
placed in OUTH to repeat itself over and over again. This, in turn, causes the 7-segment
LED display to repeat its circular pattern over and over again.
Update_Display:
IF index = 6 THEN index = 0
'
BAFG.CDE
LOOKUP index, [ %01000000,
%10000000,
%00000100,
%00000010,
%00000001,
%00100000 ], OUTH
index = index + 1
RETURN
Your Turn – Adjusting the Meter’s Hardware and Software
There are two ways to change the sensitivity of the meter. First the “software,” which is
the PBASIC program, can be changed to adjust the speed. As mentioned earlier, dividing
the time variable in the Delay subroutine’s PAUSE time / 3 command by numbers
larger than 3 will speed up the display, and smaller numbers will slow it down. To really
slow it down, time can also be multiplied by values with the multiply * operator, and for
fine tuning, there’s the */ operator.
When you connect capacitors in parallel, their values add up. So, if you plug in a second
0.01 μF capacitor right next to the first one as shown in Figure 7-14 and Figure 7-15, the
Page 224 · What’s a Microcontroller?
capacitance will be 0.02 μF. With twice the capacitance, the decay measurement for the
same light level will take twice as long.
9 Connect the second 0.01 μF capacitor right next to the first one in the light
sensor portion of the light meters circuit in Figure 7-14 and Figure 7-15.
9 Run LightMeter.bs2 and observe the result.
Since the time measurements will be twice as large, the 7-segment LED’s circular pattern
should rotate half as fast.
Figure 7-14
Two 0.01 μF Capacitors
in Parallel = 0.02 μF
Figure 7-15
Light Meter Circuits with
Two 0.01 μF Capacitors
in Parallel
Instead of half the speed of a 0.01 μF capacitor, how about one tenth the speed? You can
do this by replacing the two 0.01 μF capacitors with a 0.1 μF capacitor. It will work okay
in brightly lit rooms, but will likely be a little slow for normal lighting. Remember that
when you use a capacitor that is ten times as large, the RC-time measurement will take
ten times as long.
Measuring Light · Page 225
9 Replace the 0.01 µF capacitors with a 0.1 µF capacitor.
9 Run the program and see if the predicted effect occurred.
9 Before continuing, restore the circuit to one 0.01 µF capacitor in parallel with
the phototransistor as shown in Figure 7-9 and Figure 7-10, starting on page 215.
9 Test your restored circuit to verify that it works before continuing.
Which is better, adjusting the software or the hardware? You should always try to use
the best of both worlds. Pick a capacitor that gives you the most accurate measurements
over the widest range of light levels. Once your hardware is the best it can be, use the
software to automatically adjust the light meter so that it works well for the user under the
widest range of conditions. This takes a considerable amount of testing and refinement, but
that’s all part of the product design process.
ACTIVITY #5: ON/OFF PHOTOTRANSISTOR OUTPUT
Before microcontrollers were common in products, photoresistors were used in circuits
that varied in their voltage output. When the voltage passed below a threshold value
indicating nighttime, other circuits in the device turned the lights on. When the voltage
passed above the threshold, indicating daytime, the other circuits in the device turned the
lights off. This binary light switch behavior can be emulated with the same BASIC
Stamp and the RC decay circuit by simply modifying the PBASIC program.
Alternatively, the circuit can be modified so that it sends a 1 or 0 to an I/O pin depending
on the amount of voltage supplied to the pin, similar to the way a pushbutton does. In
this activity, you will try both these approaches.
Adjusting the Program for On/Off States
PhototransistorAnalogToBinary.bs2 takes the range of phototransistor measurements and
compares it to the half way point between the largest and smallest measurements. If the
measurement is above the half way point, it displays “Turn light on”; otherwise, it
displays “Turn light off.” The program uses constant directives to define the largest and
smallest measurements the program should expect from the phototransistor circuit.
valMax
valMin
CON
CON
4000
100
Page 226 · What’s a Microcontroller?
The program also uses MIN and MAX operators to ensure that values stay within these
limits before using them to make any decisions. If time is greater than valMax (4000 in
the example program), the statement sets time to valMax = 4000. Likewise if time is
less than valMin (100 in the example program), the statement sets time to valMin =
100.
time = time MAX valMax MIN valMin
An IF...THEN...ELSE statement converts the range of digitized analog values into a
binary output that takes the form of light-on or light-off messages.
IF time > (valMax - valMin) / 2 THEN
DEBUG CR, "Turn light on "
ELSE
DEBUG CR, "Turn light off"
ENDIF
Before this program will work properly, you have to calibrate your lighting conditions as
follows:
9 Check your phototransistor circuit to make sure it has just one 0.01 μF capacitor
(labeled 103).
9 Enter PhototransistorAnalogToBinary.bs2 into the BASIC Stamp Editor. Make
sure to add an extra space after the "n" in the "Turn light on " message.
Otherwise, you’ll get a phantom "f" from the "Turn light off" message, which
has one more character in it.
9 Load the program into the BASIC Stamp.
9 Watch the Debug Terminal as you apply the darkest and brightest lighting
conditions that you want to test, and make notes of the resulting maximum and
minimum time values.
9 Enter those values into the program’s valMax and valMin CON directives.
Now, your program is ready to run and test.
9 Load the modified program into the BASIC Stamp.
9 Test to verify that dim lighting conditions result in the “Turn Light on” message
and bright lighting conditions result in the “Turn light off” message.
Measuring Light · Page 227
' What's a Microcontroller - PhototransistorAnalogToBinary.bs2
' Change digitized analog phototransistor measurement to a binary result.
' {$STAMP BS2}
' {$PBASIC 2.5}
valMax
valMin
CON
CON
4000
100
time
VAR
Word
PAUSE 1000
DO
HIGH 2
PAUSE 100
RCTIME 2, 1, time
time = time MAX valMax MIN valMin
DEBUG HOME, "time
IF time > (valMax
DEBUG CR, "Turn
ELSE
DEBUG CR, "Turn
ENDIF
= ", DEC5 time
- valMin) / 2 THEN
light on "
light off"
LOOP
Your Turn – Different Thresholds for Light and Dark
If you try to incorporate PhototransistorAnalogToBinary.bs2 into an automatic lighting
system, it has a potential defect. Let’s say it’s dark enough outside to cause the time
measurement to pass above (valMax – valMin) / 2, so the light turns on. But if the
sensor detects that light, it would cause the measurement to pass back below (valMax –
valMin) / 2, so the light would turn off again. This lights-on/lights-off cycle could
repeat rapidly all night!
Page 228 · What’s a Microcontroller?
Figure 7-16 shows how this could work in a graph. As the light level drops, the value of
the time variable increases, and when it crosses the threshold, the automatic lights turn
on. Then, since the phototransistor senses the light that just turned on, the time
variable’s measurement drops back below the threshold, so the lights turn off. Then, the
time variable’s value increases again, and it passes above the threshold, so the lights turn
on, and the time variable drops below the threshold again, and so on…
valMax
"Turn light on "
(valMax - valMin) / 2
"Turn light off"
Figure 7-16
Oscillations
Above/Below a
Threshold
valMin
One remedy for this problem is to add a second threshold, as illustrated in Figure 7-17.
The “Turn light on” threshold only turns the light on after it’s gotten pretty dark, and the
“Turn light off” threshold only turns the light back off after it’s gotten pretty bright.
With this system, the light comes on after time passed into the “Turn light on” range.
The light turning on made it brighter, so time dropped slightly, but since it didn’t fall
clear down past the “Turn light off” threshold, nothing changed, and the light stays on as
it should. The term hysteresis is used to describe this type of system, which has two
different input thresholds that affect its output along with a no-transition zone between
them.
valMax
"Turn light on "
(valMax - valMin) / 4 * 3
No transition
(valMax - valMin) / 4
"Turn light off"
valMin
Figure 7-17
Using Different High
and Low Thresholds
to Prevent
Oscillations
Measuring Light · Page 229
You can implement this two-threshold system in your PBASIC code by modifying
PhotransistorAnalogToBinary.bs2’s IF...THEN...ELSEIF statement.
Here is an
example:
IF time > (valMax - valMin) / 4 * 3 THEN
DEBUG CR, "Turn light on "
ELSEIF time < (valMax - valMin ) / 4 THEN
DEBUG CR, "Turn light off"
ENDIF
The first IF...THEN code block displays "Turn lights on " when the time variable stores
a value that’s more than ¾ of the way to the highest time (lowest light) value. The
ELISIF code block only displays "Turn lights off" when the time variable stores a value
that’s less than ¼ of the way above the smallest time (brightest) value.
9 Save PhototransistorAnalogToBinary.bs2 as PhotransistorHysteresis.bs2.
9 Before modifying PhotransistorHysteresis.bs2, test it to make sure the existing
threshold works. If the lighting has changed, repeat valMin and valMax
calibration steps (before the PhototransistorAnalogToBinary.bs2 example code).
9 Replace PhotransistorHysteresis.bs2’s IF...ELSE...ENDIF statement with the
IF...ELSEIF...ENDIF just discussed.
9 Load the PhotransistorHysteresis.bs2 into the BASIC Stamp.
9 Test and verify that the “Turn light on” threshold is darker, and the “Turn light
off” threshold is lighter.
If you add an LED circuit and modify the code so that it turns the LED on and off, some
interesting things might happen. Especially if you put the LED right next to the
phototransistor, you might still see that on/off behavior when it gets dark, even with the
hysteresis programmed. How far away from the phototransistor does the LED have to be
to get the two thresholds to prevent the on/off behavior? Assuming that valMin and
valMax are the same in both programs, how much farther away does the LED have to be
for the unmodified PhototransistorAnalogToBinary.bs2 to work properly?
Page 230 · What’s a Microcontroller?
TTL Vs. Schmitt Trigger
Your BASIC Stamp I/O pin sends and receives signals using transistor-transistor logic (TTL).
As an output, the I/O pin sends a 5 V high signal or a 0 V low signal. The left side of Figure
7-18 shows how the I/O pin behaves as an input. The I/O pin’s IN register (IN0, IN1, IN2,
etc.) stores a 1 if the voltage applied is above 1.4 V, or a 0 if it’s below 1.4 V. These are
shown as Logic 1 and Logic 0 in the figure.
A Schmitt trigger is a circuit represented by the symbol in the center of Figure 7-18. The
right side of Figure 7-18 shows how an I/O pin set to input would behave if it had a Schmitt
trigger circuit built-in. Like the PBASIC code with two thresholds, the Schmitt trigger has
hysteresis. The input value stored by the I/O pin’s INx register doesn’t change from 0 to 1
until the input voltage goes above 4.25 V. Likewise, it doesn’t change from 1 to 0 until the
input voltage passes below 0.75 V. The BASIC Stamp 2px has a PBASIC command that
allows you to configure its input pins to Schmitt trigger.
Figure 7-18 TTL Vs. Schmitt Trigger Thresholds and Symbol
TTL
Threshold
Schmitt Trigger
Symbol
5V
Schmitt Trigger
Threshold
5V
≈4.25 V
Logic 1
Logic 1
No Change
≈1.4 V
Logic 0
0V
≈0.75 V
0V
Logic 0
Adjusting the Circuit for On/Off States
As mentioned in Chapter 5, Activity #2, the voltage threshold for a BASIC Stamp I/O pin
is 1.4 V. When an I/O pin is set to input, voltages above 1.4 V applied to the I/O pin
result in a binary 1 and voltages below 1.4 V result in a binary 0. The Vo node in the
circuit shown in Figure 7-19 varies in voltage with light. This circuit can be connected to
a BASIC Stamp I/O pin, and with low light, the voltage will pass below the BASIC
Stamp’s 1.4 V threshold, and the I/O pin’s input register will store a 0. In bright light
conditions, Vo rises above 1.4 V, and the I/O pin’s input register will store a 1.
Measuring Light · Page 231
Figure 7-19
Voltage Output Light
Circuit
+
V
–
=R
I
The reason the voltage at Vo changes with light levels is because of Ohm’s Law, which
states that the voltage across a resistor (V in Figure 7-19) is equal to the current passing
through that resistor (I) , multiplied by the resistor’s resistance (R).
V=I×R
Remember that a phototransistor lets more current pass through when exposed to more
light, and less current when exposed to less light. Let’s take a closer look at the example
circuit in Figure 7-19 and calculate how much current would have to pass though the
resistor to create a 1.4 V drop across the resistor. First, we know the value of the resistor
is 10 kΩ, or 10,000 Ω. We also know that we want the voltage to be equal to 1.4 V, so
we need to modify Ohm’s Law to solve for I. To do this, divide both sides of the
V = I × R equation by R, which results in I = V ÷ R. Then, substitute the values you
know (V = 1.4 V and R = 10 kΩ), and solve for I.
I
=
=
=
=
=
=
V÷R
1.4 V ÷ 10 kΩ
1.4 V ÷ 10,000 Ω
0.00014 V/Ω
0.00014 A
0.14 mA
Page 232 · What’s a Microcontroller?
Now, what if the transistor allows twice that much current through because it’s bright,
and what would the voltage be across the resistor? For twice the current, I = 0.28 mA,
and the resistance is still 10 kΩ, so now we are back to solving V from the original V = I
× R equation with I = 0.28 mA and R = 10 kΩ:
V
=
=
=
=
=
I×R
0.28 mA × 10 kΩ
0.00028 A × 10,000 Ω
2.8 AΩ
2.8 V
With 2.8 V applied to an I/O pin, its input register would store a 1 since 2.8 V is above
the I/O pin’s 1.4 V threshold voltage.
Your Turn – More calculations
What if the phototransistor allowed half the threshold voltage current (0.07 mA) through
the circuit, what would the voltage across the resistor be? Also, what would the I/O pin’s
input register store?
Test the Binary Light Sensor
Testing the binary light sensor circuit is a lot like testing the pushbutton circuit from
Chapter 3. When the circuit is connected to an I/O pin, the voltage will either be above
or below the BASIC Stamp I/O pin’s 1.4 V threshold, which will result in a 1 or a 0,
which can then be displayed with the Debug Terminal.
Analog to Binary Light Sensor Parts
(1) Phototransistor
(1) Resistor – 220 Ω (red-red-brown)
(1) Resistor – 10 kΩ (brown-black-orange)
(1) Resistor – 2 kΩ (red-black-red)
(1) Resistor – 4.7 kΩ (yellow-violet-red)
(1) Resistor – 100 kΩ (brown-black-yellow)
(2) Jumper wires
Measuring Light · Page 233
Analog to Binary Light Sensor Circuit
With the circuit shown in Figure 7-20, the circuit behaves like a shadow controlled
pushbutton. Shade results in in2 = 0, bright light results in in2 = 1. Keep in mind
that an I/O pin set to input does not affect the circuit it monitors because it doesn’t source
or sink any current. This makes both the I/O pin and 220 Ω resistor essentially invisible
to the circuit. So, the voltage results of our circuit calculations from the previous section
will be the same with or without the 220 Ω resistor and I/O pin connected.
9 Build the circuit shown in Figure 7-20.
Figure 7-20: Schematic and Wiring Diagram for Analog to Binary Light Sensor Circuit
connected to an I/O pin
Analog to Binary Light Sensor Test Code
TestBinaryPhototransistor.bs2 is a modified version of ReadPushbuttonState.bs2 from
Chapter 3, Activity #2. Aside from adjusting the comments, the one change to the actual
program is the DEBUG ? IN2 line, which was DEBUG ? IN3 in the pushbutton example
program because the pushbutton was connected to P3 instead of P2.
9 Review Chapter 3, Activity #2 (page 65).
9 Use TestBinaryPhototransistor.bs2 below to verify that bright light on the
phototransistor results in a 1 while darkness results in 0. You might need pretty
bright light. If your indoor lighting still results in a 0, try sunlight or a flashlight
up close. An alternate remedy for low lighting is to replace the 10 kΩ resistor
with a 100 kΩ resistor.
Page 234 · What’s a Microcontroller?
' What's a Microcontroller - TestBinaryPhototransistor.bs2
' Check the phototransistor circuit's binary output state every 1/4 second.
' {$STAMP BS2}
' {$PBASIC 2.5}
DO
DEBUG ? IN2
PAUSE 250
LOOP
Testing Series Resistance
Take a look at the V = I × R calculations earlier in this activity. If the series resistor is
1/5 the value, the voltage across the resistor will be 1/5th the value for the same light
conditions. Likewise, a resistor that is 10 times as large will cause the voltage to be ten
times as large.
What does this do for your circuit? A 100 kΩ resistor in place of a 10 kΩ resistor means
the phototransistor only has to conduct 1/10th the current to cross the BASIC Stamp I/O
pin’s 1.4 V threshold, which in turn means it takes less light to get trigger a binary 1 in
the I/O pin’s input register. This might work as a sensor in an environment that is
supposed to stay dark since it will be sensitive to small amounts of light. In contrast, 1/5
the resistance value means that the phototransistor has to conduct 5 times as much current
to get the voltage across the resistor to cross the 1.4 V threshold, which in turn means that
it takes more light to trigger the binary 1 in the I/O pin’s input register. So, this circuit
would be better for detecting brighter light.
9 Experiment with 2 kΩ, 4.7 kΩ, 10 kΩ, and 100 kΩ resistors and compare the
changes in sensitivity to light with each resistor.
Your Turn – Low Light Level Indicator
9 Choose a resistor with the best 1/0 response to low light levels in your work area.
9 Add the LED featured in Chapter 3, Activity #3 to your phototransistor threshold
circuit.
9 Put something between the LED and phototransistor so that the phototransistor
cannot “see” the LED. This eliminates potential crosstalk between the two
devices.
9 Modify the program so that it makes the light blink when a shadow is cast over
the phototransistor.
Measuring Light · Page 235
ACTIVITY #6: FOR FUN—MEASURE OUTDOOR LIGHT WITH AN LED
As mentioned earlier, the circuit introduced in Activity #1 is designed for indoor light
measurements. What if your application needs to take light measurements outdoors?
One option would be to find a phototransistor that generates less current for the same
amount of light. Another option would be to use a one of the other light sensors in the
What’s a Microcontroller kit. They are disguised as LEDs, and they perform particularly
well for bright light measurements.
When electric current passes through the LED, it emits light, so what do you think
happens when light shines on an LED? Yes indeed, it can cause electric current to flow
through a circuit. Figure 7-21 shows an LED circuit for detecting light levels outdoors,
and in other very brightly light areas. While the phototransistor allows current to pass
through provided electrical pressure (voltage) is applied, the LED is more like a tiny solar
panel and it creates its own voltage to supply the current. As far as the RC decay circuit
is concerned, the result with an LED is about the same. The LED conducts more current
and drains the capacitor of its charge more quickly with more light, and it conducts less
current and drains the capacitor less quickly with less light.
Yellow
Figure 7-21
Schematic for LED in
Light-Sensing RC-Time
Circuit
Why is the LED plugged in backwards? In Chapter 2, the LED’s anode was connected to
the 220 Ω resistor, and the cathode was connected to ground. That circuit made the LED
emit light as a result of electric current passing through the LED when voltage was applied
to the circuit. When light is shining on the LED, it will create a small voltage that generates a
small current in the opposite direction. So, the LED has to be plugged in backwards so that
the current it conducts allows the capacitor to drain through it for RC decay measurements.
Page 236 · What’s a Microcontroller?
LED Light Sensor Parts
(1) LED – yellow
(1) LED – green
(1) LED – red
(1) Resistor – 220 Ω (red-red-brown)
(1) Jumper wire
LED Light Sensor Circuit
One major difference between the LED and phototransistor is that the LED conducts
much less current for the same amount of light, so it takes very bright light for the LED
to conduct enough current to discharge the capacitor quickly enough for the RCTIME
measurement. Remember that the maximum time measurement the RCTIME can measure
is 65535 × 2 μs ≈ 131 ms. So for good RC decay measurements with the BASIC Stamp,
a much smaller capacitor is needed. In fact, the circuit works better without any external
capacitor. The LED has a small amount of capacitance inside it, called junction
capacitance, and the metal clips that hold wires you plug into the breadboard also have
capacitance. Reason being, a capacitor is two metal plates separated by an insulator
called a dielectric. So two metal clips inside the breadboard separated by plastic and air
forms a capacitor. The combination of the LED’s junction capacitance and the
breadboard’s clip capacitance makes it so that you can use the LED without any external
capacitor, as shown in Figure 7-22.
9 Build the circuit shown in Figure 7-22 and Figure 7-23, using the yellow LED.
Make sure to observe the polarity shown in the figures!
Figure 7-22
LED RCTIME Circuit
Schematic
Measuring Light · Page 237
Flat spot and
shorter cathode pin
Figure 7-23
LED RCTIME Circuit
Wiring Diagram
Longer anode pin
Testing LED Light Sensor with Code
The LED light sensing circuit can be tested in a brightly lit room or outdoors during the
day. In a dimly lit room the measurement times are likely to exceed 65535, in which case
RCTIME will store zero in the result variable. For most situations, the code is the same
code from Activity #1, TestPhototransistor.bs2.
If you are in a brightly lit room try this:
9 Run TestPhototransistor.bs2 from Activity #1.
9 Point the LED at the brightest light source by facing your board toward it.
9 Gradually rotate the board away from the brightest light source in the room; the
values displayed by the Debug Terminal should get larger as the light gets
dimmer.
If you have a bright flashlight, try this:
9 Run TestPhototransistor.bs2 from Activity #1.
9 Eliminate most bright light sources such as sunlight streaming into the windows.
9 Turn on the flashlight and point it into the top of the LED at a distance of about 4
inches (about 10 cm). If possible, turn off some of the fluorescent lights so that
the ambient light levels are low.
9 Watch the measurements the Debug Terminal displays as you gradually increase
the distance of the flashlight from the top of the LED. It will allow you to
determine the flashlight’s distance from the LED.
Page 238 · What’s a Microcontroller?
If you are in a room with only fluorescent lights and no bright light sources:
9 Run TestPhototransistor.bs2 from Activity #1.
9 Eliminate most bright light sources such as sunlight streaming into the windows.
If possible, turn off some of the fluorescent lights so that the light levels are low.
9 Point the LED into your computer monitor so that it is almost touching the
monitor, and see if the measurements make it possible to distinguish between
various colors on the display.
Outdoor tests:
9
9
9
9
9
9
9
Run StoreLightMeasurementsInEeprom.bs2 from Activity #2.
Disconnect the programming cable and take your board outside.
Face your board so that the LED is pointing directly at the sun.
Press and release your board’s Reset button to restart the datalogging program.
Gradually rotate your board away from the sun over 2 ½ minutes.
Take your board back inside and reconnect to the PC.
Run ReadLightMeasurementsFromEeprom.bs2, and examine the light
measurements. Since you gradually turned the LED away from the sun,
successive measurements should get larger.
Your Turn – Can your BASIC Stamp Tell Red from Green?
In Figure 7-2, green is in the middle of the spectrum, and red is to the right. If you
download the color PDF version of this textbook from www.parallax.com, you can place
the green and then the red LED against the screen and record light measurements across
the color spectrum. Then, by comparing the lowest measurements with each LED, you
can detect whether the LED is placed against green or red on the screen.
9 Start with a green LED in the Figure 7-22 and Figure 7-23 light detection circuit.
9 Download the PDF version of What’s a Microcontroller? from
www.parallax.com/go/WAM.
9 Display the color spectrum shown in Figure 7-2 (page 197) on your monitor, and
zoom in on the image.
9 With the TestPhototransistor.bs2 program displaying measurements in the
Debug Terminal, hold your board so that the green LED’s dome is pointing
directly into the monitor over the color spectrum. For best results, the dome of
the LED should be just barely touching the monitor, and the light levels in the
room should be fairly low.
Measuring Light · Page 239
9 Slide the green LED slowly along the spectrum bar displayed on the monitor,
and note which color resulted in the lowest measurement.
9 Repeat with the red LED. Did the red LED report its lowest measurements over
red while the green LED reported its lowest measurements over green?
The lowest red LED measurements should occur over the red color on the display, and
the lowest measurements for the green LED should occur over green.
SUMMARY
This chapter introduced light sensors and described how they are used in a variety of
products. Different light sensors detect different kinds of light, and their datasheets
describe their sensitivities in terms of light’s wavelength. This chapter focused on the
phototransistor, a device that controls the current through its collector and emitter
terminals by the amount of light shining on its base terminal. Because light can control
the amount of current a phototransistor conducts, the technique for measuring the
position of a potentiometer’s knob in the Chapter 5 RC circuit also works for measuring
the light shining on a phototransistor. The time it takes for a capacitor to lose its charge
through the phototransistor results in an RCTIME measurement that provides a number
that corresponds to the brightness of the light shining on the phototransistor.
Datalogging by storing light measurements in the unused portion of the BASIC Stamp
module’s EEPROM program memory was also introduced. WRITE and READ commands
were used to store values to and retrieve values from the BASIC Stamp module’s
EEPROM. The volume of numbers involved in datalogging can be difficult to analyze,
but graphing the data makes it a lot easier to see patterns, trends and events. Logged data
can be transferred to conventional spreadsheets and graphed, and certain graphing
utilities can even stand in for the Debug Terminal, and plot the values the BASIC Stamp
sends instead of displaying them as text. A light meter example application was also
developed, which demonstrated how light measurements can be used to control another
process, in this case, the rate of a circular pattern displayed by a 7-segment LED. This
application also used subroutines to perform three different jobs for the light meter
application.
The BASIC Stamp can be programmed to convert an RC decay time measurement to a
binary value with IF…THEN statements. Additionally, the program can take a range of RC
decay measurements and apply hysteresis with a “light on” threshold that’s in the darker
range of measurements and a “light off” threshold that’s in the lighter range. This can
help prevent on/off oscillations that might otherwise occur when the sensor reports
Page 240 · What’s a Microcontroller?
darkness and the device turns on an area light. Without hysteresis, the device might
sense this light, turn back off, and repeat this cycle indefinitely.
A hardware approach to sensing on/off light states is applying power to the
phototransistor in series with a resistor. In keeping with Ohm’s Law, the amount of
current the phototransistor conducts affects the voltage across the resistor. This variable
voltage can be connected to an I/O pin, and will result in a binary 1 if the voltage is above
the I/O pin’s 1.4 V threshold, or a binary 0 if it’s below the threshold.
The LED (light emitting diode) that emits light when current passes through it also
behaves like a tiny solar panel when light strikes it, generating a small voltages which in
turn can cause electric current in circuits. The currents the LED generates are small
enough that a combination of the LED’s own junction capacitance and the capacitance
inherent to the clips inside the breadboard provides enough capacitance for an RC decay
circuit with no external capacitor. While the phototransistor in the What’s a
Microcontroller kit performs better indoors, the LED is great for outdoor and bright light
measurements.
Questions
1. What are some examples of automatic lighting applications that depend on
ambient light sensors?
2. What are some examples of products that respond to changes in the brightness of
the ambient light?
3. What wavelength range does the visible light spectrum fall into?
4. What are the names of the phototransistor’s terminals, and which one controls
how much current the device allows through?
5. What does EEPROM stand for?
6. How many bytes can the BASIC Stamp module’s EEPROM store? How many
bits can it store?
7. What command do you use to store a value in EEPROM? What command do
you use to retrieve a value from EEPROM? Which one requires a variable?
8. What is a label?
9. What is a subroutine?
10. What command is used to call a subroutine? What command is used to end a
subroutine?
Measuring Light · Page 241
Exercises
1. Draw the schematic of a phototransistor RC-time circuit connected to P5.
2. Modify TestPhototransistor.bs2 to so that it works on a circuit connected to P5
instead of P2.
3. Explain how you would modify LightMeter.bs2 so that the circular pattern
displayed by the 7-segment LED display goes in the opposite direction.
Projects
1. Make a small prototype of a system that automatically closes the blinds when it
gets too bright and opens them again when it gets less bright. Use the servo for a
mechanical actuator. Hint: For code, you can add two servo control commands
to PhototransistorAnalogToBinary.bs2, and change the PAUSE 100 command to
PAUSE 1. Make sure to follow the instructions in the text for calibrating for area
light conditions before you test.
2. For extra credit, enhance your solution to Project 1 by incorporating the
hysteresis modifications discussed in Activity #5.
Solutions
Q1. Car headlights, streetlights, and outdoor security lights that automatically turn on
when it’s dark.
Q2. Laptop displays and cameras with auto exposure.
Q3. 380 nm to 750 nm according to Figure 7-2 on page 197.
Q4. Collector, base, and emitter. The base controls how much current passes into the
collector and back out of the emitter.
Q5. Electrically Erasable Programmable Read-Only Memory.
Q6. 2048 bytes. 2048 x 8 = 16,384 bits.
Q7. To store a value – WRITE ; To retrieve a value – READ; The READ command
requires a variable.
Q8. A label is a name that can be used as a placeholder in a PBASIC program.
Q9. A subroutine is a small segment of code that does a certain job.
Q10. Calling: GOSUB; ending: RETURN
Page 242 · What’s a Microcontroller?
E1. Schematic based on Figure 7-4 on page 200, with P2 changed to P5.
E2. The required changes are very similar to those explained on page 200.
DO
HIGH 5
PAUSE 100
RCTIME 5, 1, time
DEBUG HOME, "time =
LOOP
", DEC5 time
E3. To go in the opposite direction, the patterns must be displayed in the reverse
order. This can be done by switching the patterns around inside the LOOKUP
statement, or by reversing the order they get looked up. Here are two solutions
made with alternative Update_Display subroutines.
Solution 1
Solution 2
Update_Display:
IF index = 6 THEN index = 0
'
BAFG.CDE
LOOKUP index, [ %01000000,
%10000000,
%00000100,
%00000010,
%00000001,
%00100000 ],
OUTH
index = index + 1
RETURN
Index = 5 '<<Add after Index variable
Update_Display:
'
BAFG.CDE
LOOKUP index, [ %01000000,
%10000000,
%00000100,
%00000010,
%00000001,
%00100000 ], OUTH
IF (index = 0) THEN
index = 5
ELSE
index = index - 1
ENDIF
RETURN
Measuring Light · Page 243
P1. Phototransistor from Figure 7-4 on page 200, servo schematic for your board
from Chapter 4, Activity #1.
' What's a Microcontroller - Ch07Prj01_Blinds_Control.bs2
' Control servo position with light.
' {$STAMP BS2}
' {$PBASIC 2.5}
valMax
valMin
time
CON
CON
VAR
4000
100
Word
PAUSE 1000
DO
HIGH 2
PAUSE 1
RCTIME 2, 1, time
DEBUG HOME, "time =
' PAUSE 100 -> PAUSE 1
", DEC5 time
time = time MAX valMax MIN valMin
IF time > (valMax - valMin) / 2 THEN
DEBUG CR, "Open blinds "
PULSOUT 14, 500
ELSE
DEBUG CR, "Close blinds"
PULSOUT 14, 1000
ENDIF
' Modify
' Add
' Modify
' Add
LOOP
P2. Hysteresis functionality added for extra credit:
' What's a Microcontroller - Ch07Prj02_Blinds_Control_Extra.bs2
' Control servo position with light including hysteresis.
' {$STAMP BS2}
' {$PBASIC 2.5}
valMax
valMin
CON
CON
4000
100
time
VAR
Word
PAUSE 1000
DO
Page 244 · What’s a Microcontroller?
HIGH 2
PAUSE 1
RCTIME 2, 1, time
DEBUG HOME, "time =
' PAUSE 100 -> PAUSE 1
", DEC5 time
time = time MAX valMax MIN valMin
IF time > (valMax - valMin) / 4 * 3 THEN
DEBUG CR, "Open blinds "
PULSOUT 14, 500
ELSEIF time < (valMax - valMin ) / 4 THEN
DEBUG CR, "Close blinds"
PULSOUT 14, 1000
ENDIF
LOOP
' Modify
' Add
' Modify
' Add
Frequency and Sound · Page 245
Chapter 8: Frequency and Sound
YOUR DAY AND ELECTRONIC BEEPS
Here are a few examples of beeps you might hear during a normal day: The microwave
oven beeps when it’s done cooking your food. The cell phone plays different tones of
beeps resembling songs to get your attention when a call is coming in. The ATM
machine beeps to remind you not to forget your card. A store cash register beeps to let
the teller know that the bar code of the grocery item passed over the scanner was read.
Many calculators beep when the wrong keys are pressed. You may have started your day
with a beeping alarm clock.
MICROCONTROLLERS, SPEAKERS, AND ON/OFF SIGNALS
Just about all of the electronic beeps you hear during your daily routine are made by
microcontrollers connected to speakers. The microcontroller creates these beeps by
sending rapid high/low signals to various types of speakers. The rate of these high/low
signals is called the frequency, and it determines the tone or pitch of the beep. Each time
a high/low repeats itself, it is called a cycle. You will often see the number of cycles per
second referred to as hertz, and it is abbreviated Hz. For example, one of the most
common frequencies for the beeps that help machines get your attention is 2 kHz. That
means that the high/low signals repeat at 2000 times per second.
Introducing the Piezoelectric Speaker
In this activity, you will experiment with sending a variety of signals to a common, small,
and inexpensive speaker called a piezoelectric speaker. A piezoelectric speaker is
commonly referred to as a piezo speaker or piezo buzzer, and piezo is pronounced “pEA-zO.” Its schematic symbol and part drawing are shown in Figure 8-1.
Figure 8-1
Piezoelectric Speaker Part Drawing
and Schematic Symbol
Page 246 · What’s a Microcontroller?
ACTIVITY #1: BUILDING AND TESTING THE SPEAKER
In this activity, you will build and test the piezoelectric speaker circuit.
Speaker Circuit Parts
(1) Piezoelectric speaker
(2) Jumper wires
Building the Piezoelectric Speaker Circuit
The negative terminal of the piezoelectric speaker should be connected to Vss, and the
positive terminal should be connected to an I/O pin. The BASIC Stamp will then be
programmed to send high/low signals to the piezoelectric speaker’s positive terminal.
9 If your piezo speaker has a sticker on it, just remove it (no washing needed).
9 Build the circuit shown in Figure 8-2.
Vdd
Vin
Vss
X3
P9
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
+
Figure 8-2
Piezoelectric Speaker
Circuit Schematic and
Wiring Diagram
How the Piezoelectric Speaker Circuit Works
When a guitar string vibrates, it causes changes in air pressure. These changes in air
pressure are what your ear detects as a tone. The faster the changes in air pressure, the
higher the pitch, and the slower the changes in air pressure, the lower the pitch. The
element inside the piezo speaker’s plastic case is called a piezoelectric element. When
high/low signals are applied to the speaker’s positive terminal, the piezoelectric element
vibrates, and it causes changes in air pressure just as a guitar string does. As with the
guitar string, your ear detects the changes in air pressure caused by the piezoelectric
speaker, and it typically sounds like a beep or a tone.
Frequency and Sound · Page 247
Programming Speaker Control
The FREQOUT command is a convenient way of sending high/low signals to a speaker to
make sound. The BASIC Stamp Manual shows the command syntax as this:
FREQOUT Pin, Duration, Freq1 {, Freq2}
As with most of the other commands used in this book, Pin is a value you can use to
choose which BASIC Stamp I/O pin to use. The Duration argument is a value that tells
the FREQOUT command how long the tone should play, in milliseconds. The Freq1
argument is used to set the frequency of the tone, in hertz. There is an optional Freq2
argument that can be used to play two different tones at the same time.
Here is how to send a tone to I/O pin P9 that lasts for 1.5 seconds and has a frequency of
2 kHz:
FREQOUT 9, 1500, 2000
Example Program: TestPiezoWithFreqout.bs2
This example program sends the 2 kHz tone to the speaker on I/O pin P9 for 1.5 seconds.
You can use the Debug Terminal to see when the speaker should be beeping and when it
should stop.
9 Enter and run TestPiezoWithFreqout.bs2.
9 Verify that the speaker makes a clearly audible tone during the time that the
Debug Terminal displays the message “Tone sending…”
' What's a Microcontroller - TestPiezoWithFreqout.bs2
' Send a tone to the piezo speaker using the FREQOUT command.
'{$STAMP BS2}
'{$PBASIC 2.5}
PAUSE 1000
DEBUG "Tone sending...", CR
FREQOUT 9, 1500, 2000
DEBUG "Tone done."
Page 248 · What’s a Microcontroller?
Your Turn – Adjusting Frequency and Duration
9
9
9
9
Save TestPiezoWithFreqout.bs2 under a different name.
Try some different values for the Duration and Freq1 arguments.
After each change, run the program and make a note of the effect.
As the Freq1 argument gets larger, does the tone’s pitch go up or down? Try
values of 1500, 2000, 2500 and 3000 to answer this question.
ACTIVITY #2: ACTION SOUNDS
Many toys contain microcontrollers that are used to make “action sounds.” Action
sounds tend to involve rapidly changing the frequency played by the speaker. You can
also get some interesting effects from playing two different tones together using the
FREQOUT command’s optional Freq2 argument. This activity introduces both techniques.
Programming Action Sounds
Action and appliance sounds have three different components:
1. Pause
2. Duration
3. Frequency
The pause is the time between tones, and you can use the PAUSE command to create it.
The duration is the amount of time a tone lasts, which you can set using the FREQOUT
command’s Duration argument. The frequency determines the pitch of the tone. The
higher the frequency, the higher the pitch, the lower the frequency, the lower the pitch.
This is, of course, determined by the FREQOUT command’s Freq1 argument.
Example Program: ActionTones.bs2
ActionTones.bs2 demonstrates a few different combinations of pause, duration, and
frequency. The first sequence of tones sounds similar to an electronic alarm clock. The
second one sounds similar to something a familiar science fiction movie robot might say.
The third is more the kind of sound effect you might hear in an old video game.
9 Enter and run ActionTones.bs2.
' What's a Microcontroller - ActionTones.bs2
' Demonstrate how different combinations of pause, duration, and frequency
' can be used to make sound effects.
Frequency and Sound · Page 249
'{$STAMP BS2}
'{$PBASIC 2.5}
duration
frequency
VAR
VAR
Word
Word
PAUSE 1000
DEBUG "Alarm...", CR
PAUSE 100
FREQOUT 9, 500, 1500
PAUSE 500
FREQOUT 9, 500, 1500
PAUSE 500
FREQOUT 9, 500, 1500
PAUSE 500
FREQOUT 9, 500, 1500
PAUSE 500
DEBUG "Robot reply...", CR
PAUSE 100
FREQOUT 9, 100, 2800
FREQOUT 9, 200, 2400
FREQOUT 9, 140, 4200
FREQOUT 9, 30, 2000
PAUSE 500
DEBUG "Hyperspace...", CR
PAUSE 100
FOR duration = 15 TO 1 STEP 1
FOR frequency = 2000 TO 2500 STEP 20
FREQOUT 9, duration, frequency
NEXT
NEXT
DEBUG "Done", CR
END
How ActionTones.bs2 Works
The “Alarm” routine sounds like an alarm clock. This routine plays tones at a fixed
frequency of 1.5 kHz for duration of 0.5 s with fixed delays between tones of 0.5 s. The
“Robot reply” routine uses various frequencies for brief durations.
The “Hyperspace” routine uses no delay, but it varies both the duration and frequency.
By using FOR...NEXT loops to rapidly change the frequency and duration variables,
you can get some interesting sound effects. When one FOR...NEXT loop executes inside
another one, it is called a nested loop. Here is how the nested FOR...NEXT loop shown
Page 250 · What’s a Microcontroller?
below works. The duration variable starts at 15, then the FOR frequency... loop
takes over and sends frequencies of 2000, then 2020, then 2040, and so on, up through
2500 to the piezo speaker. When the FOR frequency... loop is finished, the FOR
duration... loop has only repeated one of its 15 passes. So it subtracts 1 from the
value of duration and repeats the FOR frequency... loop all over again.
FOR duration = 15 TO 1
FOR frequency = 2000 TO 2500 STEP 15
FREQOUT 9, duration, frequency
NEXT
NEXT
Example Program: NestedLoops.bs2
To better understand how nested FOR...NEXT loops work, NestedLoops.bs2 uses the
DEBUG command to show the value of a much less complicated version of the nested loop
used in ActionTones.bs2.
9 Enter and run NestedLoops.bs2.
9 Examine the Debug Terminal output and verify how the duration and
frequency variables change each time through the loop.
' What's a Microcontroller - NestedLoops.bs2
' Demonstrate how the nested loop in ActionTones.bs2 works.
'{$STAMP BS2}
'{$PBASIC 2.5}
duration
frequency
VAR
VAR
Word
Word
PAUSE 1000
DEBUG "Duration
"--------
Frequency", CR,
---------", CR
FOR duration = 4000 TO 1000 STEP 1000
FOR frequency = 1000 TO 3000 STEP 500
DEBUG "
" , DEC5 duration,
"
", DEC5 frequency, CR
FREQOUT 9, duration, frequency
NEXT
DEBUG CR
NEXT
END
Frequency and Sound · Page 251
Your Turn – More Sound Effects
There is pretty much an endless number of ways to modify ActionTones.bs2 to get
different sound combinations. Here is just one modification to the “Hyperspace” routine:
DEBUG "Hyperspace jump...", CR
FOR duration = 15 TO 1 STEP 3
FOR frequency = 2000 TO 2500 STEP 15
FREQOUT 9, duration, frequency
NEXT
NEXT
FOR duration = 1 TO 36 STEP 3
FOR frequency = 2500 TO 2000 STEP 15
FREQOUT 9, duration, frequency
NEXT
NEXT
9 Save your example program under the name ActionTonesYourTurn.bs2.
9 Have fun with this and other modifications of your own creation.
Two Frequencies at Once
You can play two frequencies at the same time. Remember the FREQOUT command’s
syntax from Activity #1:
FREQOUT Pin, Duration, Freq1 {, Freq2}
You can use the optional Freq2 argument to play two frequencies with a single FREQOUT
command. For example, you can play 2 kHz and 3 kHz like this:
FREQOUT 9, 1000, 2000, 3000
Each touchtone keypad tone is also an example of two frequencies combined together. In
telecommunications, that is called DTMF (Dual Tone Multi Frequency). There is also a
PBASIC command called DTMFOUT that is designed just for sending phone tones. Look up
this command in the BASIC Stamp Manual or Help for examples.
Example Program: PairsOfTones.bs2
This example program demonstrates the difference in tone that you get when you play 2
and 3 kHz together. It also demonstrates an interesting phenomenon that occurs when
you add two sound waves that are very close in frequency. When you play 2000 Hz and
2001 Hz at the same time, the tone will fade in and out once every second (at a frequency
Page 252 · What’s a Microcontroller?
of 1 Hz). If you play 2000 Hz with 2002 Hz, it will fade in and out twice a second
(2 Hz), and so on.
Beat is when two tones very close in frequency are played together causing the tone you
hear to fade in and out. The frequency of that fading in and out is the difference between
the two frequencies. If the difference is 1 Hz, the tone will fade in and out at 1 Hz. If the
difference is 2 Hz, the tone will fade in and out at 2 Hz.
The variations in air pressure made by the piezoelectric speaker are called sound waves.
When the tone is loudest, the variations in air pressure caused by the two frequencies are
adding to each other (called superposition). When the tone is at its quietest, the variations
in air pressure are canceling each other out (called interference).
9 Enter and run PairsOfTones.bs2.
9 Keep an eye on the Debug Terminal as the tones play, and note the different
effects that come from mixing the different tones.
' What's a Microcontroller - PairsOfTones.bs2
' Demonstrate some of the things that happen when you mix two tones.
'{$STAMP BS2}
'{$PBASIC 2.5}
PAUSE 1000
DEBUG "Frequency = 2000", CR
FREQOUT 9, 4000, 2000
DEBUG "Frequency = 3000", CR
FREQOUT 9, 4000, 3000
DEBUG "Frequency = 2000 + 3000", CR
FREQOUT 9, 4000, 2000, 3000
DEBUG "Frequency = 2000 + 2001", CR
FREQOUT 9, 4000, 2000, 2001
DEBUG "Frequency = 2000 + 2002", CR
FREQOUT 9, 4000, 2000, 2002
DEBUG "Frequency = 2000 + 2003", CR
FREQOUT 9, 4000, 2000, 2003
DEBUG "Frequency = 2000 + 2005", CR
FREQOUT 9, 4000, 2000, 2005
Frequency and Sound · Page 253
DEBUG "Frequency = 2000 + 2010", CR
FREQOUT 9, 4000, 2000, 2010
DEBUG "Done", CR
END
Your Turn – Condensing the Code
PairsOfTones.bs2 was written to demonstrate some interesting things that can happen
when you play two different frequencies at once using the FREQOUT command’s optional
Freq2 argument. However, it is extremely inefficient.
9 Modify PairsOfTones.bs2 so that it cycles through the Freq2 arguments ranging
from 2001 to 2005 using a word variable and a loop.
ACTIVITY #3: MUSICAL NOTES AND SIMPLE SONGS
Figure 8-3 shows the rightmost 25 keys of a piano keyboard. It also shows the
frequencies at which each wire inside the piano vibrates when that piano key is struck.
Figure 8-3: Rightmost Piano Keys and Their Frequencies
Page 254 · What’s a Microcontroller?
The keys and their corresponding notes are labeled C6 through C8. These keys are
separated into groups of 12, made up of 7 white keys and 5 black keys. The sequence of
notes repeats itself every 12 keys. Notes of the same letter are related by frequency,
doubling with each higher octave. For example, C7 is twice the frequency of C6, and C8
is twice the frequency of C7. Likewise, if you go one octave down, the frequency will be
half the value; for example, A6 is half the frequency of A7.
If you’ve ever heard a singer practice his/her notes by singing the Solfege, “Do Re Mi Fa
Sol La Ti Do,” the singer is attempting to match the notes that you get from striking the
white keys on a piano keyboard. These white keys are called natural keys, and the name
“octave” relates to the frequency doubling with every eighth natural key. A black key on
a piano can either be called sharp or flat. For example, the black key between the C and
D keys is either called C-sharp (C#) or D-flat (Db). Whether a key is called sharp or flat
depends on the particular piece being played, and the rules for that are better left to the
music classes.
Internet search for “musical scale”: By using the words "musical scale" you will find lots
of fascinating information about the history, physics and psychology of the subject. The 12
note per octave scale is the main scale of western music. Other cultures use scales that
contain 2 to 35 notes per octave.
Tuning Method: The keyboard in Figure 8-3 uses a method of tuning called equal
temperament. The frequencies are determined using a reference note, then multiplying it by
(n/12)
2
for values of n = 1, 2, 3, etc. For example, you can take the frequency for A6, and
(1/12)
(2/12)
multiply by 2
to get the frequency for A6#. Multiply it by 2
to get the frequency for
B6, and so on. Here is an example of calculating the frequency for B6 using A6 as a
reference frequency:
The frequency of A6 is 1760
2
(2/12)
= 1.1224
1760 X 1.224 = 1975.5
1975.5 is the frequency of B6
Programming Musical Notes
The FREQOUT command is also useful for musical notes. Programming the BASIC Stamp
to play music using a piezospeaker involves following a variety of rules used in playing
music using any other musical instrument. These rules apply to the same elements that
were used to make sound effects: frequency, duration, and pause. This next example
program plays some of the musical note frequencies on the piezospeaker, each with a
duration of half a second.
Frequency and Sound · Page 255
Example Program: DoReMiFaSolLaTiDo.bs2
9 Enter and run DoReMiFaSolLaTiDo.bs2
' What's a Microcontroller - DoReMiFaSolLaTiDo.bs2
' Send an octave of half second notes using a piezoelectric speaker.
'{$STAMP BS2}
'{$PBASIC 2.5}
PAUSE 1000
'Solfege
Tone
Note
DEBUG "Do...", CR:
FREQOUT 9,500,1047
' C6
DEBUG "Re...", CR:
FREQOUT 9,500,1175
' D6
DEBUG "Mi...", CR:
FREQOUT 9,500,1319
' E6
DEBUG "Fa...", CR:
FREQOUT 9,500,1396
' F6
DEBUG "Sol..", CR:
FREQOUT 9,500,1568
' G6
DEBUG "La...", CR:
FREQOUT 9,500,1760
' A6
DEBUG "Ti...", CR:
FREQOUT 9,500,1976
' B6
DEBUG "Do...", CR:
FREQOUT 9,500,2093
' C7
END
Your Turn – Sharp/Flat Notes
9 Use the frequencies shown in Figure 8-3 to add the five sharp/flat notes to
DoReMiFaSolLaTiDo.bs2
9 Modify your program so that it plays the next octave up. Hint: Save yourself
some typing and just use the * 2 operation after each Freq1 argument. For
example, FREQOUT 9, 500, 1175 * 2 will multiply D6 by 2 to give you D7,
the D note in the 7th octave.
Storing and Retrieving Sequences of Musical Notes
A good way of saving musical notes is to store them using the BASIC Stamp module’s
EEPROM. Although you could use many WRITE commands to do this, a better way is to
use the DATA directive. This is the syntax for the DATA directive:
{Symbol} DATA {Word} DataItem {, {Word} DataItem, … }
Page 256 · What’s a Microcontroller?
Here is an example of how to use the DATA directive to store the characters that
correspond to musical notes.
Notes DATA "C","C","G","G","A","A","G"
You can use the READ command to access these characters. The letter “C” is located at
address Notes + 0, and a second letter “C” is located at Notes + 1. Then, there’s a letter
“G” at Notes + 2, and so on. For example, if you want to load the last letter “G” into a
byte variable called noteLetter, use the command:
READ Notes + 6, noteLetter
You can also store lists of numbers using the DATA directive. Frequency and duration
values that the BASIC Stamp uses for musical notes need to be stored in word variables
because they are usually greater than 255. Here is how to do that with a DATA directive.
Frequencies DATA Word 2093, Word 2093, Word 3136, Word 3136,
Word 3520, Word 3520, Word 3136
Because each of these values occupies two bytes, accessing them with the READ
command is different from accessing characters. The first 2093 is at Frequencies + 0,
but the second 2093 is located at Frequencies + 2. The first 3136 is located at
Frequencies + 4, and the second 3136 is located at Frequencies + 6.
The values in the Frequencies DATA directive correspond with the musical notes in
the Notes DATA directive.
Here is a FOR...NEXT loop that places the Notes DATA into a variable named
noteLetter, then it places the Frequencies DATA into a variable named noteFreq.
FOR index = 0 to 6
READ Notes + index, noteLetter
READ Frequencies + (index * 2), Word noteFreq
DEBUG noteLetter, " ", DEC noteFreq, CR
NEXT
Frequency and Sound · Page 257
What does the (index * 2) do?
Each value stored in the Frequencies DATA directive takes a word (two bytes), while
each character in the Notes DATA directive only takes one byte. The value of index
increases by one each time through the FOR...NEXT loop. That’s fine for accessing the
note characters using the command READ Notes + index, noteLetter. The problem
is that for every one byte in Notes, the index variable needs to point twice as far down the
Frequencies list. The command READ Frequencies + (index * 2), Word
noteFreq, takes care of this.
The next example program stores notes and durations using DATA, and it uses the
FREQOUT command to play each note frequency for a specific duration. The result is the
first few notes from the children’s song “Twinkle Twinkle Little Star.”
The “Alphabet Song” used by children to memorize their “ABCs” uses the same notes as
“Twinkle Twinkle Little Star.”
Example Program: TwinkleTwinkle.bs2
This example program demonstrates how to use the DATA directive to store lists and how
to use the READ command to access the values in the lists.
9 Enter and run TwinkleTwinkle.bs2
9 Verify that the notes sound like the song “Twinkle Twinkle Little Star.”
9 Use the Debug Terminal to verify that it works as expected.
' What's a Microcontroller - TwinkleTwinkle.bs2
' Play the first seven notes from Twinkle Twinkle Little Star.
'{$STAMP BS2}
'{$PBASIC 2.5}
Notes
DATA
"C","C","G","G","A","A","G"
Frequencies
DATA
Word 2093, Word 2093, Word 3136, Word 3136,
Word 3520, Word 3520, Word 3136
Durations
DATA
Word 500, Word 500, Word 500, Word 500,
Word 500, Word 500, Word 1000
index
noteLetter
noteFreq
noteDuration
VAR
VAR
VAR
VAR
Nib
Byte
Word
Word
Page 258 · What’s a Microcontroller?
PAUSE 1000
DEBUG
"Note
"----
Duration
--------
Frequency", CR,
---------", CR
FOR index = 0 TO 6
READ Notes + index, noteLetter
DEBUG "
", noteLetter
READ Durations + (index * 2), Word noteDuration
DEBUG "
", DEC4 noteDuration
READ Frequencies + (index * 2), Word noteFreq
DEBUG "
", DEC4 noteFreq, CR
FREQOUT 9, noteDuration, noteFreq
NEXT
END
Your Turn – Adding and Playing More Notes
This program played the first seven notes from Twinkle Twinkle Little Star. The words
go “Twin-kle twin-kle lit-tle star.” The next phrase from the song goes “How I won-der
what you are” and its notes are F, F, E, E, D, D, C. As with the first phrase, the last note
is held twice as long as the other notes. To add this phrase to the song from
TwinkleTwinkle.bs2, you will need to expand each DATA directive appropriately. Don’t
forget to change the FOR...NEXT loop so that it goes from 0 to 13 instead of from 0 to 6.
9 Modify TwinkleTwinkle.bs2 so that it plays the first two phrases of the song
instead of just the first phrase.
ACTIVITY #4: MICROCONTROLLER MUSIC
Note durations are not recorded on sheet music in terms of milliseconds. Instead, they
are described as whole, half, quarter, eight, sixteenth, and thirty-second notes. As the
name suggests, a half note lasts half as long as a whole note. A quarter note lasts one
fourth the time a whole note lasts, and so on. How long is a whole note? It depends on
the piece of music being played. One piece might be played at a tempo that causes a
whole note to last for four seconds, another piece might have a two second whole note,
and yet another might have some other duration.
Frequency and Sound · Page 259
Rests are the time between notes when no tones are played. Rest durations are also
measured as whole, half, quarter, eighth, sixteenth and thirty-second.
More about microcontroller music: After completing this activity, you will be ready to learn
how to write PBASIC musical code from sheet music. See the Playing Sheet Music with the
Piezospeaker tutorial and its accompanying video primer at www.parallax.com/go/WAM.
A Better System for Storing and Retrieving Music
You can write programs that store twice as much music in your BASIC Stamp by using
bytes instead of words in your DATA directives. You can also modify your program to
make the musical notes easier to read by using some of the more common musical
conventions for notes and durations. This activity will start by introducing how to store
musical information in a way that relates to the concepts of notes, durations, and rests.
Tempo is also introduced, and it will be revisited in the next activity.
Here is one of the DATA directives that stores musical notes and durations for the next
example program. When played, it should resemble the song “Frere Jacques.” Only the
note characters are stored in the Notes DATA directive because LOOKUP and LOOKDOWN
commands will be used to match up letters to their corresponding frequencies.
Notes
DATA
Durations
DATA
WholeNote
CON
"C","D","E","C","C","D","E","C","E","F",
"G","E","F","G","Q"
4,
2,
4,
4,
4,
4,
4,
2
4,
4,
4,
4,
4,
4,
2000
The first number in the Durations DATA directive tells the program how long the first
note in the Notes Data directive should last. The second duration is for the second note,
and so on. The durations are no longer in terms of milliseconds. Instead, they are much
smaller numbers that can be stored in bytes, so there is no Word prefix in the DATA
directive. Compared to storing values in terms of milliseconds, these numbers are more
closely related to sheet music.
Page 260 · What’s a Microcontroller?
Here is a list of what each duration means.
•
•
•
•
•
•
1 – whole note
2 – half note
4 – quarter note
8 – eighth note
16 – sixteenth note
32 – thirty-second note
After each value is read from the Durations DATA directive, it is divided into the
WholeNote value to get the Duration used in the FREQOUT command. The amount of time
each note lasts depends on the tempo of the song. A faster tempo means each note lasts
for less time, while a slower tempo means each note lasts longer. Since all the note
durations are fractions of a whole note, you can use the duration of a whole note to set the
tempo.
What does the "Q" in Notes DATA mean? "Q" is for quit, and a DO UNTIL...LOOP
checks for "Q" each time through the loop and will repeat until it is found.
How do I play a rest? You can insert a rest between notes by inserting a "P". The Your
th
Turn section has the first few notes from Beethoven’s 5 Symphony, which has a rest in it.
How do I play sharp/flat notes? NotesAndDurations.bs2 has values in its lookup tables for
sharp/flat notes. When you use the lower-case version of the note, it will play the flat note.
For example, if you want to play B-flat, use “b” instead of “B”. Remember that this is the
same frequency as A-sharp.
Example Program: NotesAndDurations.bs2
9 Enter and run NotesAndDurations.bs2.
9 How does it sound?
Frequency and Sound · Page 261
' What's a Microcontroller - NotesAndDurations.bs2
' Play the first few notes from Frere Jacques.
'{$STAMP BS2}
'{$PBASIC 2.5}
DEBUG "Program Running!"
Notes
DATA
"C","D","E","C","C","D","E","C","E","F",
"G","E","F","G","Q"
Durations
DATA
WholeNote
CON
2000
index
offset
VAR
VAR
Byte
Nib
noteLetter
noteFreq
noteDuration
VAR
VAR
VAR
Byte
Word
Word
4,
2,
4,
4,
4,
4,
4,
2
4,
4,
4,
4,
4,
DO UNTIL noteLetter = "Q"
READ Notes + index, noteLetter
LOOKDOWN noteLetter, [
LOOKUP offset,
"A",
"D",
"G",
"b",
"e",
"a",
"B",
"E",
"P",
noteDuration = WholeNote / noteDuration
FREQOUT 9, noteDuration, noteFreq
LOOP
END
"d",
"g",
], offset
[ 1760, 1865, 1976, 2093, 2217,
2349, 2489, 2637, 2794, 2960,
3136, 3322,
0,
0
], noteFreq
READ Durations + index, noteDuration
index = index + 1
"C",
"F",
"Q"
4,
Page 262 · What’s a Microcontroller?
How NotesAndDurations.bs2 Works
The Notes and Durations DATA directives were discussed before the program. These
directives combined with the WholeNote constant are used to store all the musical data
used by the program.
The declarations for the five variables used in the program are shown below. Even
though a FOR...NEXT loop is no longer used to access the data, there still has to be a
variable (index) that keeps track of which DATA entry is being read in Notes and
Durations. The offset variable is used in the LOOKDOWN and LOOKUP commands to
select a particular value. The noteLetter variable stores a character accessed by the
READ command. LOOKUP and LOOKDOWN commands are used to convert this character into
a frequency value. This value is stored in the noteFreq variable and used as the
FREQOUT command’s Freq1 argument. The noteDuration variable is used in a READ
command to receive a value from the Durations DATA. It is also used to calculate the
Duration argument for the FREQOUT command.
index
offset
VAR
VAR
Byte
Nib
noteLetter
noteFreq
noteDuration
VAR
VAR
VAR
Byte
Word
Word
The main loop keeps executing until the letter “Q” is read from the Notes DATA.
DO UNTIL noteLetter = "Q"
A READ command gets a character from the Notes DATA, and stores it in the noteLetter
variable. The noteLetter variable is then used in a LOOKDOWN command to set the value
of the offset variable. Remember that offset stores a 1 if “b” is detected, a 2 if “B” is
detected, a 3 if “C” is detected, and so on. This offset value is then used in a LOOKUP
command to figure out what the value of the noteFreq variable should be. If offset is
1, noteFreq will be 1865, if offset is 2, noteFreq will be 1976, if offset is 3,
noteFreq is 2093, and so on.
READ Notes + index, noteLetter
LOOKDOWN noteLetter, [
"A",
"D",
"G",
"b",
"e",
"a",
"B",
"E",
"P",
"C", "d",
"F", "g",
"Q" ], offset
Frequency and Sound · Page 263
LOOKUP offset,
[ 1760, 1865, 1976, 2093, 2217,
2349, 2489, 2637, 2794, 2960,
3136, 3322,
0,
0 ], noteFreq
The note’s frequency has been determined, but the duration still has to be figured out.
The READ command uses the value of index to place a value from the Durations DATA
into noteDuration.
READ Durations + index, noteDuration
Then, noteDuration is set equal to the WholeNote constant divided by the
noteDuration. If noteDuration starts out as 4 from a READ command, it becomes
2000 ÷ 4 = 500. If noteDuration is 8, it becomes 2000 ÷ 8 = 250.
noteDuration = WholeNote / noteDuration
Now that noteDuration and noteFreq are determined, the FREQOUT command plays the
note.
FREQOUT 9, noteDuration, noteFreq
Each time through the main loop, the index value must be increased by one. When the
main loop gets back to the beginning, the first thing the program does is read the next
note, using the index variable.
index = index + 1
LOOP
Your Turn – Experimenting with Tempo and a Different Tune
The length of time that each note lasts is related to the tempo. You can change the tempo
by adjusting the WholeNote constant. If you increase it to 2250, the tempo will decrease,
and the song will play slower. If you decrease it to 1750, the tempo will increase and the
song will play more quickly.
9 Save NotesAndDurations.bs2 under the name NotesAndDurationsYourTurn.bs2.
9 Modify the tempo of NotesAndDurationsYourTurn.bs2 by adjusting the value of
WholeNote. Try values of 1500, 1750, 2000, and 2250.
9 Re-run the program after each modification, and decide which one sounds best.
Page 264 · What’s a Microcontroller?
Entering musical data is much easier when all you have to do is record notes and
durations. Here are the first eight notes from Beethoven’s Fifth Symphony.
Notes
DATA "G","G","G","e","P","F","F","F","D","Q"
Durations DATA
8,
WholeNote CON
2000
8,
8,
2,
8,
8,
8,
8,
2
9 Save your modified program as Beethoven’sFifth.bs2.
9 Replace the Notes and Durations DATA directives and the WholeNote constant
declaration with the code above.
9 Run the program. Does it sound familiar?
Adding Musical Features
The example program you just finished introduced notes, durations, and rests. It also
used the duration of a whole note to determine tempo. Here are three more features we
can add to a music-playing program:
•
•
•
Play “dotted” notes
Determine the whole note duration from the tempo
Play notes from more than one octave
The term dotted refers to a dot used in sheet music to indicate that a note should be
played 1 ½ times as long as its normal duration. For example, a dotted quarter note
should last for the duration of a quarter note, plus an eighth note. A dotted half note lasts
for a half plus a quarter note’s duration. You can add a data table that stores whether or
not a note is dotted. In this example, a zero means there is no dot while a 1 means there
is a dot:
Dots
DATA
0,
0,
0,
0,
0,
0,
0,
1,
0,
0
0,
1,
0,
0,
0,
0,
Music-playing programs typically express the tempo for a song in beats per minute. This
is the same as saying quarter notes per minute.
BeatsPerMin
CON
200
Figure 8-4 is a repeat of Figure 8-3 from page 253. It shows the 6th and 7th octaves on the
piano keyboard. These are the two octaves that sound the clearest when played by the
Frequency and Sound · Page 265
piezospeaker. Here is an example of a DATA directive you will use in the Your Turn
section to play notes from more than one octave using the Notes DATA directive.
Octaves
DATA
6,
6,
7,
6,
6,
6
6,
6,
6,
6,
6,
6,
7,
6,
Figure 8-4: Rightmost Piano Keys and Their Frequencies
Example Program: MusicWithMoreFeatures.bs2
This example program plays the first few notes from “For He’s a Jolly Good Fellow.”
All the notes come from the same (7th) octave, but some of the notes are dotted. In the
Your Turn section, you will try an example that uses notes from more than one octave,
and dotted notes.
9 Enter and run MusicWithMoreFeatures.bs2.
9 Count notes and see if you can hear the dotted (1 ½ duration) notes.
9 Also listen for notes in octave 7. Try changing one of those notes to octave 6.
The change in the way the music sounds is pretty drastic.
Page 266 · What’s a Microcontroller?
' What's a Microcontroller - MusicWithMoreFeatures.bs2
' Play the beginning of For He's a Jolly Good Fellow.
'{$STAMP BS2}
'{$PBASIC 2.5}
DEBUG "Program Running!"
Notes
DATA
"C","E","E","E","D","E","F","E","E","D","D",
"D","C","D","E","C","Q"
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7
4, 2, 4, 4, 4, 4, 2, 2, 4, 2, 4,
4, 4, 4, 2, 2
0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0,
0, 0, 0, 1, 0
Octaves
DATA
Durations
DATA
Dots
DATA
BeatsPerMin
CON
320
index
offset
VAR
VAR
Byte
Nib
noteLetter
noteFreq
noteDuration
noteOctave
noteDot
VAR
VAR
VAR
VAR
VAR
Byte
Word
Word
Nib
Bit
wholeNote
VAR
Word
wholeNote = 60000 / BeatsPerMin * 4
DO UNTIL noteLetter = "Q"
READ Notes + index, noteLetter
LOOKDOWN noteLetter,
LOOKUP offset,
[ "C",
"F",
"b",
"d",
"g",
"B",
"D",
"G",
"P",
"e",
"a",
"Q"
"E",
"A",
], offset
[ 4186, 4435, 4699, 4978, 5274,
5588, 5920, 6272, 6645, 7040,
7459, 7902,
0,
0
], noteFreq
READ Octaves + index, noteOctave
noteOctave = 8 - noteOctave
noteFreq = noteFreq / (DCD noteOctave)
READ Durations + index, noteDuration
noteDuration = WholeNote / noteDuration
READ Dots + index, noteDot
IF noteDot = 1 THEN noteDuration = noteDuration * 3 / 2
Frequency and Sound · Page 267
FREQOUT 9, noteDuration, noteFreq
index = index + 1
LOOP
END
How MusicWithMoreFeatures.bs2 Works
Below is the musical data for the entire song. For each note in the Notes DATA directive,
there is a corresponding entry in the Octaves, Durations, and Dots DATA directives.
For example, the first note is a C note in the 7th octave; it’s a quarter note and it’s not
dotted. Here is another example: the second from the last note (not including “Q”) is an
E note, in the 7th octave. It’s a half note, and it is dotted. There is also a BeatsPerMin
constant that sets the tempo for the song.
Notes
DATA
Octaves
DATA
Durations
DATA
Dots
DATA
BeatsPerMin
CON
"C","E","E","E","D","E","F","E","E","D","D",
"D","C","D","E","C","Q"
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7
4, 2, 4, 4, 4, 4, 2, 2, 4, 2, 4,
4, 4, 4, 2, 2
0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0,
0, 0, 0, 1, 0
320
In the previous example program, WholeNote was a constant. This time, it’s a variable
that will hold the duration of a whole note in ms. After this value is calculated,
WholeNote will be used to determine all the other note durations, just like in the previous
program. The index, offset, noteLetter, and noteDuration variables are also used
in the same manner they were in the previous program. The noteFreq variable is
handled a little differently since now it has to be adjusted depending on the octave the
note is played in. The noteOctave and noteDot variables have been added to handle
the octave and dot features.
wholeNote
VAR
Word
index
offset
VAR
VAR
Byte
Nib
noteLetter
noteFreq
VAR
VAR
Byte
Word
Page 268 · What’s a Microcontroller?
noteDuration
noteOctave
noteDot
VAR
VAR
VAR
Word
Nib
Bit
The wholeNote variable is calculated using BeatsPerMin. The tempo of the song is
defined in beats per minute, and the program has to divide BeatsPerMin into 60000 ms,
then multiply by 4. The result is the correct value for a whole note.
wholeNote = 60000 / BeatsPerMin * 4
Math executes from left to right. In the calculation wholeNote = 60000 /
beatsPerMin * 4, the BASIC Stamp first calculates 60000 / beatsPerMin. Then, it
multiplies that result by 4.
Parentheses can be used to group operations. If you want to divide 4 into beatsPerMin
first, you can do this: wholeNote = 60000 / (beatsPerMin * 4).
This is all the same as the previous program:
DO UNTIL noteLetter = "Q"
READ Notes + index, noteLetter
LOOKDOWN noteLetter,
[ "C",
"F",
"b",
"d",
"g",
"B",
"D",
"G",
"P",
"e",
"a",
"Q"
"E",
"A",
], offset
Now that octaves are in the mix, the part of the code that figures out the note frequency
has changed. The LOOKUP command’s table of values contains note frequencies from the
8th octave. These values can be divided by 1 if you want to play notes in the 8th octave,
by 2 if you want to play notes in the 7th octave, by 4 if you want to play notes in the 6th
octave and by 8 if you want to play notes in the 5th octave. The division happens next.
All this LOOKUP command does is place a note from the 8th octave into the noteFreq
variable.
LOOKUP offset,
[ 4186, 4435, 4699, 4978, 5274,
5588, 5920, 6272, 6645, 7040,
7459, 7902,
0,
0
], noteFreq
Here is how the noteFreq variable is adjusted for the correct octave. First, the READ
command grabs the octave value stored in the Octaves DATA. This could be a value
between 5 and 8.
Frequency and Sound · Page 269
READ Octaves + index, noteOctave
Depending on the octave, we want to divide noteFreq by either 1, 2, 4, or 8. That means
that the goal is really to divide by 20 = 1, 21 = 2, 22 = 4, or 23 = 8. The statement below
takes the value of noteOctave, which could be a value between 5 and 8, and subtracts
that value from 8. If noteOctave was 8, now it’s 0. If noteOctave was 7, now it’s 1.
If noteOctave was 6, now it’s 2, and if noteOctave was 5, now it’s 3.
noteOctave = 8 - noteOctave
Now, noteOctave is a value that can be used as an exponent of 2, but how do you raise 2
to a power in PBASIC? One answer is to use the DCD operator. DCD 0 is 1, DCD 1 is 2,
DCD 2 is 4, and DCD 3 is 8. Dividing noteFreq by DCD noteOctave means you are
dividing by 1, 2, 4, or 8, which divides noteFreq down by the correct value. The end
result is that noteFreq is set to the correct octave. You will use the Debug Terminal in
the Your Turn section to take a closer look at how this works.
noteFreq = noteFreq / (DCD noteOctave)
How am I supposed to know to use the DCD operator? Keep learning and practicing.
Every time you see a new command, operator, or any other keyword used in an example,
look it up in the BASIC Stamp manual. Read about it, and try using it in a program of your
own design.
Get in the habit of periodically reading the BASIC Stamp Manual and trying the short
example programs. That’s the best way to get familiar with the various commands and
operators and how they work. By doing these things, you will develop a habit of always
adding to the list of programming tools you can use to solve problems.
The first two lines of code for determining the note duration are about the same as the
code from the previous example program. Now, however, any note could be dotted,
which means the duration might have to be multiplied by 1.5. A READ command is used
to access values stored in EEPROM by the Dots DATA directive. An IF...THEN
statement is used to multiply by 3 and divide by 2 whenever the value of the noteDot
variable is 1.
READ Durations + index, noteDuration
noteDuration = WholeNote / noteDuration
READ Dots + index, noteDot
IF noteDot = 1 THEN noteDuration = noteDuration * 3 / 2
Page 270 · What’s a Microcontroller?
Integer math The BASIC Stamp does not automatically process a number like 1.5. When
performing math, it only works with integers: …, -5, -4, -3, -2, -1, 0, 1, 2, 3, … The best
solution for multiplying by 1.5 is to multiply by 3/2. First, multiply by 3, and then divide by 2.
There are many ways to program the BASIC Stamp to handle fractional values. You can
program the BASIC Stamp to use integers to figure out the fractional portion of a number.
This is introduced in the Basic Analog and Digital Student Guide. There are also two
operators that make fractional values easier to work with, and they are: ** and */. These
are explained in detail in the Applied Sensors Student Guide and in the BASIC Stamp
Manual.
The remainder of this example program works the same way that it did in the previous
example program:
FREQOUT 9, noteDuration, noteFreq
index = index + 1
LOOP
END
Your Turn – Playing a Tune with More than One Octave
MusicWithMoreFeatures.bs2 made use of rests, but it stayed in one octave. The tune
“Take Me Out to the Ball Game” shown below plays most of its notes in the 6th octave.
There are two notes in the 7th octave, and they make a big difference to the way it sounds.
9 Save a copy of the program as MusicWithMoreFeaturesYourTurn.bs2.
9 Modify the program by replacing the four data directives and one constant
declaration with these:
Notes
DATA
Octaves
DATA
Durations
DATA
Dots
DATA
BeatsPerMin
CON
"C","C","A","G","E","G","D","P","C","C","A",
"G","E","G","Q"
6, 7, 6, 6, 6, 6, 6, 6, 6, 7, 6,
6, 6, 6
2, 4, 4, 4, 4, 2, 2, 4, 2, 4, 4,
4, 4, 2
0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0,
0, 0, 1
240
9 Run the program and verify that it sounds right.
Frequency and Sound · Page 271
Those two notes in the 7th octave are essential for making the tune sound right. It’s
interesting to hear what happens if those 7 values are changed to 6.
9 Try changing the two 7 values in the Octaves DATA directive so that they are 6.
Keep in mind, this will make “Take Me out to the Ball Game” sound weird.
9 Run the program, and listen to the effect of the wrong octaves on the song.
9 Change the Octaves DATA back to its original state.
9 Run the program again and listen to see if it sounds correct again.
ACTIVITY #5: RINGTONES WITH RTTTL
Older cell phones used to play ringtones with a piezospeaker. Ringtones were
downloaded from the web to a computer, and then uploaded from the computer to the cell
phone’s microcontroller. At the time, one of the most widely used ways of composing,
recording and posting ringtones was one that featured strings of text with characters that
describe each note in the song. Here is an example of how the first few notes from
“Beethoven’s 5th” look in one of these strings:
Beethoven5:d=8,o=7,b=125:g,g,g,2d#,p,f,f,f,2d
This format for storing musical data is called RTTTL, which stands for Ringing Tone
Text Transfer Language. The great thing about RTTTL files at the time was that they
were widely shared via the World Wide Web. Many sites had RTTTL files available for
free download. There were also free software programs that could be used to compose
and emulate these files as well as upload them to your cell phone. The RTTTL
specification is still published online. Appendix C summarizes how an RTTTL file stores
notes, durations, pauses, tempo, and dotted notes.
This activity introduces some PBASIC programming techniques that can be used to
recognize different elements of text. The ability to recognize different characters or
groups of characters and take action based on what those characters contain is extremely
useful. In fact, it’s the key to converting an RTTTL format ringtone (like Beethoven5
above) into music. At the end of this activity, there is an application program that you
can use to play RTTTL format ringtones.
Selecting which Code Block to Execute on a Case by Case Basis
The SELECT...CASE statement is probably the best programming tool for recognizing
characters or values. Keep in mind that this is one of the tools used to convert an RTTTL
ringtone into musical notes.
Page 272 · What’s a Microcontroller?
In general, SELECT...CASE is used to:
•
•
•
Select a variable or expression.
Evaluate that variable or expression on a case by case basis.
Execute different blocks of code depending on which case that variable’s value
fits into.
Here is the syntax for SELECT...CASE:
SELECT expression
CASE condition(s)
statement(s)
{ CASE ELSE
statement(s) }
ENDSELECT
You can try the next two example programs to see how SELECT...CASE works.
SelectCaseWithValues.bs2 takes numeric values you enter into the Debug Terminal and
tells you the minimum variable size you will need to hold that value.
SelectCaseWithCharacters.bs2 tells you whether the character you entered into the Debug
Terminal is upper or lower case, a digit, or punctuation.
Remember to use the Transmit windowpane in the Debug Terminal to send the characters
you type to the BASIC Stamp. The Transmit and Receive windowpanes are shown in
Figure 8-5.
Windowpanes
Transmit →
Receive →
Figure 8-5
Sending Messages
to the BASIC Stamp
Click the Transmit
(upper) windowpane
and enter the value
or characters you
want to transmit to
the BASIC Stamp.
Example Program: SelectCaseWithValues.bs2
9 Enter and run SelectCaseWithValues.bs2.
9 In the Debug Terminal, make sure that the Echo Off checkbox is clear (no
checkmark).
Frequency and Sound · Page 273
9 Click the Debug Terminal’s Transmit windowpane.
9 Enter a value between 0 and 65535, and press the Enter key.
What happens if you enter a number larger than 65535? If you enter the number 65536,
the BASIC Stamp will store the number 0. If you enter the number 65537, the BASIC
Stamp will store the number 1, and so on. When a number is too large for the variable it
fits into, it is called overflow.
9 Use Table 8-1 to verify that the example program makes the right decisions
about the size of the numbers you enter into the Debug Terminal.
Table 8-1: Variable Types and Values They Can Store
Variable type
Bit
Range of Values
0 to 1
Nib
0 to 15
Byte
0 to 255
Word
0 to 65535
' What's a Microcontroller - SelectCaseWithValues.bs2
' Enter a value and see the minimum variable size required to hold it.
'{$STAMP BS2}
'{$PBASIC 2.5}
value
PAUSE 1000
VAR
Word
DEBUG "Enter a value from", CR,
"0 to 65535: "
DO
DEBUGIN DEC value
SELECT value
CASE 0, 1
DEBUG "Bit", CR
PAUSE 100
CASE 2 TO 15
DEBUG "Nib (Nibble)", CR
PAUSE 200
Page 274 · What’s a Microcontroller?
CASE 16 TO 255
DEBUG "Byte", CR
PAUSE 300
CASE 256 TO 65535
DEBUG "Word", CR
PAUSE 400
ENDSELECT
DEBUG CR, "Enter another value: "
LOOP
How SelectCaseWithValues.bs2 Works
A word variable is declared to hold the values entered into the Debug Terminal.
value VAR Word
The DEBUGIN command takes the number you enter and places it into the value variable.
DEBUGIN DEC value
The SELECT statement chooses the value variable as the one to evaluate cases for.
SELECT value
The first case is if the value variable equals either 0 or 1. If value equals either of those
numbers, the DEBUG and PAUSE commands that follow it are executed.
CASE 0, 1
DEBUG "BIT", CR
PAUSE 100
The second case is if value equals any number from 2 to 15. If it does equal any of those
numbers, the DEBUG and PAUSE commands below it are executed.
CASE 2 to 15
DEBUG "NIB (Nibble)", CR
PAUSE 200
When all the cases are done, the ENDSELECT keyword is used to complete the
SELECT..CASE statement.
ENDSELECT
Frequency and Sound · Page 275
Example Program: SelectCaseWithCharacters.bs2
This example program evaluates each character you enter into the Debug Terminal’s
Transmit windowpane. It can recognize upper and lower case characters, digits, and
some punctuation. If you enter a character the program does not recognize, it will tell
you to try again (entering a different character).
9
9
9
9
Enter and run SelectCaseWithCharacters.bs2.
Make sure the Echo Off checkbox is clear (no checkmark).
Click the Debug Terminal’s Transmit windowpane to place the cursor there.
Enter characters into the Transmit windowpane and observe the results.
' What's a Microcontroller - SelectCaseWithCharacters.bs2
' Program that can identify some characters: case, digit, punctuation.
'{$STAMP BS2}
'{$PBASIC 2.5}
character
PAUSE 1000
VAR
Byte
DEBUG "Enter a character: ", CR
DO
DEBUGIN character
SELECT character
CASE "A" TO "Z"
DEBUG CR, "Upper case", CR
CASE "a" TO "z"
DEBUG CR, "Lower case", CR
CASE "0" TO "9"
DEBUG CR, "Digit", CR
CASE "!", "?", ".", ","
DEBUG CR, "Punctuation", CR
CASE ELSE
DEBUG CR, "Character not known.", CR,
"Try a different one."
ENDSELECT
DEBUG CR, "Enter another character", CR
LOOP
Page 276 · What’s a Microcontroller?
How SelectCaseWithCharacters.bs2 Works
When compared to SelectCaseWithValues.bs2, this example program has a few
differences. First, the name of the value variable was changed to character, and its
size was changed from word to byte. This is because all characters in PBASIC are byte
size. The SELECT statement chooses the character variable for case-by-case evaluation.
SELECT character
The quotation marks are used to tell the BASIC Stamp Editor that you are referring to
characters. We can treat the following groups of characters and punctuation marks the
same way as a range of numbers, since the BASIC Stamp recognizes them by their ASCII
numeric equivalents—see the ASCII chart in the BASIC Stamp Editor Help.
SELECT character
CASE "A" TO "Z"
DEBUG CR, "Upper case", CR
CASE "a" TO "z"
DEBUG CR, "Lower case", CR
CASE "0" TO "9"
DEBUG CR, "Digit", CR
CASE "!", "?", ".", ","
DEBUG CR, "Punctuation", CR
There is also one different CASE statement that was not used in the previous example:
CASE ELSE
DEBUG CR, "Character not known.", CR,
"Try a different one."
This CASE statement tells the SELECT code block what to do if none of the other cases are
true. You can get this case to work by entering a character such as % or $.
Your Turn – Selecting Special Characters
9 Modify the SELECT...CASE statement in SelectCaseWithCharacters.bs2 so that
it displays “Special character” when you enter one of these characters: @, #, $,
%, ’^’ , &, *, (, ), _, or +.
Frequency and Sound · Page 277
RTTTL Ringtone Player Application Program
Below is the RTTTL file that contains the musical information used in the next example
program. There are five more RTTTL_File DATA directives that you can try in the Your
Turn section. This program plays a tune called “Reveille” which is the bugle call played
at military camps first thing in the morning. You may have heard it in any number of
movies or television shows.
RTTTL_File
DATA "Reveille:d=4,o=7,b=140:8g6,8c,16e,16c,8g6,8e,",
"8c,16e,16c,8g6,8e,8c,16e,16c,8a6,8c,e,8c,8g6,",
"8c,16e,16c,8g6,8e,8c,16e,16c,8g6,8e,8c,16e,",
"16c,8g6,8e,c,p,8e,8e,8e,8e,g,8e,8c,8e,8c,8e,8c,",
"e,8c,8e,8e,8e,8e,8e,g,8e,8c,8e,8c,8g6,8g6,c."
Example Program: MicroMusicWithRtttl.bs2
This application program is pretty long, and it’s a good idea to download the latest
version from the www.parallax.com/go/WAM page. Downloading the program and
opening it with the BASIC Stamp Editor should save you a significant amount of time.
The alternative, of course, is to hand enter and debug four pages of code.
9 With the BASIC Stamp Editor, open your downloaded MicroMusicWithRtttl.bs2
file, or hand enter the example below very carefully.
9 Run the program, and verify that the piece is recognizable as the Reveille bugle
call.
9 Go to the Your Turn section and try some more tunes (RTTTL_File DATA
directives).
' What's a Microcontroller - MicroMusicWithRtttl.bs2
' Play Nokia RTTTL format ringtones using DATA.
'{$STAMP BS2}
'{$PBASIC 2.5}
DEBUG "Program Running!"
' -----[ I/O Definitions ]------------------------------------------------SpeakerPin
PIN
9
' Piezospeaker connected to P9.
' -----[ Variables ]------------------------------------------------------counter
char
index
VAR
VAR
VAR
Word
Byte
Word
' General purpose counter.
' Variable stores characters.
' Index for pointing at data.
Page 278 · What’s a Microcontroller?
noteLetter
noteFreq
noteOctave
VAR
VAR
VAR
Byte
Word
Word
' Stores note character.
' Stores note frequency.
' Stores note octave.
duration
tempo
VAR
VAR
Word
Word
' Stores note duration.
' Stores tempo.
default_d
default_o
default_b
VAR
VAR
VAR
Byte
Byte
Word
' Stores default duration.
' Stores default octave.
' Stores default beats/min.
' -----[ EEPROM Data ]----------------------------------------------------RTTTL_File
DATA
"Reveille:d=4,o=7,b=140:8g6,8c,16e,16c,8g6,8e,",
"8c,16e,16c,8g6,8e,8c,16e,16c,8a6,8c,e,8c,8g6,",
"8c,16e,16c,8g6,8e,8c,16e,16c,8g6,8e,8c,16e,",
"16c,8g6,8e,c,p,8e,8e,8e,8e,g,8e,8c,8e,8c,8e,8c,",
"e,8c,8e,8e,8e,8e,8e,g,8e,8c,8e,8c,8g6,8g6,c."
Done
DATA
",q,"
Notes
DATA
Octave8
DATA
"p",
"a",
"#",
"b",
"c",
"#",
"d",
"#",
"e",
"f",
"#",
"g",
"#"
Word 0,
Word 3520, Word 3729, Word 3951,
Word 4186, Word 4435, Word 4699, Word 4978,
Word 5274, Word 5588, Word 5920, Word 6272,
Word 6645
' -----[ Initialization ]-------------------------------------------------counter = 0
' Initialize counter.
GOSUB
GOSUB
GOSUB
GOSUB
GOSUB
GOSUB
'
'
'
'
'
'
FindEquals
ProcessDuration
FindEquals
ProcessOctave
FindEquals
GetTempo
Find first '=' in file.
Get default duration.
Find next '='.
Get default octave.
Find last '='.
Get default tempo.
' -----[ Program Code ]---------------------------------------------------DO UNTIL char = "q"
GOSUB ProcessDuration
GOSUB ProcessNote
GOSUB CheckForDot
GOSUB ProcessOctave
GOSUB PlayNote
LOOP
'
'
'
'
'
'
'
Loop until 'q' in DATA.
Get note duration.
Get index value of note.
If dot, 3/2 duration.
Get octave.
Get freq, play note, next.
End of main loop.
END
' End of program.
Frequency and Sound · Page 279
' -----[ Subroutine - Find Equals Character ]----------------------------FindEquals:
DO
READ RTTTL_File + counter, char
counter = counter + 1
LOOP UNTIL char = "="
'
'
'
'
'
Go through characters in
RTTTL file looking for
'='. Increment counter
until '=' is found, then
return.
RETURN
'
'
'
'
'
'
-----[ Subroutine - Read Tempo from RTTTL Header ]---------------------Each keyboard character has a unique number called an ASCII value.
The characters 0, 1, 2,...9 have ASCII values of 48, 49, 50,...57.
You can always convert from the character representing a digit to
to its value by subtracting 48 from the variable storing the digit.
You can examine this by comparing DEBUG DEC 49 and DEBUG 49.
GetTempo:
default_b = 0
DO
READ RTTTL_File + counter, char
IF char = ":" THEN
default_b = default_b / 10
counter = counter + 1
EXIT
ENDIF
default_b = default_b + char - 48
counter = counter + 1
default_b = default_b * 10
LOOP UNTIL char = ":"
RETURN
'
'
'
'
'
'
'
'
'
'
'
'
'
'
'
'
Parse RTTTL file for Tempo.
Convert characters to
digits by subtracting 48
from each character's ASCII
value. Iteratively multiply
each digit by 10 if there
is another digit, then add
the most recent digit to
one's column.
For example, the string
"120" is (1 X 10 X 10)
+ (2 X 10) + 0. The '1'
is converted first, then
multiplied by 10. The '2'
is then converted/added.
0 is converted/added, done.
' -----[ Subroutine - Look up Octave ]-----------------------------------ProcessOctave:
READ RTTTL_File + counter, char
SELECT char
CASE "5" TO "8"
noteOctave = char - "0"
counter = counter + 1
CASE ELSE
noteOctave = default_o
ENDSELECT
IF default_o = 0 THEN
default_o = noteOctave
ENDIF
'
'
'
'
'
'
'
'
'
'
'
'
'
'
Octave may or may not be
included in a given note
because any note that is
played in the default
octave does not specify
the octave. If a char
from '5' to '8' then use
it, else use default_o.
Characters are converted
to digits by subtracting
'0', which is the same as
subtracting 48. The first
time this subroutine is
called, default_o is 0.
Page 280 · What’s a Microcontroller?
RETURN
' If 0, then set default_o.
' -----[ Subroutine - Find Index of Note ]-------------------------------ProcessNote:
READ RTTTL_File + counter, char
SELECT char
CASE "p"
index = 0
counter = counter + 1
CASE "a" TO "g"
FOR index = 1 TO 12
READ Notes + index, noteLetter
IF noteLetter = char THEN EXIT
NEXT
counter = counter + 1
READ RTTTL_File + counter, char
SELECT char
CASE "#"
index = index + 1
counter = counter + 1
ENDSELECT
ENDSELECT
'
'
'
'
'
'
'
'
'
'
'
'
'
'
'
Set index value for lookup
of note frequency based on
note character. If 'p',
index is 0. If 'a' to 'g',
read character values in
DATA table and find match.
Record index value when
match is found. If next
char is a sharp (#), add
1 to the index value to
increase the index (and
frequency) by 1 notch.
As with other subroutines,
increment counter for each
character that is processed.
RETURN
' -----[ Subroutine - Determine Note Duration ]--------------------------ProcessDuration:
READ RTTTL_File + counter, char
'
'
'
'
'
'
'
'
Check to see if characters
form 1, 2, 4, 8, 16 or 32.
If yes, then convert from
ASCII character to a value
by subtracting 48. In the
case of 16 or 32, multiply
by 10 and add the next
digit to the ones column.
'
'
'
'
If default_d not defined
(if default_d = 0), then
set default_d = to the
duration from the d=#.
SELECT char
CASE "1", "2", "3", "4", "8"
duration = char - 48
counter = counter + 1
READ RTTTL_File + counter, char
SELECT char
CASE "6", "2"
duration = duration * 10 + char - 48
counter = counter + 1
ENDSELECT
CASE ELSE
' If no duration, use
duration = default_d
' use default.
ENDSELECT
IF default_d <> 0 THEN
duration = 60000/default_b/duration*3
ELSE
default_d = duration
ENDIF
Frequency and Sound · Page 281
RETURN
' -----[ Subroutine - Check For '.' Indicating 1.5 Duration ]------------CheckForDot:
READ RTTTL_File + counter, char
SELECT char
CASE "."
duration = duration * 3 / 2
counter = counter + 1
ENDSELECT
'
'
'
'
'
'
Check for dot indicating
multiply duration by 3/2.
If dot found, multiply by
3/2 and increment counter,
else, do nothing and
return.
RETURN
' -----[ Subroutine - Find Comma and Play Note/Duration ]----------------PlayNote:
'
'
'
'
'
Find last comma in the
current note entry. Then,
fetch the note frequency
from data, and play it, or
pause if frequency = 0.
READ RTTTL_File + counter, char
SELECT char
CASE ","
counter = counter + 1
READ Octave8 + (index * 2), Word noteFreq
noteOctave = 8 - noteOctave
noteFreq = noteFreq / (DCD noteOctave)
IF noteFreq = 0 THEN
PAUSE duration
ELSE
FREQOUT SpeakerPin, duration, noteFreq
ENDIF
ENDSELECT
RETURN
How MicroMusicWithRtttl.bs2 Works
This example program is fun to use, and it shows the kind of code you will be able to
write with some practice. However, it was included in this text more for fun than for the
coding concepts it employs. If you examine the code briefly, you might notice that you
have already used all of the commands and operators in the program, except one!
Page 282 · What’s a Microcontroller?
Here is a list of the elements in this program that should, by now, be familiar:
•
•
•
•
•
•
•
•
•
•
•
•
Comments to help explain your code
Constant and variable declarations
DATA declarations
READ commands
IF...ELSE...ENDIF blocks
DO...LOOP both with and without WHILE and UNTIL
Subroutines with GOSUB, labels, and RETURN
FOR...NEXT loops
LOOKUP and LOOKDOWN commands
The FREQOUT and PAUSE commands
The SELECT...CASE command
EXIT is new, but it simply allows the program to “exit” a loop before it is
finished, and is often used in IF...THEN statements.
Your Turn – Different Tunes
9 Try replacing the RTTTL_File DATA directive in MicroMusicWithRTTTL.bs2
with each of the five different music files below.
Only one RTTTL_File DATA directive at a time! Make sure to replace, not add, your
new RTTTL_File DATA directive.
9 Run MicroMusicWithRTTTL.bs2 to test each RTTTL file.
RTTTL_File
DATA
"TwinkleTwinkle:d=4,o=7,b=120:c,c,g,g,a,a,2g,f,",
"f,e,e,d,d,2c,g,g,f,f,e,e,2d,g,g,f,f,e,e,2d,c,c,",
"g,g,a,a,2g,f,f,e,e,d,d,1c"
RTTTL_File
DATA
"FrereJacques:d=4,o=7,b=125:c,d,e,c,c,d,e,c,e,f",
",2g,e,f,2g,8g,8a,8g,8f,e,c,8g,8a,8g,8f,e,c,c,g6",
",2c,c,g6,2c"
RTTTL_File
DATA
"Beethoven5:d=8,o=7,b=125:g,g,g,2d#,p,f,f,f,2d"
RTTTL_File
DATA
"ForHe'sAJollyGoodFellow:d=4,o=7,b=320:c,2e,e,e,",
"d,e,2f.,2e,e,2d,d,d,c,d,2e.,2c,d,2e,e,e,d,e,2f,",
"g,2a,a,g,g,g,2f,d,2c"
Frequency and Sound · Page 283
RTTTL_File
DATA
"TakeMeOutToTheBallgame:d=4,o=7,b=225:2c6,c,a6,",
"g6,e6,2g.6,2d6,p,2c6,c,a6,g6,e6,2g.6,g6,p,p,a6",
",g#6,a6,e6,f6,g6,a6,p,f6,2d6,p,2a6,a6,a6,b6,c,",
"d,b6,a6,g6"
Downloading RTTTL Files: There are RTTTL files available for download from various
sites on the World Wide Web. These files are contributed by ring-tone enthusiasts, many of
whom are not music experts. Some phone tones are pretty good, others are barely
recognizable. If you want to download and play some more RTTTL files, make sure to
remove any spaces from between characters, then insert the text file between quotes.
SUMMARY
This chapter introduced techniques for making sounds and musical tones with the BASIC
Stamp and a piezoelectric speaker. The FREQOUT command can be used to send a
piezoelectric speaker high/low signals that cause it to make sound effects and/or musical
notes. The FREQOUT command has arguments that control the I/O Pin the signal is sent
to, the Duration of the tone, and the frequency of the tone (Freq1). The optional Freq2
argument can be used to play two tones at once.
Sound effects can be made by adjusting the frequency and duration of tones and the
pauses between them. The value of the frequency can also be swept across a range of
values to create a variety of effects.
Making musical notes also depends on frequency, duration, and pauses. The value of the
FREQOUT command’s Duration argument is determined by the tempo of the song and the
duration of the note (whole, half, quarter, etc.). The Freq1 value of the note is determined
by the note’s letter and octave. Rests between notes are used to set the duration of the
PAUSE command.
Playing simple songs using the BASIC Stamp can be done with a sequence of FREQOUT
commands, but there are better ways to store and retrieve musical data. DATA directives
along with their optional Symbol labels were used to store byte values using no prefix and
word values using the Word prefix. The READ command was used to retrieve values
stored by DATA directives. In this chapter’s examples, the READ command’s Location
argument always used the DATA directive’s optional Symbol label to differentiate between
different types of data. Some the Symbol labels that were used were Notes, Durations,
Dots, and Octaves.
Page 284 · What’s a Microcontroller?
Musical data can be stored in formats that lend themselves to translation from sheet
music. The sheet music style data can then be converted into frequencies using the
LOOKUP and LOOKDOWN commands. Mathematic operations can also be performed on
variable values to change the octave of a note by dividing its frequency by a power of
two. Mathematic operations are also useful for note durations given either the tempo or
the duration of a whole note.
SELECT...CASE was introduced as a way of evaluating a variable on a case by case
basis. SELECT...CASE is particularly useful for examining characters or numbers when
there are many choices as to what the variable could be and many different sets of actions
that need to be taken based on the variable’s value. A program that converts strings of
characters that describe musical tones for older cell phones (called RTTTL files) was
used to introduce a larger program that makes use of all the programming techniques
introduced in this text. SELECT...CASE played a prominent role in this program because
it is used to examine characters selected in an RTTTL file on a case-by-case basis.
Questions
1. What causes a tone to sound high-pitched? What causes a tone to sound lowpitched?
2. What does FREQOUT 15, 1000, 3000 do? What effect does each of the
numbers have?
3. How can you modify the FREQOUT command from Question 2 so that it sends
two frequencies at once?
4. If you strike a piano’s B6 key, what frequency does it send?
5. How do you modify a DATA directive or READ command if you want to store and
retrieve word values?
6. Can you have more than one DATA directive? If so, how would you tell a READ
command to get data from one or the other DATA directive?
7. If you know the frequency of a note in one octave, what do you have to do to
that frequency to play it in the next higher octave?
8. What does SELECT...CASE do?
Exercises
1. Modify the “Alarm…” tone from ActionTones.bs2 so that the frequency of the
tone it plays increases by 500 each time the tone repeats.
2. Explain how to modify MusicWithMoreFeatures.bs2 so that it displays an alert
message in the Debug Terminal each time a dotted note is played.
Frequency and Sound · Page 285
Project
1. Build a pushbutton-controlled tone generator. If one pushbutton is pressed, the
speaker should make a 2 kHz beep for 1/5 of a second. If the other pushbutton is
pressed the speaker should make a 3 kHz beep for 1/10 of a second.
Solutions
Q1. Our ears detect changes in air pressure as tones. A high-pitched tone is from
faster changes in air pressure, a low pitched tone from slower changes in air
pressure.
Q2. FREQOUT 15, 1000, 3000 sends a 3000 Hz signal out P15 for one second
(1000 ms). The effect of each number: 15 – I/O pin P15; 1000 – duration of tone
equals 1000 ms or one second; 3000 – the frequency of the tone, in hertz, so this
sends a 3000 Hz tone.
Q3. Use the optional Freq2 argument. To play 3000 Hz and say, 2000 Hz, we simply
add the second frequency to the command, after a comma:
FREQOUT 15, 1000, 3000, 2000
Q4. 1975.5 Hz, see Figure 8-3 on page 253.
Q5. Use the optional Word modifier before each data item.
Q6. Yes. Each DATA directive can have a different optional Symbol parameter. To
specify which DATA directive to get the data from, include the Symbol parameter
after the READ keyword. For example: READ Notes, noteLetter. In this
example, Notes is the Symbol parameter.
Q7. To get a given note in the next higher octave, multiply the frequency by two.
Q8. SELECT...CASE selects a variable or expression, evaluates it on a case by case
basis, and executes different blocks of code depending on which case the
variable's value fits into.
Page 286 · What’s a Microcontroller?
E1. This problem can be solved either by manually increasing each tone by 500, or
by utilizing a FOR...NEXT loop with a STEP value of 500.
Utilizing FOR...NEXT loop:
Manually increasing tone:
DEBUG "Increasing Alarm...",CR
DEBUG "Increasing alarm...", CR
PAUSE 100
PAUSE 100
FREQOUT 9, 500, 1500
FOR frequency = 1500 TO 3000 STEP 500
PAUSE 500
FREQOUT 9, 500, frequency
FREQOUT 9, 500, 2000
PAUSE 500
PAUSE 500
NEXT
FREQOUT 9, 500, 2500
PAUSE 500
FREQOUT 9, 500, 3000
PAUSE 500
E2. Modify the lines that check for the dotted note:
READ Dots + index, noteDot
IF noteDot = 1 THEN noteDuration = noteDuration * 3 / 2
Add a DEBUG command to the IF...THEN. Don't forget the ENDIF.
READ Dots + index, noteDot
IF noteDot = 1 THEN
noteDuration = noteDuration * 3 / 2
DEBUG "Dotted Note!", CR
ENDIF
P1. Use the piezospeaker circuit from Figure 8-2 on page 246; pushbutton circuits
from Figure 4-26 on page 130.
' What's a Microcontroller - Ch8Prj01_PushButtonToneGenerator.bs2
' P4 Pressed: 2 kHz beep for 1/5 second. 2 kHz = 2000 Hz.
'
1/5 s = 1000 / 5 ms = 200 ms
' P3 Pressed: 3 kHz beep for 1/10 second. 3 kHz = 3000 Hz.
'
1/10 s = 1000 / 10 ms = 100 ms
'{$STAMP BS2}
'{$PBASIC 2.5}
DEBUG "Program Running!"
DO
IF (IN4 = 1) THEN
FREQOUT 9, 200, 2000
ELSEIF (IN3 = 1) THEN
FREQOUT 9, 100, 3000
ENDIF
LOOP
' 2000 Hz for 200 ms
' 3000 Hz for 100 ms
Electronic Building Blocks · Page 287
Chapter 9: Electronic Building Blocks
THOSE LITTLE BLACK CHIPS
You need look no further than your BASIC Stamp (see Figure 9-1) to find examples of
“those little black chips.” Each of these chips has a special function. The upper-right
chip is the voltage regulator. This chip takes the battery voltage and converts it to almost
exactly 5.0 V, which is what the rest of the components on the BASIC Stamp need to run
properly. The upper-left chip is the BASIC Stamp module’s EEPROM. PBASIC
programs are condensed to numbers called tokens that are downloaded to the BASIC
Stamp. These tokens are stored in the EEPROM, and you can view them by clicking Run
and then Memory Map in the BASIC Stamp Editor. The largest chip is called the
Interpreter chip. It is a microcontroller pre-programmed with the PBASIC Interpreter
that fetches the tokens from the EEPROM and then interprets the PBASIC command that
the token represents. Then, it executes the command, fetches the next token, and so on.
This process is called fetch and execute.
2K EEPROM stores
PBASIC code and logged
data
→
5V Regulator
converts input
voltage to
regulated 5
volts
←
Figure 9-1
Integrated
Circuits on
the BASIC
Stamp 2
PBASIC Interpreter chip
(a pre-programmed
microcontroller)
→
Page 288 · What’s a Microcontroller?
People use the term “integrated circuit” (IC) to talk about little black chips. The
integrated circuit is actually a tiny silicon chip that’s contained inside the black plastic or
ceramic case. Depending on the chip, it may have anywhere between hundreds and
millions of transistors. A transistor is the basic building block for integrated circuits, and
you will have the opportunity to experiment with a transistor in this chapter. Other
familiar components that are designed into silicon chips include diodes, resistors and
capacitors.
Take a moment to think about the activities you’ve tried in this book so far. The list
includes switching LEDs on and off, reading pushbuttons, controlling servos, reading
potentiometers, measuring light, controlling displays, and making sounds. Even though
that’s just the beginning, it’s still pretty impressive, especially considering that you can
combine these activities to make more complex gadgets. The core of the system that
made all those activities possible is comprised of just the three integrated circuits shown
in Figure 9-1 and a few other parts. It just goes to show how powerful integrated circuits
can be when they are designed to work together.
EXPAND YOUR PROJECTS WITH PERIPHERAL INTEGRATED CIRCUITS
There are thousands of integrated circuits designed to be used with microcontrollers.
Sometimes different integrated circuit manufacturers make chips that perform the same
function. Sometimes each chip’s features differ slightly, and other times the chips are
almost identical but one might cost a little less than the other. Each one of the thousands
of different integrated circuits can be used as a building block for a variety of designs.
Companies publish information on how each of their integrated circuits work in
documents called datasheets and make them available on their web sites. These
manufacturers also publish application notes, which show how to use their integrated
circuit in unique or useful ways that make it easier to design products. The integrated
circuit manufacturers give away this information in hopes that engineers will use it to
build their chip onto the latest must-have toy or appliance. If thousands of toys are sold,
it means the company sells thousands of their integrated circuits.
In this chapter, you will experiment with a transistor, and a special-purpose integrated
circuit called a digital potentiometer. As mentioned earlier, the transistor is the basic
building block for integrated circuits. It’s also a basic building block for lots of other
circuits as well. The digital potentiometer also has a variety of uses. Keep in mind that
for each activity you have done, there are probably hundreds of different ways that you
could use each of these integrated circuits.
Electronic Building Blocks · Page 289
ACTIVITY #1: CONTROL CURRENT FLOW WITH A TRANSISTOR
In this activity, you will use a transistor as a way to control the current passing through an
LED. You can use the LED to monitor the current since it glows more brightly when
more current passes through it and less brightly when less current passes through it.
Introducing the Transistor
Figure 9-2 shows the schematic symbol and part drawing of the 2N3904 transistor. There
are many different types of transistors. This one is called NPN, which refers to the type
of materials used to manufacture the transistor and the way those materials are layered on
the silicon. The best way to get started thinking about a transistor is to imagine it as a
valve that is used to control current. Different transistors control how much current
passes through by different means. This transistor controls how much current passes into
C (collector) and back out of E (emitter). It uses the amount of current allowed into the B
(base) terminal to control the current passing from C through E. With a very small
amount of current allowed into B, a current flow of about 416 times that amount flows
through the transistor into C and out of E.
C
C
B
E
Figure 9-2
2N3904 Transistor
B
2N3904
E
The 2N3904 Part Datasheet: As mentioned earlier, semiconductor manufacturers publish
documents called datasheets for the parts they make.
These datasheets contain
information engineers use to design the part into a product. To see an example of a part
datasheet for the 2N3904: Go to www.fairchildsemi.com. Enter “2N3904” into the Search
field on Fairchild Semiconductor’s home page, and click Go. One of the search results
should be a link to the 2N3904 product information. Follow it and look for a Datasheet link.
Most web browsers display the datasheet by opening it with Adobe Acrobat Reader.
Page 290 · What’s a Microcontroller?
Transistor Example Parts
(1) Transistor – 2N3904
(2) Resistors – 100 kΩ (brown-black-yellow)
(1) LED – any color
(1) Potentiometer – 10 kΩ
(3) Jumper wires
Building and Testing the Transistor Circuit
Figure 9-3 shows a circuit that you can use to manually control how much current the
transistor allows through the LED. By twisting the knob on the potentiometer, the circuit
will deliver different amounts of current to the transistor’s base. This will cause a change
in the amount of current the transistor allows to pass from its collector to its emitter. The
LED will give you a clear indication of the change by glowing more or less brightly.
9 Build the circuit shown in Figure 9-3.
o Make sure that the LED’s anode (longer) pin is connected to Vdd.
o Double-check your transistor circuit. Note that the transistor’s flat sice is
facing to the right in the wiring diagram.
9 Turn the knob on the potentiometer and verify that the LED changes brightness
in response to a change in the position of the potentiometer’s wiper terminal.
Vdd
Vdd
LED
100 kΩ
POT
10 kΩ
100 kΩ
Vss
Vss
Figure 9-3
Manual
PotentiometerControlled
Transistor
Circuit
Electronic Building Blocks · Page 291
Your Turn – Switching the Transistor On/Off
If all you want to do is switch a transistor on and off, you can use the circuit shown in
Figure 9-4. When the BASIC Stamp sends a high signal to this circuit, it will make it so
that the transistor conducts as much current as if you adjusted the potentiometer for
maximum brightness. When the BASIC Stamp sends a low signal to this circuit, it will
cause the transistor to stop conducting current, and the LED should emit no light.
What’s the difference between this and connecting an LED circuit to an I/O pin?
BASIC Stamp I/O pins have limitations on how much current they can deliver. Transistors
have limitations too, but they are much higher. In the Process Control Student Guide, a
transistor is used to drive a small DC fan. It is also used to supply large amounts of current
to a small resistor that is used as a heating element. Either of these two applications would
draw so much current that they would quickly damage the BASIC Stamp, but the transistor
takes it in stride.
9 Build the circuit shown in Figure 9-4.
9 Write a program that sends high and low signals to P8 twice every second.
HINT: LedOnOff.bs2 from Chapter 2 needs only to be modified to send
high/low signals to P8 instead of P14. Remember to save it under a new name
before making the modifications.
9 Run the program and verify that it gives you on/off control of the LED.
Vdd
Vdd
Vin
Vss
X3
LED
P8
100 kΩ
100 kΩ
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Figure 9-4
Circuit giving
BASIC Stamp
On/Off Control
of Current to
LED with a
Transistor
Page 292 · What’s a Microcontroller?
ACTIVITY #2: INTRODUCING THE DIGITAL POTENTIOMETER
In this activity, you will replace the manually adjusted potentiometer with an integrated
circuit potentiometer that is digitally adjusted. You will then program the BASIC Stamp
to adjust the digital potentiometer, which will in turn adjust the LED’s brightness in the
same way the manual potentiometer did in the previous activity.
Introducing the Digital Potentiometer
Figure 9-5 shows a pin map of the digital potentiometer you will use in this activity. This
chip has 8 pins, four on each side that are spaced to make it easy to plug into a
breadboard (1/10 inch apart). The manufacturer places a reference notch on the plastic
case so that you can tell the difference between pin 1 and pin 5. The reference notch is a
small half-circle in the chip’s case. You can use this notch as a reference for the pin
numbers on the chip. The pin numbers on the chip count upwards, counterclockwise
from the reference notch.
Part Substitutions: It is sometimes necessary for Parallax to make a part substitution. The
part will function the same, but the label on it may be different. If you find that the digital
potentiometer included in your What’s a Microcontroller Parts Kit is not labeled AD5220, rest
assured that it will still work the same way and perform correctly in this activity.
Reference
Notch
Figure 9-5
AD5220 Pin Map
1 CLK
Vdd 8
2 U/D
CS 7
3 A1
B1 6
4 GND
W1 5
AD5220
Use the reference notch to make
sure you have the AD5220 right-sideup when building it into your circuit on
the breadboard.
Electronic Building Blocks · Page 293
Here is a summary of each of the AD5220’s pins and functions:
1. CLK: The pin that receives clock pulses (low-high-low signals) to move the
wiper terminal.
2. U/D: The pin that receives a high signal to make the wiper (W1) terminal move
towards A1, and a low signal to make it move towards B1. This pin just sets the
direction, the wiper terminal doesn’t actually move until a pulse (a low – high –
low signal) is sent to the CLK pin.
3. A1: The potentiometer’s A terminal.
4. GND: The ground connection. The ground on the Board of Education and
BASIC Stamp HomeWork Board is the Vss terminal.
5. W1: The potentiometer’s wiper (W) terminal.
6. B1: The potentiometer’s B terminal.
7. CS: The chip select pin. Apply a high signal to this pin, and the chip ignores all
control signals sent to CLK and U/D. Apply a low signal to this pin, and it acts
on any control signals it receives.
8. Vdd: Connect to +5 V, which is Vdd on the Board of Education and BASIC
Stamp HomeWork Board.
The AD5220 Part Datasheet: To see the part datasheet for the AD5220: Go to
www.analog.com. Enter “AD5220” into the Search field on Analog Devices’ home page, and
click the Search button. Click the Data Sheets link. Click the link that reads “AD5220:
Increment/Decrement Digital Potentiometer Datasheet”.
Digital Pot Controlled Transistor Parts
(1) Transistor – 2N3904
(2) Resistors – 100 kΩ (brown-black-yellow)
(1) LED – any color
(1) Digital potentiometer – AD5220
(10) Jumper wires
Building the Digital Potentiometer Circuit
Figure 9-6 shows a circuit schematic with the digital potentiometer used in place of a
manual potentiometer, and Figure 9-7 shows a wiring diagram for the circuit. The
BASIC Stamp can control the digital potentiometer by issuing control signals to P5 and
P6.
9 Build the circuit shown in Figure 9-6 and Figure 9-7.
Page 294 · What’s a Microcontroller?
Vdd
Vdd
Vdd
AD5220
P6
1
CLK
Vdd 8
P5
2
U/D
CS 7
3
A1
B1 6
GND
4
W1 5
Figure 9-6
Digital Potentiometer
Controlled Transistor
Circuit Schematic
100 kΩ
100 kΩ
Vss
Vss
Vdd
Vin
Vss
X3
Figure 9-7
Wiring Diagram for
Figure 9-6
AD5220
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Programming Digital Potentiometer Control
Imagine that the knob on the manual potentiometer from the previous exercise has 128
positions. Imagine also that the potentiometer is in the middle of its range of motion.
That means you could rotate the knob one direction by 63 steps and the other direction by
64 steps.
Let’s say you turn the potentiometer’s knob one step clockwise. The LED will get only
slightly brighter. This would be the same as sending a high signal to the AD5220’s U/D
pin and sending one pulse (high-low-high) to the CLK pin.
HIGH 5
PULSOUT 6, 1
Electronic Building Blocks · Page 295
Imagine next that you turn your manual potentiometer 3 steps counterclockwise. The
LED will get a little bit dimmer. This would be the same as sending a low signal to the
U/D pin on the AD5220 and sending three pulses to the CLK pin.
LOW 5
FOR counter = 1 TO 3
PULSOUT 6, 1
PAUSE 1
NEXT
Imagine next that you turn the potentiometer all the way clockwise. That’s the same as
sending a high signal to the AD5220’s U/D pin and sending 65 pulses to the CLK pin.
Now the LED should be shining brightly.
HIGH 5
FOR counter = 1 TO 65
PULSOUT 6, 1
PAUSE 1
NEXT
Finally, imagine that you turn your manual potentiometer all the way counterclockwise.
The LED should emit no light. That’s the same as sending a low signal to the U/D pin,
and applying 128 pulses to the CLK pin
LOW 5
FOR counter = 0 TO 127
PULSOUT 6, 1
PAUSE 1
NEXT
Example Program: DigitalPotUpDown.bs2
This example program adjusts the potentiometer up and down, from one end of its range
to the other, causing the LED to get gradually brighter, then gradually dimmer.
9 Enter and run DigitalPotUpDown.bs2.
Page 296 · What’s a Microcontroller?
' What's a Microcontroller - DigitalPotUpDown.bs2
' Sweep digital pot through values.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
counter
VAR
Byte
DO
LOW 5
FOR counter = 0 TO 127
PULSOUT 6, 1
PAUSE 10
NEXT
HIGH 5
FOR counter = 0 TO 127
PULSOUT 6, 1
PAUSE 10
NEXT
LOOP
Your Turn – Changing the Rate and Condensing the Code
You can increase or decrease the rate at which the LED gets brighter and dimmer by
changing the PAUSE command’s Duration argument.
9 Modify and re-run the program using PAUSE 20 and note the difference in the
rate that the LED gets brighter and dimmer.
9 Repeat for PAUSE 5.
You can also use a command called TOGGLE to make this program simpler. TOGGLE
changes the state of a BASIC Stamp I/O pin. If the I/O pin was sending a high signal,
TOGGLE makes it send a low signal. If the I/O pin was sending a low signal, TOGGLE
makes it send a high signal.
9 Save DigitalPotUpDown.bs2 as DigitalPotUpDownWithToggle.bs2.
9 Modify the program so that it looks like the one by that name shown below.
9 Run the program and verify that it functions the same way as the
DigitalPotUpDown.bs2.
9 Compare the number of lines of code it took to do the same job.
Electronic Building Blocks · Page 297
Running out of program memory is a problem some people encounter when their BASIC
Stamp projects get large and complicated. Using TOGGLE instead of two FOR...NEXT
loops is just one example of many techniques that can be used to do the same job with half
the code.
' What's a Microcontroller - DigitalPotUpDownWithToggle.bs2
' Sweep digital pot through values.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
counter
LOW 5
VAR
Byte
DO
FOR counter = 0 TO 127
PULSOUT 6,5
PAUSE 10
NEXT
TOGGLE 5
LOOP
Looking Inside the Digital Potentiometer
Figure 9-8 shows a diagram of the potentiometer inside the AD5220. The AD5220 has
128 resistive elements, each of which is 78.125 Ω (nominal value). All 128 of these add
up to 10,000 Ω or 10 kΩ.
3
A1
78 Ω
Ad5220
pos. 127
1 CLK
2 U/D
5
7 CS
78 Ω
pos. 126
W1
40 Ω
78 Ω
pos. 125
…
…
78 Ω
pos. 1
78 Ω
B1
pos. 0
6
Figure 9-8
Inside the AD5220
Page 298 · What’s a Microcontroller?
A nominal value means a named value. Parts like resistors and capacitors typically have
a nominal value and a tolerance. Each of the AD5220’s resistive elements has a nominal
value of 78.125 Ω, with a tolerance of 30% (23.438 Ω) above or below the nominal value.
Between each of these resistive elements is a switch, called a tap. Each switch is actually
a group of transistors that are switched on or off to let current pass or not pass. Only one
of these switches can be closed at one time. If one of the upper switches is closed (like
pos. 125, 126, or 127), it’s like having the manual potentiometer knob turned most or all
the way clockwise. If pos. 0 or 1 is closed, it’s like having a manual potentiometer turned
most or all the way counterclockwise.
Imagine that Pos. 126 is closed. If you want to set the tap to 125, (open pos. 126 and
close pos. 125), set U/D low, then apply a pulse to CLK. If you want to set the tap to Pos
127, set U/D high, and apply 2 pulses. If you want to bring the tap down to 1, set U/D
low, and apply 126 pulses.
This next example program uses the Debug Terminal to ask you which tap setting you
want. Then it decides whether to set the U/D pin high or low, and applies the correct
number of pulses to move the tap from its old setting to the new setting.
With the exception of EEPROM Data, the next example program also has all the sections
you could normally expect to find in an application program:
•
•
•
•
•
•
•
•
Title – comments that include the filename of a program, its description, and the
Stamp and PBASIC directives
EEPROM Data – DATA declarations that store predefined lists of values in
portions of EEPROM memory that are not needed for program storage
I/O Definitions – PIN directives that name I/O pins
Constants – CON declarations that name values in the program
Variables – VAR declarations that assign names to portions of BASIC Stamp’s
RAM memory for storing values
Initialization – a routine that gets the program started on the right foot. In this
next program, the potentiometer’s tap needs to be brought down to zero
Main – the routine that handles the primary jobs the program has to do
Subroutines – the segments of code that do specific jobs, either for each other, or
in this case, for the main routine
Electronic Building Blocks · Page 299
Example Program: TerminalControlledDigtialPot.bs2
You can use this example program and the Debug Terminal to set the digital pot’s tap.
By changing the tap setting on the digital pot, you change the brightness of the LED
connected to the transistor that the digital pot controls. Figure 9-9 shows an example of
entering the value 120 into the Debug Terminal’s Transmit windowpane while the
program is running. Since the old tap setting was 65, the LED becomes nearly twice as
bright when it is adjusted to 120.
Windowpanes
Figure 9-9
Sending Messages
to the BASIC Stamp
Transmit →
Click the Transmit
(upper) windowpane
and enter the
numbers for the new
tap setting.
Receive →
9
9
9
9
Enter and run TerminalControlledDigtialPot.bs2.
Make sure the Echo Off box is clear (no checkmark).
Click the Debug Terminal’s Transmit windowpane to place the cursor there.
Enter values between 0 and 127 into the Debug Terminal. Make sure to press
the enter key after you type in the digits.
' -----[ Title ]----------------------------------------------------------' What's a Microcontroller - TerminalControlledDigitalPot.bs2
' Update digital pot's tap based on Debug Terminal user input.
' {$STAMP BS2}
' {$PBASIC 2.5}
' -----[ EEPROM Data ]-----------------------------------------------------
' -----[ I/O Definitions ]------------------------------------------------UdPin
ClkPin
PIN
PIN
5
6
' Set values of I/O pins
' connected to CLK and U/D.
Page 300 · What’s a Microcontroller?
' -----[ Constants ]------------------------------------------------------DelayPulses
DelayReader
CON
CON
10
2000
' Delay to observe LED fade.
' -----[ Variables ]------------------------------------------------------counter
oldTapSetting
newTapSetting
VAR
VAR
VAR
Byte
Byte
Byte
' Counter for FOR...NEXT.
' Previous tap setting.
' New tap setting.
' -----[ Initialization ]-------------------------------------------------oldTapSetting = 0
newTapSetting = 0
' Initialize new and old
' tap settings to zero.
LOW UdPin
FOR counter = 0 TO 127
PULSOUT 6,5
PAUSE 1
NEXT
PAUSE 1000
' Set U/D pin for Down.
' Set tap to lowest position.
' Wait 1 s before 1st message
' -----[ Main Routine ]---------------------------------------------------DO
GOSUB Get_New_Tap_Setting
GOSUB Set_Ud_Pin
GOSUB Pulse_Clk_pin
' User display and get input.
' Set U/D pin for up/down.
' Deliver pulses.
LOOP
' -----[ Subroutines ]----------------------------------------------------Get_New_Tap_Setting:
' Display instructions and
' get user input for new
' tap setting value.
DEBUG CLS, "Tap setting is: ",
DEC newTapSetting, CR, CR
DEBUG "Enter new tap", CR, "setting (0 TO 127): "
DEBUGIN DEC newTapSetting
RETURN
Set_Ud_Pin:
IF newTapSetting > oldTapSetting THEN
HIGH UdPin
oldTapSetting = oldTapSetting + 1
ELSEIF newTapSetting < oldTapSetting THEN
LOW UdPin
oldTapSetting = oldTapSetting - 1
'
'
'
'
'
Examine new and old tap values
to decide value of U/D pin.
Notify user if values are
equal.
Increment for Pulse_Clk_pin.
' Decrement for Pulse_Clk_pin.
Electronic Building Blocks · Page 301
ELSE
DEBUG CR, "New and old settings", CR,
"are the same, try ", CR,
"again...", CR
PAUSE DelayReader
ENDIF
' Give reader time to view
' Message.
RETURN
Pulse_Clk_pin:
' Deliver pulses from old to new values. Keep in mind that Set_Ud_Pin
' adjusted the value of oldTapSetting toward newTapSetting by one.
' This keeps the FOR...NEXT loop from executing one too many times.
FOR counter = oldTapSetting TO newTapSetting
PULSOUT ClkPin, 1
PAUSE DelayPulses
NEXT
oldTapSetting = newTapSetting
RETURN
' Keep track of new and old
' tapSetting values.
Page 302 · What’s a Microcontroller?
SUMMARY
This chapter introduced integrated circuits and how they can be used with the BASIC
Stamp. A transistor was used as a current valve, and a digital potentiometer was used to
control the amount of current passing through the transistor. Examining the digital
potentiometer introduced the reference notch and pin map as important elements of
electronic chips. The function of each of the digital potentiometer pins was discussed, as
well as the device’s internal structure. The PBASIC command TOGGLE was introduced as
a means to save program memory.
Questions
1. What are the names of the terminals on the transistor you used in this chapter?
2. Which terminal controls the current passing through the transistor?
3. What can you do to increase or decrease the current passing through the
transistor?
Exercise
1. Write a program that adjusts the tap in the digital pot to position 0 regardless of
its current setting.
Project - Advanced Challenge
1. Add a phototransistor to your project and cause the brightness of the LED to
adjust with the brightness seen by the phototransistor. Note: the solution given is
worth reading, as it demonstrates a useful approach to scaling an input to another
output.
Electronic Building Blocks · Page 303
Solutions
Q1. Emitter, base, and collector.
Q2. The base controls the current passing through the transistor.
Q3. Increase or decrease the current allowed into the transistor's base.
E1. To solve this exercise, look at TerminalControlledDigitalPot.bs2. The first thing
it does, in the Initialization section, is to set the tap to the lowest position. This
exact code is used in the solution below.
' What's a Microcontroller - Ch9Ex01_SetTapToZero.bs2
' Turn tap on digital pot all the way down to zero
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
UdPin
ClkPin
counter
PIN
PIN
VAR
LOW UdPin
FOR counter = 0 TO 128
PULSOUT ClkPin,5
PAUSE 1
NEXT
5
6
Byte
' Set values of I/O pins
' connected to CLK and U/D.
' Counter for FOR...NEXT.
' Set U/D pin for Down.
' Set tap to lowest position.
P1. Use the digital potentiometer circuit from Figure 9-6 on page 294 and the
phototransistor circuit from Figure 7-4 on page 200.
This solution builds on TerminalControlledDigitalPot.bs2, and incorporates
elements from PhototransistorAnalogToBinary.bs2 from Chapter 7, Activity #5.
It also applies some algebra to solve a scaling problem that makes the range of
values you could get from the phototransistor RCTIME measurement fit into a
range of 0 to 128 for the digital potentiometer. Keep in mind that this is one
example solution, and by no means the only solution or approach.
The GOSUB Get_New_Tap_Setting subroutine call from the program
TerminalControlledDigitalPot.bs2 is replaced by two other subroutine calls:
GOSUB Read_Phototransistor and GOSUB Scale_Phototransistor.
Likewise, the Get_New_Tap_Setting subroutine is replaced by
Read_Phototransistor
and
Scale_Phototransistor
subroutines.
Read_Phototransistor is a subroutine version of the commands that take the
phototransistor RCIME measurement and limit its input range in
Page 304 · What’s a Microcontroller?
PhototransistorAnalogToBinary.bs2. The pin, constant and variable names have
been adjusted, and the PAUSE 100 for a 10-times-per-second display was
changed to PAUSE 1, which is all that’s needed to charge the capacitor before
taking the RCTIME measurement. After this subroutine stores a value in the
lightReading variable, it will be somewhere between ValMin (100) and
ValMax (4000). Make sure to test and adjust these values for your own lighting
conditions.
The problem we now have is that there are only 128 tap settings, and 3900
possible phototransistor RCTIME measurements. To fix this, we need to divide
the phototransistor RCTIME measurement by some value to make it fit into the 0
to 127 range. So, we know we need to divide the range of input values by some
value to make it fit into 128 values. It looks like this in an equation:
Range of Possible Phototransistor Measurements
= 128 Possible Tap Settings
Scale Divisor
To solve this, multiply both sides of the equation by Scale Divisor, and then
divide both sides by 128 Possible Tap Settings.
Scale Divisor =
Range of Possible Phototransistor Measurements
128 Possible Tap Settings
In the code, the range of possible phototransistor measurements is ValMax –
ValMin, scaleDivisor is a variable, and 128 is a constant. So, this code from
the Declarations and Initialization section figures out the value of
scaleDivisor like this:
scaleDivisor = (valMax – valMin) / 128
After every phototransistor RCTIME measurement, the Scale_Phototransistor
subroutine subtracts valMin from lightReading and then divides the
measurement by scaleDivisor. The result maps the 100 to 4000 input
measurement range to a 0 to 127 output tap setting range.
Scale_Phototransistor:
lightReading = (lightReading – valMin) / scaleDivisor
RETURN
Electronic Building Blocks · Page 305
Assuming ValMin is 100 and ValMax is 4000, the lightReading variable could
store 3900 possible values. What if the input range was ValMin = 10,000 to
ValMax = 13,900? When you subtract ValMin = 10,000, there are still 3900
possible values, and dividing scaleDivisor into it will correctly map the
measurement to the corresponding digital pot tap setting. If your code didn’t
first subtract ValMin, the resulting scaled value would be completely out of the 0
to 128 range for the digital pot.
'
'
'
'
What's a Microcontroller - Ch9Prj01_PhotoControlledDigitalPot.bs2
Update digital pot's tap based on phototransistor reading
{$STAMP BS2}
{$PBASIC 2.5}
' -----[ Declarations and Initialization]-----------------------------UdPin
PIN
5
' Set values of I/O pins
ClkPin
PIN
6
' connected to CLK and U/D.
PhotoPin
PIN
2
' Phototransistor on pin P2
DelayPulses
CON
10
' Delay to observe LED fade.
DelayReader
CON
2000
valMax
CON
4000
' Max phototransistor val
valMin
CON
100
' Min phototransistor val
counter
oldTapSetting
newTapSetting
lightReading
scaleDivisor
VAR
VAR
VAR
VAR
VAR
Byte
Byte
Byte
Word
Word
'
'
'
'
'
Counter for FOR...NEXT.
Previous tap setting.
New tap setting.
reading from phototransistor
For scaling values
' Set up a value that can be divided into the phototransistor RCTIME
' measurement to scale it to a range of 0 to 128
scaleDivisor = (valMax - valMin) / 128
oldTapSetting = 0
newTapSetting = 0
' Initialize new and old
' tap settings to zero.
LOW UdPin
FOR counter = 0 TO 127
PULSOUT ClkPin,5
PAUSE 1
NEXT
' Set U/D pin for Down.
' Set tap to lowest position.
PAUSE 1000
' 1 sec. before 1st message
' -----[ Main Routine ]-----------------------------------------------DO
GOSUB Read_Phototransistor
GOSUB Scale_Phototransistor
Page 306 · What’s a Microcontroller?
newTapSetting = lightReading MIN 1 MAX 127
DEBUG HOME, DEC5 lightReading
GOSUB Set_Ud_Pin
' Set U/D pin for up/down.
GOSUB Pulse_Clk_pin
' Deliver pulses.
LOOP
' -----[ Subroutines ]------------------------------------------------Set_Ud_Pin:
' Examine old and new
IF newTapSetting > oldTapSetting THEN
' tap values to decide
HIGH UdPin
ELSEIF newTapSetting < oldTapSetting THEN
' value of UdPin. Notify
LOW UdPin
' user if values are
ENDIF
' equal.
RETURN
Pulse_Clk_pin:
' Deliver pulses
FOR counter = oldTapSetting TO newTapSetting ' from old to new
PULSOUT ClkPin, 1
' values.
PAUSE DelayPulses
NEXT
oldTapSetting = newTapSetting
' Keep track of new and old
RETURN
' tapSetting values.
Read_Phototransistor:
HIGH PhotoPin
PAUSE 1
RCTIME PhotoPin, 1, lightReading
lightReading = lightReading MAX valMax MIN valMin
RETURN
Scale_Phototransistor:
lightReading = (lightReading - valMin) / scaleDivisor
RETURN
Prototyping Your Own Inventions · Page 307
Chapter 10: Prototyping Your Own Inventions
This text introduced the basics of integrating an onboard computer into projects and
inventions. Common circuit ingredients in everyday products that you now have some
experience with include: indicator lights, buttons, servos, dials, digital displays, light
sensors, speakers, transistors, and other integrated circuits. You also now have
experience connecting these circuits to the BASIC Stamp microcontroller and writing
code to test each of them as well as integrate them into small applications.
At this point, you may be interested in using your new skills to invent something, or to
learn more, or maybe both. What you have learned in this book can get you well down
the road to making prototypes for a wide variety of inventions. In this chapter, we’ll use
a micro alarm system as an example prototype of a familiar device. Along the way, we’ll
cover some important prototyping techniques and habits, including:
•
•
•
•
•
•
•
Suggestions for early development of your design ideas and inventions
An example of how to build and test each sub-system in the prototype
Examples of how to incorporate test code into the project code
Good practices for code commenting and file versioning
Examples of using familiar parts as stand-ins for devices with similar interfaces
Tips and tricks for getting past design hurdles
Where to go next to find more Stamps in Class projects and interesting devices
APPLY WHAT YOU KNOW TO OTHER PARTS AND COMPONENTS
The pushbutton circuit from Chapter 3 is an example of a very simple input device that
converts a physical condition (whether or not someone has pressed a button) to a high or
low signal the BASIC Stamp can detect and process. You have also used pushbuttons in
applications that controlled light blinking, servo positions and speaker tones. There are
many sensors that detect a physical condition other than “contact” that also send high or
low signals a BASIC Stamp I/O pin can monitor. A few examples include gas, motion,
and sound sensors, and there are many, many more. Since you now have experience
making the BASIC Stamp monitor a pushbutton circuit, monitoring a sound or motion
sensor is very similar, and certainly a reasonable next step.
Another technique from this book is measuring RC decay with the RCTIME command to
sense potentiometer knob position and light levels with both a phototransistor and an
LED. These examples are just the tip of the iceberg in terms of sensors you can use with
Page 308 · What’s a Microcontroller?
an RC decay circuit. Other examples include humidity, temperature, and pressure, and
that’s still just the beginning. The LED indicator light provides still another example
circuit that’s representative of a variety of circuits with different functions. The LED
circuit is controlled by high/low BASIC Stamp I/O pin output. With additional support
circuits, you can use high/low signals to run electric motors forwards and backwards, turn
lights on/off, turn heating elements on/off, and more.
Now, think about all the other devices you have experimented with in this book. Each of
them is just one example in a list of devices with similar interfaces that you can use to
prototype all manner of inventions.
PROTOTYPING A MICRO SECURITY SYSTEM
In this chapter, we’ll use parts from the What’s a Microcontroller kit to make a very
small security system prototype you could use in a desk, dresser, tool chest, or closet. It
could come in handy for those of you who suspect siblings or coworkers of borrowing
your stuff without asking. With this prototype, we’ll also investigate other parts and
components you could substitute in your security system that operate on the same
principles as the familiar kit parts, but could give your system greatly enhanced
functionality. From there, we’ll look at how to find, understand, test and incorporate
other parts that you may never have worked with before.
ACTIVITY #1: FROM IDEA TO PROOF OF CONCEPT
Many products start out as an idea, in some cases an invention that could be “really cool,”
and in other cases it’s something that solves a problem. This idea can be developed into a
concept with drawings and specifications, and some early design work. The next step is
typically to develop a working prototype. It might not be pretty, but it should reliably
demonstrate that a device can be made that works according to the concept and
specification. In companies that develop products, this proof of concept is typically
required to get management approval and funding to continue developing the product.
Idea, Concept, and Functional Description
Let’s say you have a cabinet with a door on a hinge and a drawer, and it needs a very
small alarm system. Or maybe you want to design a special cabinet with built-in
security. Figure 10-1 shows a sketch of how a potentiometer and electrical contact
similar to a pushbutton could be used to detect when either the door or drawer is open.
This sketch is similar to a concept diagram, which focuses only on conveying what the
product or invention does.
Prototyping Your Own Inventions · Page 309
Figure 10-1
Concept
Sketch of a
Cabinet Micro
Security
System
The functional description is important. When you have a better idea of what your device
is supposed to do at the beginning, it prevents problems that can happen if you have to
redesign the device to accommodate something you didn’t think about. Designers and
companies that create custom devices for customers have to be very careful to crossexamine their customers to understand what they expect. Especially for customengineered devices, redesigns can be hugely expensive and time consuming.
Here is an example of a very brief functional description we can use for our simple
system: Develop a circuit and program prototype for a micro alarm system that can
monitor one small door that’s on a hinge and a drawer. If armed, an alarm should sound
if either door or drawer is opened. A status LED should glow green when the alarm is
not armed, and red when it is armed. A prototype may be armed and disarmed by
computer control. A time delay should be incorporated after the device has been armed
to allow the user to close the cabinet.
Page 310 · What’s a Microcontroller?
Specification
Beyond the functional description, a specification typically accounts for as many aspects
of the proposed device as possible, including: cost, power consumption, voltage supply,
dimensions, weight, speaker volume, and many other details.
Initial Design
Often, the initial design involves brainstorming for approaches that “might” solve the
design problem, and many of these ideas have to be tested to find out if they really are
feasible. Other portions of the design might involve fairly standard or common parts and
design practices. Our micro alarm fits in this category, at least for the prototype. A
pushbutton could be mounted in the cabinet so that when the drawer is closed, it presses
the pushbutton. For the door on a hinge, a potentiometer could be attached so that it
twists with the door and can sense the door’s position. The bicolor LED is a familiar
indicator, and the piezospeaker is certainly a well-known alarm noise maker.
So, now we know the circuits we need for our micro security cabinet prototype: bicolor
LED, pushbutton, potentiometer, and piezospeaker. Here is a list of chapters and
activities where each of these circuits was introduced:
•
•
•
•
Bicolor LED: Chapter 2, Activity #5
Pushbutton: Chapter 3, Activity #2
Potentiometer: Chapter 5, Activity #3
Piezospeaker: Chapter 8, Activity #1
Cabinet Alarm Parts List:
Going back into each chapter and putting all the parts together results in this parts list:
(3) Resistors – 220 Ω (red-red-brown)
(1) Resistors – 10 kΩ (brown-black-orange)
(1) LED – bicolor
(1) Pushbutton – normally open
(1) Piezospeaker
(1) Capacitor – 0.01 µF
(1) Potentiometer – 10 kΩ
(4) Jumper wires
Cabinet Alarm Schematic
The schematic in Figure 10-2 is arranged to give all the components plenty of space on
the breadboard, so not all the I/O pin connections are the same as they were in earlier
chapters. You’ll have to keep this in mind when you harvest code examples from the
earlier chapters to test each of the circuits.
Prototyping Your Own Inventions · Page 311
Figure 10-2: Alarm System Prototype Schematic
ACTIVITY #2: BUILD AND TEST EACH CIRCUIT INDIVIDUALLY
Whenever possible, test each subsystem individually before trying to make them work
together. If you follow this rule, your projects will go more smoothly, and it’ll save a lot
of troubleshooting time. For example, if all the circuits are built but not tested, people
have a natural tendency to spend too much time examining code and forget to check each
circuit. So, the most important time savings in this procedure is in making sure that there
are no circuit mistakes trying to trick you into thinking they are coding errors.
Page 312 · What’s a Microcontroller?
Building and Testing Each Circuit
This activity demonstrates focusing on individual subsystems by building and testing
each circuit. Once the pushbutton circuit is built and tested, we’ll build and test the
speaker circuit. After repeating this process with the potentiometer and bicolor LED, the
circuits will all be “known good” and ready for some application code.
9 Find test code in Chapter 3, Activity #2 that you can adapt to testing the Figure
10-2 pushbutton circuit.
9 Change the I/O pin references so that it works with the circuit in Figure 10-2.
9 Test the code and correct any bugs or wiring errors before continuing.
9 Repeat this same process for:
o Piezospeaker circuit from Chapter 8, Activity #1
o Potentiometer circuit from Chapter 5, Activity #3
o Bicolor LED circuit from Chapter 2, Activity #5
9 Make sure to save each modified program under a new name, preferably in a
separate folder, maybe named “WAM Chapter 10.”
Your Turn – System Test
Now that all the circuits are tested and all the test programs saved on your PC, it’s time to
build up a system test that displays debug messages indicating which circuit is being
tested as it executes the test code. This is a useful exercise because typical alarm systems
have self-test and diagnostic modes that utilize all the features in one routine.
9 Combine elements in your test programs into a single program that it:
o Starts by displaying the color of the bicolor LED in the Debug Terminal
as it updates the color...
o Then displays a message that the piezospeaker is making sound while it
beeps...
o Finally enters a loop that repeatedly reports the pushbutton drawer
sensor and potentiometer hinged door sensor status in the Debug
Terminal.
9 Test for and fix any bugs before continuing.
Prototyping Your Own Inventions · Page 313
ACTIVITY #3: ORGANIZE CODING TASKS INTO SMALL PIECES
Just as each circuit should be built and tested before making them work together, each
feature of the code should also be developed and tested individually before incorporating
it into the larger application. MicroAlarmProto(Dev-009).bs2 is an example of a program
that’s on its way to a proof of concept. Its Debug Terminal user interface is mostly in
place, and the alarm system correctly cycles through its various modes or states,
including not armed, arming, armed, and triggered.
At this point, the Alarm_Arming subroutine at the end of the program is still under
construction. It has code in place that triggers the alarm if the pushbutton is released,
which indicates that the drawer has been opened, but it does not yet monitor the hinged
door. Potentiometer code needs to be added to the Check_Sensors subroutine that
measures its position. If its position is beyond a certain threshold, 15 for example, the
state variable should be changed to Triggered. Two additional tasks that remain are to
turn the bicolor LED green when the alarm is not armed, and red when it is armed. These
remaining tasks are indicated by comments in the code that look like this:
' To-do: bicolor LED green
...
' To-do: bicolor LED red
...
' To-do: Check if Potentiometer is over threshold
value. If yes, then, trigger alarm
9 Hand-enter MicroAlarmProto(Dev-009).bs2 into the BASIC Stamp Editor
(recommended), or download it from www.parallax.com/go/WAM and open it
with the BASIC Stamp Editor.
9 Examine the program and note how each subroutine is modular, and does a
specific job. This is part of organizing coding tasks into small pieces.
9 If you do not remember how to use the Debug Terminal’s Transmit and Receive
windowpanes, review Figure 9-9 on page 299.
9 Load MicroAlarmProto(Dev-009).bs2 into the BASIC Stamp and use the Debug
Terminal’s Transmit windowpane to type the character A to arm the alarm, and
D to disarm the alarm. The system does a brief countdown before arming the
alarm. Make sure to press and hold the pushbutton before the alarm arms.
9 While the alarm is armed, release the button. You will have a chance to disarm
the alarm after a few seconds of alarm tone.
9 Arm the alarm again. This time, type “D” to disarm the alarm before releasing
the button.
Page 314 · What’s a Microcontroller?
' -----[ Title ]----------------------------------------------------------'What's a Microcontroller - MicroAlarmProto(Dev-009).bs2
'Test cabinet alarm system.
' {$STAMP BS2}
' {$PBASIC 2.5}
' Target = BASIC Stamp 2
' Language = PBASIC 2.5
' -----[ Constants ]------------------------------------------------------NotArmed
CON 0
' Alarm system states
Arming
CON 1
Armed
CON 3
Triggered CON 4
' -----[ Variables ]------------------------------------------------------seconds
VAR Word
' Stores second count
counter
VAR Byte
' For counting
char
VAR Byte
' Stores characters
state
VAR Nib
' Stores alarm system state
' -----[ Initialization ]-------------------------------------------------PAUSE 1000
' Wait 1 second
DEBUG "Program running..."
' Display running message
state = NotArmed
' Initialize alarm state
' -----[ Main Routine ]---------------------------------------------------DO
' Main loop
SELECT state
' Evaluate state case by case
CASE NotArmed
' If state = not armed
' To-do: bicolor LED green
GOSUB Prompt_to_Arm
'
call Prompt_to_Arm
CASE Arming
' If state = Arming
GOSUB Alarm_Arming
'
call Alarm_Arming
CASE Armed
' If state = Armed
' To-do: bicolor LED red
GOSUB Check_Sensors
'
Call Check_Sensors
GOSUB Prompt_to_Disarm
'
Call Prompt_to_Disarm
CASE Triggered
' If state = Triggered
GOSUB Alarm_Triggered
'
Call Alarm_Triggered
ENDSELECT
' Done evaluating char
LOOP
' Repeat main loop
' =====[ Subroutines ]=====================================================
' -----[ Subroutine - Prompt_To_Arm ]-------------------------------------Prompt_to_Arm:
DEBUG CLS, "Type A to arm", CR, ">"
' Display message
GOSUB Get_User_Input
' Call Get_User_Input
RETURN
' Return from Prompt_to_Arm
Prototyping Your Own Inventions · Page 315
' -----[ Subroutine - Prompt_to_Disarm ]----------------------------------Prompt_to_Disarm:
DEBUG CLS, "Type D to disarm", CR, ">"
' Display message
GOSUB Get_User_Input
' Call Get_User_Input
RETURN
' Return from Prompt_to_Disarm
' -----[ Subroutine - Alarm_Arming ]--------------------------------------Alarm_Arming:
DEBUG CLS, "Close the cabinet.",
' Warn user to secure cabinet
CR, "You have"
FOR seconds = 8 TO 0
' Count down seconds left
DEBUG CRSRX, 9, DEC seconds, CLREOL,
' Display time remaining
" seconds left..."
PAUSE 1000
' Wait 1 second
NEXT
' Repeat count down
state = Armed
' Set state variable to Armed
RETURN
' Return from Alarm_Arming
' -----[ Subroutine - Alarm_Armed ]---------------------------------------Alarm_Armed:
DO
' Armed loop
GOSUB Prompt_To_disarm
' Check for user input
GOSUB Check_Sensors
' Check sensors
LOOP UNTIL state <> Armed
' Repeat until state not armed
RETURN
' Return from Alarm_Armed
' -----[ Subroutine - Alarm_Triggered ]-----------------------------------Alarm_Triggered:
DO
' Alarm triggered loop
DEBUG CLS, "Alarm triggered!!!"
' Display warning
FOR counter = 1 TO 15
' Sound 15 alarm tones
FREQOUT 6, 100, 4500
PAUSE 100
NEXT
FOR seconds = 1 TO 6
' 3 sec. for user to disarm
IF state <> triggered THEN EXIT
GOSUB Prompt_to_Disarm
NEXT
LOOP UNTIL state <> triggered
' Repeat until disarmed
' -----[ Subroutine - Get_User_Input ]------------------------------------Get_User_Input:
char = 0
' Clear char variable
SERIN 16, 84, 500, Timeout_Label, [char] ' Wait 0.5 sec. for key press
GOSUB Process_Char
' If key, call Process_Char
Timeout_Label:
' If no key, skip call
RETURN
' Return from Get_User_Input
' -----[ Subroutine - Process_Char ]--------------------------------------Process_Char:
SELECT char
' Evaluate char case by case
CASE "A", "a"
' If "A" or "a"
Page 316 · What’s a Microcontroller?
state = Arming
CASE "D", "d"
state = NotArmed
CASE ELSE
DEBUG "Wrong character, try again"
PAUSE 2000
ENDSELECT
RETURN
'
'
'
'
'
'
'
'
Change state var to Arming
Else if "D" or "d"
Change state var to NotArmed
else if no "A", "a", "D", "d"
Display error message
Give user 2 sec. to read
Done with evaluating char
Return from Process_Char
' -----[ Subroutine - Check_Sensors ]-------------------------------------Check_Sensors:
' To-do: Check if Potentiometer is over threshold value.
' If yes, then, trigger alarm
IF IN0 = 0 THEN state = Triggered
' Btn released? Trigger alarm.
RETURN
' Return from Check_Sensors
New Coding Techniques in the Example Code
Take a look at the second FOR...NEXT loop in the Alarm_Triggered subroutine:
FOR seconds = 1 TO 6
IF state <> triggered THEN EXIT
GOSUB Prompt_to_Disarm
NEXT
If a call to the Prompt_to_Disarm subroutine results in a change in the state variable,
the IF...THEN statement uses EXIT to get out of the FOR...NEXT loop before the 6
repetitions are done.
Another new command called SERIN appears in the Get_User_Input subroutine. DEBUG
and DEBUGIN are special versions of the more general SEROUT and SERIN commands. To
see how this works, try replacing the command DEBUG "Program running..." with
SEROUT 16, 84, ["Program running..."]. Unlike the DEBUG and DEBUGIN
commands, SEROUT and SERIN can communicate on any I/O pin, or pin 16 for
communication with the DEBUG terminal. They also have special codes you can use to
select the baud rate that are described in the SERIN and SEROUT command’s Baud Rate
tables in the BASIC Stamp Manual.
Get_User_Input:
char = 0
SERIN 16, 84, 500, Timeout_Label, [char]
GOSUB Process_Char
Timeout_Label:
RETURN
Prototyping Your Own Inventions · Page 317
The Get_User_Input subroutine starts by setting the char variable to 0 to clear any old
values char might be storing. Then, it executes the SERIN command, with its optional
Timeout value set to 500 ms (half a second), and its optional timeout label set to
Timeout_Label, which is two lines below. If the SERIN command does receive a
character within 500 ms, it stores the result in the char variable and moves on to the next
line, which calls the Process_Char subroutine. If it doesn’t get a character in 500 ms, it
instead jumps to Timeout_Label, which causes it to skip over the subroutine call.
Your Turn – Next Steps Toward the Proof of Concept
It’s time to get this program functioning as a proof of concept.
9 Save a copy of MicroAlarmProto(Dev-009).bs2 as MicroAlarmProto(Dev010).bs2
9 Use segments of your tested code from Activity #2 to complete the three “Todo” items.
9 Test your modified code, and when you get it working right, save a copy of the
code as MicroAlarmProto(Dev-011).bs2
ACTIVITY #4: DOCUMENT YOUR CODE!
MicroAlarmProto(Dev-011).bs2 is not quite finished because it still needs some
documentation and other changes that make the program easier to modify and maintain.
For example, in the Alarm_Triggered subroutine, the command FREQOUT 6, 100,
4500 has what some coders call “mystery numbers.” Mystery numbers are values that are
used in a way the casual observer might not be able to easily discern. You could rewrite
this command as FREQOUT SpeakerPin, BeepTime, AlarmTone. Then, you can add a
Pin Directives section above the Constants section, and declare SpeakerPin PIN 6.
Also, in the Constants section, declare BeepTime CON 100, and AlarmTone CON 4500.
Not every constant in a given program has to be named. Keep in mind that mystery
numbers are values that are used in a way the casual observer might not be able to easily
discern. Another example from the Alarm_Triggered subroutine is:
FOR seconds = 1 TO 6
' 3 sec. for user to disarm.
The numbers 1 and 6 are not mystery numbers because it’s clear that they make the
FOR...NEXT loop repeat six times, and the comment to its right indicates that six
repetitions lasts for three seconds. Not all supervisors may agree with this interpretation,
Page 318 · What’s a Microcontroller?
and some might heatedly proclaim that the 1 and the 6 really are mystery numbers. If
you end up coding at work and your boss is a stickler for naming all constants, it’s
probably a good idea to just adhere to whatever coding style is required.
9 Go through MicroAlarmProto(Dev-011).bs2 and document mystery numbers by
declaring pin directives and constants, and substituting their names for numbers
in the program.
9 One exception to PIN directives is the SERIN command’s Pin argument, which
should be declared as a constant and not a pin. Pin arguments are for I/O pins
and range from P0 to P15. The Pin argument 16 causes the SERIN command to
listen to the BASIC Stamp module’s SIN pin, which is connected to your board’s
programming port.
Another area where MicroAlarmProto(Dev-011).bs2’s documentation is still weak is in
the comments that explain each routine and subroutine. Each subroutine should have
comments that explain what it does, any variables it depends on to do its job, and any
variables that the subroutine uses to store results before its RETURN. Here is an example
of good documentation added to the beginning of the Process_Char subroutine.
'
'
'
'
'
'
'
-----[ Subroutine - Process_Char ]--------------------------Updates the state variable based on the contents of the
char variable. If char contains "A" or "a", the Armed
constant gets stored in state. If char contains "D" or "d",
the NotArmed constant gets stored in state.
Process_Char:
'... code omitted here
RETURN
' Return from...
9 Update descriptions between the subroutine titles and their labels, and repeat for
the main routine as well.
9 When you are done, save a copy of your code with the name
MicroAlarmProofOfConcept(v1.0).bs2.
Save Copies and Increment Version Numbers after Each Small Change
Make sure to continue saving copies of your code with each small adjustment. This
makes it easy to take small steps backward to working code if your change(s) cause bugs.
For
example,
before
your
next
modification,
save
the
file
as
Prototyping Your Own Inventions · Page 319
MicroAlarmProofOfConcept(v1.01).bs2, or maybe even v1.01a. When your next feature
is fully implemented, chose a reasonable revision step. If it’s a smaller revision, try v1.1;
if it’s a big revision, up it to v2.0.
ACTIVITY #5: GIVE YOUR APP AMAZING NEW FUNCTIONALITY
As mentioned earlier, each circuit you have worked with in this text is really an example
from a group of components and modules that the BASIC Stamp can interact with in the
same way. Figure 10-3 shows some part substitutions you could make to convert your
current mini-enclosure security system into one that will protect an object sitting out in
the open. This modified system can instead detect motions in the room, and also detect if
someone lifts up the object you want to protect:
•
•
Pushbutton: high-low output → replace with PIR Motion Sensor
Potentiometer: variable resistor → replace with FlexiForce Sensor
The PIR sensor detects changing patterns of passive infrared light in the surrounding
area, and sends a high signal to indicate that motion is detected, or a low signal to
indicate no motion. The FlexiForce sensor’s resistance varies with force applied to the
round dot on the end (such as an object sitting on it), so it can be measured in an RC
circuit with the RCTIME command.
Figure 10-3: Sensors to Upgrade our Mini Alarm System
PIR Motion
Sensor
FlexiForce Sensor
9 Go to www.parallax.com and type “motion detection” into the Search field, then
click the Go button.
9 Find the PIR Sensor in the search results and go to its product page.
9 Download PIR Sensor Documentation (.pdf), and optionally watch the PIR
Sensor video clip. The PDF will be in the page’s Downloads section.
Page 320 · What’s a Microcontroller?
9 Read the documentation’s explanations, schematic, and PIR_Simple.bs2
example code. Could you substitute this sensor for a pushbutton?
9 Go back to your search results (or back to the Parallax home page) and type
pressure into the Search field. Then, follow the FlexiForce sensor link.
9 Find and un-zip the FlexiForce Documentation and Source Code (.zip).
9 In the un-zipped folder, open and read the documentation, schematic, and
FlexiForce Simple.bs2 source code. Could you substitute this sensor for a
potentiometer?
For a step-by-step example that demonstrates how the enhancements in both this
and the next activity can be incorporated into your Micro Alarm application, follow the
Stamps in Class “Mini Projects” link at: www.parallax.com/Education.
ACTIVITY #6 : HOW TO JUMP OVER DESIGN HURDLES
Now that you’re just about done with What’s a Microcontroller? one of the most
important next steps you can take is finding answers for tasks you don’t already know
how to solve with your microcontroller. Here are the general steps:
Step 1: Look for components or circuits that could solve your problem.
Step 2: Read about the component/circuit, and find out how it works. Pay
special attention to how the BASIC Stamp would need to interact with
the component/circuit.
Step 3: Check to find out if example code is available for the circuit or
component. That’ll make it a lot easier to incorporate into your
application.
Let’s say that the next step in your project is to display the system’s status without the
computer connection. Here’s an example of how you can find and evaluate a component
for your application.
9 (Step 1) Go to www.parallax.com and try the term “display” in the Search field.
From the home page, you may need to click the Go button instead of just
pressing Enter. Go to the product pages of the various result items in the search
and see if you can find one that’s relatively inexpensive and capable of
displaying a couple lines of text.
If you decided the Parallax Serial 2x16 LCD in Figure 10-4 is a good candidate, you’re
on the right track. However, just about any of the displays are fair game.
Prototyping Your Own Inventions · Page 321
Figure 10-4
Parallax 2x16 Serial LCD
9 (Step 2) Go to the Parallax Serial 2x16 LCD product page. If you haven’t
already done so, read the product description. Then, find the link to the Parallax
Serial 2x16 LCD’s PDF Documentation. It’ll be in the page’s Downloads &
Resources section, probably labeled “Parallax Serial 2x16 LCD Documentation
v2.0 (pdf).” The 2.0 version number might be higher by the time you try this.
9 (Step 3) Check for example code in the Parallax Serial 2x16 LCD’s PDF
documentation as well as links to code in the product web page’s Downloads &
Resources section. Look for a nice, short, simple example program that displays
a test message because it usually provides a good starting point.
After the brief introduction to SERIN and SEROUT that followed this chapter’s example
program, example code for the Parallax Serial LCD, which relies on SEROUT, might look
rather familiar.
If you follow the Smart Sensors and Applications link, you can download the Smart
Sensors and Applications textbook, which has an entire chapter about controlling this
display with the BASIC Stamp 2.
Three Examples out of How Many?
The PIR and FlexiForce Sensors and the Parallax Serial LCD are three examples of
modules and components you can use to greatly increase your prototype’s functionality.
These three are just a drop in the bucket compared to what’s available.
Page 322 · What’s a Microcontroller?
Figure 10-5 shows a few more modules and components, and it still represents just a
small sample. The examples in the figure are: (a) RF module for radio communication,
(b) gyro for detecting rotation speed, (c) compass for finding direction, (d) vibration
sensor, (e) accelerometer for detecting tilt and speed changes, (f) ultrasonic sensor for
detecting distance, (g) light intensity sensor, (h) servo controller, (i) DC motor controller,
(j) Darlington array for driving stepper motor coils, and (k) stepper motor. You can find
any of these devices at www.parallax.com with a keyword search. For example, to find
out more about (f), enter “ultrasonic sensor” into the Parallax home page’s Search field
and then click the Go button.
Figure 10-5: More Module and Accessory Examples
Motor Control
Sensors
Communication
b
e
h
j
a
c
f
k
i
d
g
Your Turn – Investigating More Resources
If you have a project in mind and need to find a circuit and code to support one of your
project’s features, the search procedure just discussed provides a good starting point, but
it only finds product pages on www.parallax.com, and there are a number of design
questions that product pages won’t necessarily answer. Fortunately, there are lots more
resources, including:
Prototyping Your Own Inventions · Page 323
•
•
•
•
•
Stamps in Class PDF textbooks
Parallax PDF product documentation
Nuts and Volts of BASIC Stamps columns
Answers to questions and articles at forums.parallax.com
BASIC Stamp articles published on the Internet
When you are looking for components and information about how to use them with the
BASIC Stamp, it falls in the category of “application information.” When searching for
application information, it’s best to start with the manufacturer’s web site, then expand
the search to include forums, and if you still haven’t found a good solution, expand it
further to include the World Wide Web at large. Figure 10-6 shows an example of a
Google keywords search that will search for the terms “infrared” and “remote” in PDF
documents and product pages at www.parallax.com. The important part here is that the
Google searches PDF documents instead of just product pages. Make sure there are no
spaces in site:www.parallax.com.
Figure 10-6
Google Search of the site
www.parallax.com
You can modify the search to include questions and answers on the Parallax support
forums by changing the “www” to “forums” like this:
infrared remote site:forums.parallax.com
This searches for all questions, answers and short articles that contain the words
“infrared” and “remote” at forums.parallax.com. To find an application specific to the
BASIC Stamp, change your search to the terms below. Make sure the words BASIC
Stamp are in quotes because it will filter out postage stamp collecting results.
Here is a summary of the Google search sequences for “BASIC Stamp” infrared remote
9 infrared remote site:www.parallax.com
o Searches for the terms “infrared” and “remote” in PDF and product
pages at www.parallax.com
Page 324 · What’s a Microcontroller?
9 infrared remote site:fourms.parallax.com
o Searches for the terms “infrared” and “remote” in discussions at
forums.parallax.com
9 “BASIC Stamp” infrared remote
o Searches the web at large for the words “infrared” and “remote” in the
same page or PDF with the phrase “BASIC Stamp.”
Let’s say that the next step for your Micro Alarm project is a keypad, but the
documentation and examples you found with a simple product page search at
parallax.com turned out to be sparse and devoid of example circuits and code. Since
some more searching would be in order, let’s try a Google search of the Parallax site for
all references to keypad. Remember, the Google search includes PDF documents.
9 Go to www.google.com.
9 Type “keypad site:www.parallax.com” into the Search field and then press Enter.
The results may take some patience and persistence to sift through, and there may be
many pages of results. There’s usually enough of an excerpt from each search result to
get some context for each link. This will give some idea of which ones to skip and which
ones to look at more closely. After a few pages, you might find and follow a link to an
IR Remote Parts Kit, shown in Figure 10-7. This might not be a solution you were
expecting, but after examining the price, documentation, and example code, it might have
a lot of potential for your enhanced micro security system keypad.
Figure 10-7: IR Remote Parts Kit
If after all that, you still haven’t found the information you need, it’s time to ask at
forums.parallax.com. When you post a question there, it will be seen by experts in a
variety of fields as well as by teachers, hobbyists, and students. The collective expertise
of the Parallax Forums should be able to help get you past just about any design hurdle!
Prototyping Your Own Inventions · Page 325
Processor Memory and Speed Design Hurdles
In some cases, programs for larger projects can grow long enough to exceed the BASIC
Stamp 2’s program memory. This design hurdle can sometimes be jumped by rewriting
code that does more work with fewer commands. Another option is to upgrade to a
BASIC Stamp model with a larger program memory. In other cases, the project might
involve storing more variable values than the BASIC Stamp 2 can accommodate. There
are also BASIC Stamp 2 models that feature scratchpad RAM for variable values. Other
projects might need to do more tasks in less time than the BASIC Stamp 2 is designed to
take, so some models of BASIC Stamp 2 are designed with faster processing speeds.
Figure 10-8 shows all of the different BASIC Stamp models. For details about one,
follow the “Compare BASIC Stamp Modules” link at www.parallax.com/basicstamp.
Figure 10-8: The Complete Lineup of BASIC Stamp Models
From left: BS1, BS2, BS2e, BS2sx, BS2p24, BS2p40, BS2pe, BS2px
BS1:
BS2:
BS2e:
BS2sx:
BS2p24:
BS2p40:
BS2pe:
BS2px:
Affordable yet capable, perfect for small projects or tight spaces.
Ideal for beginners with a vast resource base of sample code; the core of the
Stamps in Class program.
Perfect for BS2 users who need more program and variable space.
Supports the BS2 command set with more variable and program space at more
than twice the execution speed.
In addition to more speed and variable space, special commands support I/O
polling, character LCDs and I2C and 1-wire protocols.
All the features of the BS2p24 with a bank of 16 additional I/O pins.
Supports the BS2p24 command set paired with lower power consumption and
more memory for battery-powered datalogging applications.
The fastest BASIC Stamp model supports all BS2p24 commands, plus special
I/O configuration features.
Page 326 · What’s a Microcontroller?
One thing to keep in mind if you upgrade to a faster model of BASIC Stamp is
differences in units for time-sensitive commands like RCTIME and FREQOUT. Since
different models’ processors run at different speeds, units for Duration and Frequency and
other arguments might be different. For example, the when the BS2 executes FREQOUT
th
6, 100, 4500, it sends a high pitched alarm tone to P6 for 100 ms (1/10 of a second)
at a frequency of 4500 Hz. The same command executed by the BS2px sends a tone that
only last 16.6 ms at a frequency of 27,135 Hz, which is so high-pitched that it’s not even
audible to human ears! For the complete descriptions of how each command works on
each model, and for tips on converting BS2 programs to perform correctly on other
models, see the BASIC Stamp Editor Help.
High-performance Parallel Processing
Some complex applications require processing agility and memory that’s well beyond the
BASIC Stamp 2 line’s capabilities. These are the kind of projects that the Propeller
microcontroller is designed for. This uniquely capable microcontroller has eight much
higher speed processors in one chip, along with 32 I/O pins and ample program memory
and RAM. The processors can all operate at the same time, both independently and cooperatively, sharing access to global memory and a system clock. Each processor also has
its own memory, and additional hardware to perform complex tasks like high-speed I/O
pin state monitoring or generating signals for a television or computer display.
The Propeller Education Kit shown in Figure 10-9 is a good way to get started with the
Propeller microcontroller. This kit is not necessarily the best next step after What’s a
Microcontroller? The next activity has some good recommendations for next book/kit
steps. However, when you notice that your projects are getting more ambitious and
challenging, remember the Propeller microcontroller and Propeller Education Kit.
Figure 10-9
Propeller Education Kit
(left)
PE Platform (right)
Prototyping Your Own Inventions · Page 327
ACTIVITY #7: WHAT’S NEXT?
Now that you are just about finished with What’s a Microcontroller? it’s time to think
about what to learn next. Before continuing, take a moment to consider what you’re most
interested in. Some general categories you could delve further into include:
•
•
•
•
•
•
Robotics
Electronics
Sensors
Automation
Hobby projects
Earth sciences and climate measurement
This activity inventories resources you can use to move forward with each these
categories.
The resources, kits, and components discussed in this activity are current as of when
this chapter was written (Fall 2009). Newer and better versions of resources, kits, and
components may become available that replace the ones presented here. Make sure to
check www.parallax.com for the latest information.
What’s a Microcontroller Sequels
Figure 10-10 shows the books and kits that make the best sequels to this book. Robotics
with the Boe-Bot is a lot of fun and a great learning experience because you get to apply
many of the techniques from this book to robotics applications with the rolling Boe-Bot®
robot. Smart Sensors and Applications was written to be “What’s a Microcontroller, Part
2.” It was renamed because all the nifty sensors and the liquid crystal display shown in
the center of Figure 10-10 have coprocessors that communicate with the BASIC Stamp.
The coprocessors make them “smart” sensors. Understanding Signals is great because it
allows you to “see” interactions between the BASIC Stamp and circuits with a Parallax
oscilloscope that you plug into your computer’s USB port.
Page 328 · What’s a Microcontroller?
Figure 10-10: Great Next Steps after What’s a Microcontroller?
Boe-Bot Robot Kit
Smart Sensors
and Applications
Parts and Text
Understanding
Signals Parts
and Text
More Stamps in Class Kits and Textbooks
Figure 10-11 shows a flowchart that outlines all the Stamps in Class kits and textbooks
available at the time of this writing. It’s accessible through the Stamps in Class Program
Overviews and Flowchart link at www.parallax.com/Education, and you can click each
picture to visit the product page for the book and its accompanying kit. What’s a
Microcontroller? is at the top-left of the figure. From there, the flowchart indicates that
you can either jump to Robotics with the Boe-Bot or any text/kit in the Sensors or Signals
series.
Full PDF Textbook Downloads: You can download the entire full-color PDF of each
Stamps in Class book at www.parallax.com. Click on any of the chart’s pictures to navigate
to the Text + Kit page, and you will find the PDF link in the page’s Downloads section.
Prototyping Your Own Inventions · Page 329
Figure 10-11
Stamps in Class
Flowchart
If the category you are interested in is:
•
•
•
•
•
Robotics, then the next step is definitely Robotics with the Boe-Bot.
Sensors, inventing, or hobby projects, then your next step would be Smart
Sensors and Applications.
Electronics (signals), then your next step would be Understanding Signals.
Automation, then your next step would be Process Control.
Earth science and climate measurement, then your next step would be Applied
Sensors (originally named Earth Measurements).
Page 330 · What’s a Microcontroller?
Additional Stamps In Class Resources
Above and beyond what’s in the Stamps in Class Textbooks, there are Stamps in Class
“Mini Projects” linked at www.parallax.com/Education. Some projects utilize just the
stock parts from a given kit but demonstrate new ways to use them along with new
concepts. Many of these projects are like complete Stamps in Class textbook chapters
with activities, schematics, wiring diagrams, and complete code listings that can be
downloaded. Some even have accompanying video tutorials. Figure 10-12 is taken from
the video for the “Build Your Own Mini Timer” project, which can be done with just the
parts you have been using in this book. Whether you are looking for more information or
creative inspiration, you might find it here.
Figure 10-12: Example Stamps in Class “Mini Project”
Prototyping Your Own Inventions · Page 331
SUMMARY
This book introduced a variety of circuits and techniques, all of which are building blocks
in common products as well as in inventions. This book also introduced techniques for
orchestrating the various building blocks with the BASIC Stamp microcontroller. This
chapter demonstrated how to incorporate these techniques and building blocks into a
prototype, and it also recommended some next steps for learning more in your area of
interest.
The approach for making the BASIC Stamp interact with a given circuit can be applied to
a variety of other circuits and modules to accomplish an even wider range of tasks. Two
examples applied to the micro alarm prototype were: (1) a motion sensor with an
interface similar to the pushbutton and (2) a pressure sensor with an interface similar to
the potentiometer.
While developing code for your application, make sure to save your work frequently
under incremented revision names. Also, make sure to use meaningful names for I/O
pins and numbers with PIN and CON directives. Finally, add lots of comments to your
code explaining what it does and how it does it. Subroutines should include comments
that explain what the subroutine does along with any variables with values it uses to do
its job as well as variables that results are stored in when the subroutine is done.
This chapter also introduced a variety of research techniques for implementing features in
your prototype. Even if you start with no clue about how to make a particular feature
work, you can use search terms to find useful component, circuit, and code examples.
Stamps in Class textbooks and kits also feature a wealth of circuits and useful design
techniques, and they are a great place to learn more in the fields of robotics, sensors,
electronics, automation, earth science, and more. All the textbooks that come with
Stamps in Class kits are free downloads.
Now that you have reached the end of this book, take a moment now to think about four
things: (1) the techniques you have learned, (2) your next invention, project or prototype,
(3) how what you have learned here can be applied to it, and (4) what you want to learn
next.
9 Now, it’s time to get started on your next project or prototype.
9 Make sure to keep studying and learning new techniques as you go.
9 Have fun, and good luck!
Page 332 · What’s a Microcontroller?
Appendix A: Parts List and Kit Options · Page 333
Appendix A: Parts List and Kit Options
What’s a Microcontroller Parts & Text Kit #28152, Parts Only #28122
Parts and quantities subject to change without notice
Parallax Part #
Description
Quantity
150-01020
Resistor, 5%, 1/4W, 1 kΩ
150-01030
Resistor, 5%, 1/4W, 10 kΩ
4
150-01040
Resistor, 5%, 1/4W, 100 kΩ
2
150-02020
Resistor, 5%, 1/4W, 2 kΩ
2
150-02210
Resistor, 5%, 1/4W, 220 Ω
6
150-04710
Resistor, 5%, 1/4W, 470 Ω
6
152-01031
Potentiometer - 10 kΩ
1
200-01031
Capacitor, 0.01 μF
2
200-01040
Capacitor, 0.1 μF
2
201-01080
Capacitor, 1000 μF
1
201-03080
Capacitor 3300 μF
1
What’s a Microcontroller? text (in #28152 only)
1
LED - Green - T1 3/4
2
350-00005
LED - Bicolor - T1 3/4
1
350-00006
LED - Red - T1 3/4
2
350-00007
LED - Yellow - T1 3/4
2
350-00027
7-segment LED Display
1
350-00029
Phototransistor, 850 nm, T1 3/4
1
28123
350-00001
10
400-00002
Pushbutton – Normally Open
2
451-00303
3 Pin Header – Male/Male
1
500-00001
Transistor – 2N3904
1
604-00010
10 kΩ digital potentiometer
1
800-00016
3” Jumper Wires – Bag of 10
2
900-00001
Piezo Speaker
1
900-00005
Parallax Standard Servo
1
Page 334 · What’s a Microcontroller?
COMPLETE KIT OPTIONS
There are several kit options available that include a BASIC Stamp 2 microcontroller
development board and all of the electronic components to complete the activities in this
text:
•
BASIC Stamp Activity Kit (#90005) includes:
o BASIC Stamp HomeWork Board with surface-mount BS2
o USB to Serial Adapter with USB A to Mini-B Cable (#28031)
o What’s a Microcontroller? Parts and Text (#28152)
•
BASIC Stamp Discovery Kit (Serial #27207 or USB #27807) includes:
o Board of Education (Serial #28150 or USB #28850)
o BASIC Stamp 2 microcontroller module (#BS2-IC)
o Programming Cable (Serial #800-00003 or USB A to Mini-B
#805-00006)
o What’s a Microcontroller? Parts and Text (#28152)
o BASIC Stamp Manual (#27218)
•
•
What’s a Microcontroller Parts & Text Kit (#28152). PLUS
Board of Education Full Kit (Serial #28103 or USB #28803) includes:
o Board of Education (Serial #28150 or USB #28850)
o BASIC Stamp 2 microcontroller module (#BS2-IC)
o Programming cable (Serial #800-00003 or USB A to Mini-B
#805-00006)
o Jumper Wires (1 pack of 10)
A note to Educators: Quantity discounts are available for all of the kits listed above; see
each kit’s product page at www.parallax.com for details. In addition, the BASIC Stamp
HomeWork Board is available separately in packs of 10 as an economical solution for
classroom use, costing significantly less than the Board of Education + BASIC Stamp 2
module (#28158). Please contact the Parallax Sales Team toll free at (888) 512-1024 for
higher quantity pricing.
Appendix B: More about Electricity · Page 335
Appendix B: More about Electricity
What’s an electron? An electron is one of the three fundamental parts of an atom; the
other two are the proton and the neutron. One or more protons and neutrons stick together
in the center of the molecule in an area called the nucleus. Electrons are very small in
comparison to protons and neutrons, and they orbit around the nucleus. Electrons repel
each other, and electrons and protons attract to each other.
What’s charge? The tendency of an electron to repel from another electron and attract to a
nearby proton is called negative charge. The tendency for a proton to repel from another
proton and attract an electron is called positive charge. When a molecule has more
electrons than protons, it is said to be negatively charged. If a molecule has fewer electrons
than protons, it is said to be positively charged. If a molecule has the same number of
protons and electrons, it is called neutrally charged.
What’s voltage? Voltage is like electrical pressure. When a negatively charged molecule is
near a positively charged molecule, the extra electron on the negatively charged molecule
tries to get from the negatively charged molecule to the positively charged molecule.
Batteries keep a compound with negatively charged molecules separated from a compound
with positively charged molecules. Each of these compounds is connected to one of the
battery’s terminals; the positively charged compound is connected to the positive (+)
terminal, and the negative compound is connected to the negative (-) terminal.
The volt is a measurement of electrical pressure, and it’s abbreviated with a capital V. You
may already be familiar with the nine volt (9 V) battery used to supply power to the Board of
Education or HomeWork Board. Other common batteries include the 12 V batteries found in
cars and the 1.5 V AA batteries used in calculators, handheld games, and other devices.
What’s current? Current is a measure of the number of electrons per second passing
through a circuit. Sometimes the molecules bond in a chemical reaction that creates a
compound (that is neutrally charged). Other times, the electron leaves the negatively
charged molecule and joins the positively charged molecule by passing though a circuit like
the one you just built and tested. The letter most commonly used to refer to current in
schematics and books is capital “I.”
What’s an amp? An amp (short for ampere) is the basic unit of current, and the notation for
the amp is the capital “A.” Compared to the circuits you are using with the BASIC Stamp, an
amp is a very large amount of current. It’s a convenient value for describing the amount of
current that a car battery supplies to headlights, the fan that cool a car’s engine, and other
high power devices. Milliamp (mA) and microamp (μA) measurements are more convenient
for discussing the BASIC Stamp module’s supply current as well as currents between I/O
pins and circuits. 1 mA = 1/1,000 A, and 1 μA = 1/1,000,000 A.
What’s resistance? Resistance is the tendency of an element in a circuit to resist the flow
of electrons (the current) from a battery’s negative terminal to its positive terminal.
The ohm is the basic measurement of resistance. It has already been introduced and it’s
abbreviated with the Greek letter omega (Ω).
What’s a conductor? Copper wire has almost no resistance, and it’s called a conductor.
Page 336 · What’s a Microcontroller?
BONUS ACTIVITY: OHM’S LAW, VOLTAGE, AND CURRENT
This activity applies some of the definitions just discussed.
Ohm’s Law Parts
(1) Resistor – 220 Ω (red-red-brown)
(1) Resistor – 470 Ω (yellow-violet-brown)
(1) Resistor – 1 kΩ (brown-black-red)
(1) Resistor – 2 kΩ (red-black-red)
(1) LED – any color
Test Circuit
The resistance value of Ri in Figure B-1 can be changed. Lower resistance allows more
current through the LED, and it glows more brightly. Higher resistance values will cause
the LED to look dim because they do not allow as much current to pass through the
circuit.
9 Disconnect power from your Board of Education or HomeWork Board whenever
you modify the circuit.
9 Build the circuit shown in Figure B-1 starting with a 220 Ω resistor.
9 Modify the circuit by replacing the 220 Ω resistor with a 470 Ω resistor. Was
the LED less bright?
9 Repeat using the 1 kΩ resistor, then the 2 kΩ resistor, checking the change in
brightness each time.
Vdd
X3
Vdd
R1 R2 R3 R4
Ri
LED
Vss
R1 = 220 Ω
R2 = 470 Ω
R3 = 1 kΩ
R4 = 2 kΩ
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vin
Vss
+
Figure B-1
LED Current Monitor
Appendix B: More about Electricity · Page 337
If you are using a 9 V battery, you can also compare the brightness of a different voltage
source, Vin. Vin is connected directly to the 9 V battery’s + terminal, and Vss is
connected directly to the battery’s negative terminal. On our system, Vdd is regulated
5 V. That’s about half the voltage of the 9 V battery.
9 If you are not using a 9 V battery, stop here and skip to the Calculating the
Current section below. Otherwise, continue.
9 Start with the circuit shown in Figure B-1, but use a 1 kΩ resistor.
9 Make a note of how bright the LED is.
9 Disconnect power.
9 Modify the circuit by disconnecting the resistor lead from Vdd and plugging it
into Vin.
9 When you plug the power back in, is the LED brighter? How much brighter?
DO NOT try the Vin experiment with a 220 or 470 Ω resistor, it will supply the LED with
more current than it is rated for.
Calculating the Current
The BASIC Stamp Manual has some rules about how much current I/O pins can supply
to circuits. If you don’t follow these rules, you may end up damaging your BASIC
Stamp. The rules have to do with how much current an I/O pin is allowed to deliver and
how much current a group of I/O pins is allowed to deliver.
Current Rules for BASIC Stamp I/O Pins
•
An I/O pin can “source” up to 20 mA. In other words, if you send a HIGH signal to
an I/O pin, it should not supply the LED circuit with more than 20 mA.
•
If you rewire the LED circuit so that the BASIC Stamp makes the LED turn on
when you send the LOW command, an I/O pin can “sink” up to 25 mA.
•
P0 through P7 can only source up to 40 mA. Likewise with P8 through P15.
40 mA is also the I/O supply current budget for the BASIC Stamp 2 module’s 5 V
regulator, so the total current draw for all I/O pins should never exceed 40 mA. If
you have lots of LED circuits, you will need larger resistors so that the circuits
don’t draw too much current.
•
For more information, consult the BASIC Stamp 2 Pin Descriptions table in the
BASIC Stamp Manual.
Page 338 · What’s a Microcontroller?
If you know how to calculate how much current your circuit will use, then you can decide
if it’s OK to make your LEDs glow that brightly. Every component has rules for what it
does with voltage, resistance, and current. For the light emitting diode, the rule is a value
called the diode forward voltage. For the resistor, the rule is called Ohm’s Law. There
are also rules for how current and voltage add up in circuits. These are called Kirchhoff’s
Voltage and Current Laws.
Vdd – Vss = 5 V The voltage (electrical pressure) from Vdd to Vss is 5 V. This is called
regulated voltage, and it works about the same as a battery that supplies exactly 5 V.
(Batteries are not typically 5 V, though four 1.2 V rechargeable nickel-cadmium batteries in
series can add up to 4.8 V.) The Board of Education and BASIC Stamp HomeWork Board
both have 5 V regulators that convert the 6 to 9 V battery supply voltage to regulated 5 V for
the Vdd sockets above the breadboard. The BASIC Stamp also has a built-in voltage
regulator that converts the 6 to 9 V input to 5 V for its components.
Vin – Vss = 9 V If you are using 9 V battery, the voltage from Vin to Vss is 9 V. Be careful.
If you are using a voltage regulator that plugs into the wall, even if it says 9 V, it could go as
high as 18 V.
Ground and/or reference refer to the negative terminal of a circuit. When it comes to the
BASIC Stamp and Board of Education, Vss is considered the ground reference. It is zero
volts, and if you are using a 9 V battery, it is that battery’s negative terminal. The battery’s
positive terminal is 9 V. Vdd is 5 V (above the Vss reference of 0 V), and it is a special
voltage made by a voltage regulator chip to supply the BASIC Stamp with power.
Ohm’s Law: V = I × R The voltage measured across a resistor’s terminals (V) equals the
current passing through the resistor (I) times the resistor’s resistance (R).
Diode Forward Voltage: The voltage across a diode’s anode and cathode as current
passes through it from anode to cathode. For the green LED in the Figure 2-6 circuit on
page 33, you can assume that the forward voltage across the LED is approximately 2.1 V for
the sake of making circuit calculations. If the LED is yellow, assume 2.0 V, and if it’s red,
assume 1.7 V. These voltages will vary slightly with the amount of current passing through
the circuit. Smaller series resistance and/or higher voltage applied to the circuit results in
higher current flow. Larger series resistance and/or smaller applied voltage results in lower
current flow.
Kirchhoff’s Voltage Law Simplified: voltage used equals voltage supplied. If you
supply a circuit with 5 V, the number of volts all the parts use had better add up to 5 V.
Kirchhoff’s Current Law Simplified: current in equals current out. The current that
enters an LED circuit from Vdd is the same amount of current that leaves it through Vss.
Also, if you connect three LEDs to the BASIC Stamp, and each LED circuit draws 5 mA, it
means the BASIC Stamp has to supply all the circuits with a total of 15 mA.
Appendix B: More about Electricity · Page 339
Example Calculation: One Circuit, Two Circuits
Calculating how much current a red LED circuit draws takes two steps:
1. Figure out the voltage across the resistor
2. Use Ohm’s Law to figure out the current through the resistor.
Figure B-2 shows how to calculate the voltage across the resistor. The voltage supplied
is on the left; it’s 5 V. The voltages used by each component are to the right of the
circuit. The voltage we don’t know at the start is VR, the voltage across the resistor. But,
we do know that the voltage across the LED is going to be about 1.7 V (the red light
emitting diode’s forward voltage). We also know that the voltage across the parts has to
add up to 5 V because of Kirchhoff’s Voltage Law. The difference between 5 V and 1.7
V is 3.3 V, so that must be the voltage across the resistor VR.
VR + 1.7 V = 5 V
VR = 5 V − 1.7 V
VR = 3.3V
Figure B-2
Voltage Across
the Circuit,
Resistor, and
LED
Kilo is metric for 1000. The metric way of saying 1000 is kilo, and it’s abbreviated with the
lower-case k. Instead of writing 1000 Ω, you can write 1 kΩ. Either way, it’s pronounced
one-kilo-ohm. Likewise, 2000 Ω is written 2 kΩ.
Milli is metric for 1/1000, and it is abbreviated with a lower-case m. If the BASIC Stamp
supplies an LED circuit with 3.3 thousandths of an amp, that’s 3.3 milliamps, or 3.3 mA.
What’s a mA? Pronounced milliamp, it’s the notation for one-one-thousandth-of-an-amp.
The “m”’ in mA is the metric notation for milli, which stands for 1/1000. The “A” in mA stands
for amps. Put the two together, and you have milliamps, and it’s very useful for describing
the amount of current drawn by the BASIC Stamp and the circuits connected to it.
Now that we have calculated the voltage across the resistor, Figure B-3 shows an
example of how to use that value to calculate the current passing through the resistor.
Start with Ohm’s Law: V = I × R. You know the answers to V (3.3 V) and R (470 Ω).
Now, all you have to do is solve for I (the current).
Page 340 · What’s a Microcontroller?
V = I×R
3.3V = I × 470 Ω
3.3 V
I=
470 Ω
I ≈ 0.00702 V
Ω
I = 0.00702 A
7.02
I=
A
1000
I = 7.02 mA
Figure B-3
Calculating
Current through
the Resistor
Yes, it’s true ! 1 A = 1 V/Ω (One amp is one volt per ohm).
How much current is 7.02 mA? It’s the amount of current the LED circuit in Figure B-2
conducts. You can replace the 470 Ω resistor with a 220 Ω resistor, and the circuit will
conduct about 15.0 mA, and the LED will glow more brightly. If you use a 1000 Ω resistor,
the circuit will conduct 3.3 mA, and the LED will glow less brightly. A 2000 Ω resistor will
cause the LED to glow less brightly still, and the current will be 1.65 mA.
Let’s say you want to make an I/O pin turn two LEDs on at the same time. That means
that inside the BASIC Stamp, it would supply the circuits as shown in Figure B-4.
Would the circuit’s current draw exceed the I/O pin’s 20 mA limit? Let’s find out.
Remember that the simplified version of Kirchhoff’s Current Law says that the total
current drawn from the supply equals the current supplied to all the circuits. That means
that I in Figure B-4 has to equal the total of the two currents being drawn. Simply add
the two current draws, and you’ll get an answer of 14.04 mA, which you can round to
14.0 mA. Since this current draw is still below the I/O pin’s 20 mA limit, it can safely be
connected to an I/O pin and switched on/off with the BASIC Stamp.
Appendix B: More about Electricity · Page 341
I = I 1 + I 2 + ... I i
I = 7.02 mA + 7.02 mA
I = 14.04 mA ≈ 14.0 mA
Figure B-4
Total
Current
Supplied to
Two LED
Circuits
Your Turn – Modifying the Circuit
9 Repeat the exercise in Figure B-2, but use Vin – Vss = 9 V instead of Vdd – Vss
= 5 V.
Assuming the forward voltage does not change, the answer is VR = 7.3 V. The
measured resistor voltage will probably be slightly less because of a larger LED
forward voltage from more current passing through the circuit.
9 Repeat the exercise in Figure B-3, but use a 1 kΩ resistor.
Answer: I = 3.3 mA.
9 Use VR = 7.3 V to do the exercise in Figure B-3 with a 1 kΩ resistor.
Answer: I = 7.3 mA.
9 Repeat the exercise shown in Figure B-4 with one of the resistors at 470 Ω and
the other at 1 kΩ.
Answer: I = 7.02 mA + 3.3 mA = 10.32 mA.
Page 342 · What’s a Microcontroller?
Appendix C: RTTTL Format Summary · Page 343
Appendix C: RTTTL Format Summary
This is a summary intended to help make sense out of RTTTL format. The full RTTTL
specification can be found published at various web sites. With any search engine, use
the keywords “RTTTL specification” to review web pages that include the specification.
Here is an example of an RTTTL format ringtone:
TakeMeOutToTheBallgame:d=4,o=7,b=225:2c6,c,a6,g6,e6,2g.6,2d6,p,
2c6,c,a6,g6,e6,2g.6,g6,p,p,a6,g#6,a6,e6,f6,g6,a6,p,f6,2d6,p,2a6
,a6,a6,b6,c, d,b6,a6,g6
The text before the first colon is what the cell phone displays as the name of the song. In
this case, the ringtone is named:
TakeMeOutToTheBallGame:
Between the first and second colon, the default settings for the song are entered using d,
o, and b. Here is what they mean:
d – duration
o – octave
b – beats per minute or tempo.
In TakeMeOutToTheBallGame, the default settings are:
d=4,o=7,b=225:
The notes in the melody are entered after the second colon, and they are separated by
commas. If just the note letter is used, that note will be played for the default duration in
the default octave. For example, the second note in TakeMeOutToTheBallGame is:
,c,
Since it has no other information, it will be played for the default quarter note duration
(d=4), in the seventh octave (o=7).
A note could have up to five characters between the commas; here is what each character
specifies:
,duration
note
sharp
dot
octave,
Page 344 · What’s a Microcontroller?
For example:
,2g#.6,
…means play the half note G-sharp for 1 ½ the duration of a half note, and play it in the
sixth octave.
Here are a few examples from TakeMeOutToTheBallGame:
,2g.6, – half note, G, dotted, sixth octave
,a6, – default quarter note duration, A note played in the sixth octave
,g#6, – quarter duration, g note, sharp (denoted by #), sixth octave
The character:
,p,
…stands for pause, and it is used for rests. With no extra information, the p plays for the
default quarter-note duration. You could also play a half note’s worth of rest by using:
,2p,
Here is an example of a dotted rest:
,2p.,
In this case the rest would last for a half note plus a quarter note’s duration.
Index · Page 345
Index
-$-
$ (Hexadecimal formatter), 207
-%-
Applied Sensors, 329
Arguments, 39
ASCII, 276
Automation, 329
% (Binary formatter), 181
-*-
** (Multiply High operator), 270
*/ (Multiply Middle operator, 85, 270
-?-
? (symbol = x formatter), 45
-µ-
µF (microfarad), 143
-1-
16-bit rollover bug, 122
-7-
7-segment display, 169, 170, 169–71
-A-
Action sounds, 248
Active-high vs. active low, 69
AD5220 digital potentiometer, 292
Algorithm, 87
Alphabet Song, 257
Amp, 335
AND, 78
Anode, 30
7-segment display, 170
LED, 30
Apostrophe, 42
-B-
Base
Phototransistor, 198
Transistor, 289
Base-10 numbers, 183
Base-16 numbers, 183
Base-2 numbers, 67
BASIC Stamp, 11, 325
BASIC Stamp Editor, 15
BASIC Stamp model comparison, 325
Battery, 35
Beat, 252
Benjamin Franklin, 35
Bicolor LED, 50
Binary, 61
Binary numbers, 67, 179, 181
% (Binary formatter), 181
Bit, 45, 179
Variable size, 45
Boolean, 61
Breadboard, 31, 32, 259
BS1, 325
BS2, 325
BS2e, 325
BS2p24, 325
BS2p40, 325
BS2pe, 325
Page 346 · What’s a Microcontroller?
BS2px, 230, 325
BS2sx, 325
Bug, 16-bit rollover, 122
Build Your Own Mini Timer project
video, 330
Bus, parallel, 177
Byte, 45, 179
Variable size, 45
-C-
Cabinet alarm project, 310
Cadmium sulfide, 197
Capacitor, 143
Ceramic Capactior Schematic Symbol
and Parts Drawing, 150
Electrolytic, 143
Electrolytic Capacitor Schematic Symbol
and Part Drawing, 144
Junction capacitance, 236
Polar – identifying terminals, 144
Used in parallel, 224
Cathode, 30
Common cathode in &-segment display,
170
LED, 30
Charge, 335
Closed circuit, 62
CLREOL, 167
CMOS, 61
Code block, 78
Code overhead, 84
Collector
Phototransistor, 198
Transistor, 289
Color spectrum, 197
COM port, 41
Commenting code, 42
Common cathode, 170
Communication products, 322
Concept diagram, 308
Conductor, 335
Constants, 160
Control characters. See DEBUG
Control Characters
Controlling, 61
Counting, 80
CR, 25
CRSRUP, 129
Current, 28, 35, 335
Milliamp, 339
Cycle, 117, 245
-D-
DATA, 255
Datalogging, 203
DCD, 269
DEBUG, 39
DEBUG Control Characters, 129
CLREOL, 167
CR, 25
CRSRUP, 129
HOME, 76
DEBUG Formatters, 129
$ (Hexadecimal Formatter), 207
% (Binary formatter), 181
? (symbol = x formatter), 45
DEC (Decimal formatter), 120, 207
Debug Terminal
Index · Page 347
Transmit and Receive Windowpanes, 120
DEBUGIN, 119
DEC, 120, 207
Decimal formatter DEC, 207
Decimal numbers, 183, 204
Degree, 102
Device, parallel, 177
Diode, 30
Diode Forward Voltage, 338
DIRH, 178
DO…LOOP, 39, 83, 123
Dot, in music, 264
DTMFOUT, 251
Dual Tone Multi Frequency, 251
-E-
Earth Measurements, 329
Earth science, 329
Echo, 121
EEPROM, 203
Electrolytic capacitor, 143
Electron, 34, 35, 335
Embedded system, 11
Emitter
Phototransistor, 198
Transistor, 289
END, 63
EXIT, 282, 316
-F-
Farads, 164
Fetch and execute, 287
Flat notes, 254
FlexiForce Sensor, 319
FOR…NEXT, 43, 124
Formatters, DEBUG. See DEBUG
Formatters
Fractions, 85
FREQOUT, 247, 251
Frequency, 245
Functional description, 309
-G-
Google, 323
GOSUB, 216
GOTO, 217
Graphing software, 213
Ground, 31, 338
-H-
hertz, 245, 247
Hexadecimal formatter $, 207
Hexadecimal numbers, 183
Hexadecimal to decimal conversion,
205
HIGH, 39, 182
HOME, 76
HomeWork board
and the RCTIME circuit voltage divider,
155
Hysteresis, 228
-I-
I/O pin protection, 69
I/O pins
Default direction, 181
DIRH and OUTH registers, 178
I/O Pins. See Input/Output Pins
IF…ELSEIF…ELSE, 75
IF…THEN, 78
IF…THEN…ELSE, 71
IN, 67
Input/output pins. See I/O pins
Integer, 270
Page 348 · What’s a Microcontroller?
Interference, 252
Interpreter chip, 287
IR Remote Parts Kit, 324
-J-
Junction capacitance, 236
-K-
KCL, 338
kHz, 245
Kilo, 339
Kirchhoff’s Laws (Simplified)
Current, 338
Voltage, 338
Kirchhoff’s Voltage and Current Laws,
338
KVL, 338
-L-
Label, 217
LCD Display, 320
LED, 27
as a light sensor, 235
Bi-color, 50
Part Drawing and Schematic Symbol, 30
Light Emitting Diode. See LED
Light emitting diodes, 27
light meter, 214
Light meter, 214
LOOKDOWN, 186, 187
LOOKUP, 183
LOW, 39, 182
-M-
mA, 339
Main routine, 222
Math Operations, 268
Memory
Memory Map, 204
Overwriting the program, 207
Memory Map, 203, 207
Metric units of measure, 339
Microcontroller, 11
Microfarads, 143
Microsecond, 105
Milli, 339
Millipede Project, 13
Millisecond, 39, 105
Motor Control product, 322
Music
Dot, 264
Rests, 259
Tempo, 260
Mystery numbers, 317
-N-
Nanometer, 197
Natural keys, 254
nc, 171
Negative charge, 335
Nested loop, 249
Nesting subroutines, 219
Neutral, 35
Neutron, 335
Nib, 45
Variable size, 45
No-connect, 171
Nominal value, 298
NPN transistor, 289
Nucleus, 335
Numbers
Index · Page 349
Binary, 67, 179
Byte, 45
Decimal, 183
CLREOL, 167
Hexadecimal, 183
CR, 25
Nuts and Volts of BASIC Stamps
columns, 323
-O-
Octave, 254
Offset, 158
Ohm, 335
Ohm’s Law, 231, 338
Omega Ω, 28
ON GOSUB, 217
ON GOTO, 217
Open circuit, 62
OR, 78
OUTH, 178
Overflow, 273
Overwriting the program, 207
-P-
Parallax Standard Servo
Caution, 96
Parts diagram, 95
Parallel
bus, 177
device, 177
Parallel capacitors, 224
Parallel processing, 326
PAUSE, 39
PBASIC Language
CRSRUP, 129
DATA, 255
DCD, 269
DEBUG, 39
DEBUGIN, 119
DEC, 207
DEC, 120
DIRH, 178
DO…LOOP, 39, 83, 123
DTMFOUT, 251
END, 63
EXIT, 282, 316
FOR…NEXT, 43, 124
FREQOUT, 247, 251
GOSUB, 216
GOTO, 217
HIGH, 39, 182
HOME, 76
IF…ELSEIF…ELSE, 75
IF…THEN, 78
IF…THEN…ELSE, 71
IN, 67
LOOKDOWN, 186, 187
AND, 78
LOOKUP, 183
Arguments, 39
LOW, 39, 182
Bit, 45
Nib, 45
Page 350 · What’s a Microcontroller?
ON GOSUB, 217
OR, 78
OUTH, 178
PAUSE, 39
PIN, 162
PULSOUT, 105
RANDOM, 86
PIR Motion Sensor, 319
polling, 83
Polling, 80
Positive charge, 335
Potentiometer, 139
AD5220 (digital), 292
Process Control, 329
Program
RCTIME, 149, 199
Loops, nested, 249
READ, 206
Overwriting, 207
RETURN, 216
Program Listings
SELECT…CASE, 272
ActionTones.bs2, 248
SERIN, 316
Ch01Prj01_Add1234.bs2, 25
SEROUT, 316
Ch01Prj02_ FirstProgramYourTurn.bs2,
26
STEP, 124
TOGGLE, 296
UNTIL, 83, 123
WHILE, 123
Word, 206
WRITE, 206, 207
PBASIC Operators
Ch02Prj01_Countdown.bs2, 60
Ch03Prj01_TwoPlayerReactionTimer.bs2,
91
Ch04Prj01Soln1__KillSwitch.bs2, 136
Ch07Prj01_Blinds_Control.bs2, 243
Ch07Prj02_Blinds_Control_Extra.bs2,
243
** (Multiply High), 270
Ch5Prj01_ControlServoWithPot.bs2, 166
*/ (Multiply Middle, 85, 270
Ch6Prj01_FishAndChips.bs2, 193
DCD, 269
Ch8Prj01_PushButtonToneGenerator.bs2
, 286
Order of execution, 268
Parentheses, 268
Photoresistor, 197
Phototransistor, 198
Piezoelectric Speaker, 245
PIN, 162
Pin map, 170, 292
Ch9Ex01_SetTapToZero.bs2, 303
Ch9Prj01_PhotoControlledDigitalPot.bs2,
305
ControlServoWithPot.bs2, 159
DialDisplay.bs2, 189
DigitalPotUpDown.bs2, 295
Index · Page 351
DigitalPotUpDownWithToggle.bs2, 297
SimpleLookdown.bs2, 187
DisplayDigits.bs2, 179
SimpleLookup.bs2, 183
DisplayDigitsWithLookup.bs2, 184
SimpleSubroutines.bs2, 217
DoReMiFaSolLaTiDo.bs2, 255
SlowServoSignalsForLed.bs2, 113
FlashBothLeds.bs2, 49
StoreLightMeasurementsInEeprom.bs2,
208
LedOnOff.bs2, 38
LedOnOffTenTimes.bs2, 44
LightMeter.bs2, 220
MicroAlarmProto(Dev-009).bs2, 314
MicroMusicWithRtttl.bs2, 277
MusicWithMoreFeatures.bs2, 265
NestedLoops.bs2, 250
NotesAndDurations.bs2, 260
PairsOfTones.bs2, 252
PhototransistorAnalogToBinary.bs2, 227
PolledRcTimer.bs2, 147
PushbuttonControlledLed.bs2, 71
PushbuttonControlOfTwoLeds.bs2, 75
ReactionTimer.bs2, 81
ReadLightMeasurementsFromEeprom.bs
2, 210
ReadPotWithRcTime.bs2, 152
ReadPushbuttonState.bs2, 68
SegmentTestWithHighLow.bs2, 178
SelectCaseWithCharacters.bs2, 275
SelectCaseWithValues.bs2, 273
ServoCenter.bs2, 106
ServoControlWithDebug.bs2, 122
ServoVelocities.bs2, 127
TerminalControlledDigitalPot.bs2, 299
TestBiColorLed.bs2, 55
TestBinaryPhototransistor.bs2, 234
TestPhototransistor.bs2, 201
TestPiezoWithFreqout.bs2, 247
TestSecondLed.bs2, 48
ThreeServoPositions.bs2, 115
TwinkleTwinkle.bs2, 257
Proof of concept, 317
Propeller microcontroller, 326
Proton, 335
Prototyping, 307
Prototyping area, 31
Pseudo code, 72
Pseudo random, 87
Pull-up and Pull-down resistors, 69
PULSOUT, 105
Pushbutton, 62
Active-high, 69
-R-
RANDOM, 86
RCTIME, 149, 199
READ, 206
Receive Windowpane, 120
Receiving, 61
Reference, 338
Reference notch, 292
Page 352 · What’s a Microcontroller?
Remote, IR Remote Parts Kit, 324
Resistance, 335
Resistor, 28, 38
as I/O pin protection, 38
Color Code Values, 29
I/O pin protection, 69
Part drawing and schematic symbol, 29
Pull-up and Pull-down, 69
Variable, digital potentiometer, 292
Variable, Flexiforce, 319
Variable, potentiometer, 139
Rests, in music, 259
RETURN, 216
Reveille, 277
Ringing Tone Text Transfer Language,
271
Robotics with the Boe-Bot, 329
Rollover bug, 122
RTTTL, 271
-S-
Scaling, 158
Schematic, 35
Schematic symbol, 28
Schmitt trigger, 230
Seed value for pseudo random numbers,
86, 87
SELECT…CASE, 272
Sending, 61
Sensing, 61
Sensors products, 322
Serial 2x16 LCD, 320
SERIN, 316
SEROUT, 316
Servo
Caution Statement, 96
Power supply warning, 101
Timing diagram, 104
Servo Header Jumper, 97
Sharp notes, 254
Smart Sensors and Applications, 329
Smart Sensors and Applications
textbook, 321
Sockets, 31
Sound waves. See
Specification, 310
StampPlot LITE, 213
Stamps in Class Flowchart, 329
Stamps in Class Mini Projects, 330
STEP, 124
Subroutine, 216
Call, 218
Label, 217
Nesting limit, 219
Sunlight, 201
Superposition, 252
Switching, 61
-T-
Take Me Out To The Ball Game, 343
Tap, potentiometer, 298
Tempo, 260
Timing diagram, 104
TOGGLE, 296
Tokens, 203, 207, 287
Tolerance, 29, 298
Transistor, 198, 289
Schematic Symbol and Part Drawing, 289
Transistor-transistor logic (TTL), 230
Transmit Windowpane, 120
Index · Page 353
Transmitting, 61
TTL, 61
Twinkle Twinkle Little Star., 257
-U-
UNTIL, 83
USB drivers, 20
-V-
Variable range error, 122
Variable resistor, 319
potentiometer (digital), 292
potentiometer (single-turn), 139
Variables, 45
Bit, 45
Byte, 45
DIRH, 178
Initialization, 82
Naming rules, 43
Nib, 45
OUTH, 178
Overflow, 273
RAM storage, 209
Word, 45
Vdd, 338
Video tutorials, 330
Vin, 338
Virtual COM Port, 20
Visible light, 197
Volt, 335
Voltage, 35, 335
Voltage decay circuit, 145
Voltage divider, 155
Vss, 338
-W-
Wavelength, 197
WHILE, 123
Word, 45
Variable size, 45
WORD modifier, 206
WRITE, 206, 207
-Ω-
Ω omega, 28
Page 354 · What’s a Microcontroller?
Parts and quantities are subject to change without notice. Parts may differ from what is
shown in this picture. If you have any questions about your kit, please email
stampsinclass@parallax.com.
Web Site: www.parallax.com
Forums: forums.parallax.com
Sales: sales@parallax.com
Technical: support@parallax.com
Office: (916) 624-8333
Fax: (916) 624-8003
Sales: (888) 512-1024
Tech Support: (888) 997-8267
Errata for What’s a Microcontroller? Text v3.0
(#28123)
If you find what may be additional errata items not listed here, please email editor@parallax.com. We
appreciate your sharp eyes!
Text errors are noted with red strikethrough text, and corrections with blue text, in the sections below.
Formatted PDF replacement pages for each correction are appended to this document.
Page 143
How the Potentiometer Circuit Works
The total resistance in your test circuit is 220 Ω plus the resistance between the A and W terminals of the
potentiometer. The resistance between the A and W terminals increases as the knob is adjusted further
clockwise counterclockwise, which in turn reduces the current through the LED, making it dimmer.
Pages 166-167
'
'
'
'
'
'
'
'
What's a Microcontroller - Ch5Prj01_ControlServoWithPot.bs2
Read potentiometer in RC-time circuit using RCTIME command.
The time var ranges from 126 to 713, and an offset of 330 is needed.
Use RCTIME result in time variable to control servo position.
Bicolor LED on P12, P13 tells direction of servo rotation:
green for CW, red for CCW, off when servo is holding position.
{$STAMP BS2}
{$PBASIC 2.5}
PAUSE 1000
DEBUG "Program Running!"
time
prevTime
VAR
VAR
Word
Word
' time reading from pot
' previous reading
DO
prevTime = time
HIGH 7
PAUSE 10
RCTIME 7, 1, time
time = time + 350
time = time */ 185
time = time + 500
Copyright © Parallax Inc.
' Store previous time reading
' Read pot using RCTIME
' Scale pot, match servo range
' Scale by 0.724 (X 256 for */).
' Offset by 500.
Errata What’s a Microcontroller? v3.0 (#28123)
v1.0 3/22/2010 Page 1 of 3
Page 204
The memory map does not match the one for the program in Activity #1. This is the correct memory map
display.
Figure 0-1a
Memory Map
To view this window,
click Run, and select
Memory Map.
Copyright © Parallax Inc.
Errata What’s a Microcontroller? v3.0 (#28123)
v1.0 3/22/2010 Page 2 of 3
Pages 232-234
If your What’s a Microcontroller v3.0 kit did not contain a 4.7 kΩ resistor, you can instead use two 10 kΩ
resistors in parallel as shown in Figure 7-20a. The equivalent resistance of two 10 kΩ resistors in parallel is
5 kΩ. You can also use this approach with two 100 kΩ resistors in parallel for an equivalent resistance of
50 kΩ. (Since this is not a book error specifically, no replacement PDF page is provided.)
Figure 0-2a: Schematic and Wiring Diagram that Utilize two 10 kΩ Resistors in Parallel for an
Equivalent Resistance of 5 kΩ.
Equivalent Resistance for Series and Parallel Values.
When two or more resistors are connected in series, the equivalent resistance is:
REQ = R1 + R2 + R3…
(Equivalent resistance for resistors in series)
When two or more resistors are connected in parallel, their equivalent resistance is:
REQ = 1 ÷ (1/R1 + 1/R2 + 1/R3…)
(Equivalent resistance for resistors in parallel)
For two 10 kΩ resistors in parallel, that’s 1 ÷ (1/10k + 1/10k) = 1 ÷ (2/10k) = 10k/2 = 5 k.
Two equal value resistors in parallel allow twice as much current through as one of them in
its own would let through. So it stands to reason that the equivalent resistance for two equal
value resistors in parallel would be one half of the value.
Equivalent Capacitance for Series and Parallel Values.
It’s easiest to remember how to calculate equivalent capacitance if you think about it as the
reverse of series and parallel resistor calculations. So, the equivalent capacitance for
parallel capacitors adds up, and equivalent capacitance for series capacitors uses inverses.
CEQ = C1 + C2 + C3…
(Equivalent capacitance for capacitors in parallel)
In the previous activity, two capacitors were placed in parallel to double the capacitance.
For two 0.01 μF capacitors, that’s CEQ = 0.01 μF + 0.01 μF = 0.02 μF
When two or more capacitors are placed in series, their equivalent capacitance is:
CEQ = 1 / (1/C1 + 1/C2 + 1/C3…)
(Equivalent capacitance for capacitors in series)
<<<<<<END OF ERRATA LIST. FORMATTED CORRECTED PDF PAGES APPENDED>>>>>>
Copyright © Parallax Inc.
Errata What’s a Microcontroller? v3.0 (#28123)
v1.0 3/22/2010 Page 3 of 3
Measuring Rotation · Page 143
(a)
(c)
(e)
(b)
(d)
(f)
Figure 5-6
Potentiometer Knob
(a) through (f) show the
potentiometer’s wiper
terminal set to different
positions.
How the Potentiometer Circuit Works
The total resistance in your test circuit is 220 Ω plus the resistance between the A and W
terminals of the potentiometer. The resistance between the A and W terminals increases
as the knob is adjusted further counterclockwise, which in turn reduces the current
through the LED, making it dimmer.
ACTIVITY #2: MEASURING RESISTANCE BY MEASURING TIME
This activity introduces a new part called a capacitor. A capacitor behaves like a
rechargeable battery that only holds its charge for short durations of time. This activity
also introduces RC-time, which is an abbreviation for resistor-capacitor time. RC-time is
a measurement of how long it takes for a capacitor to lose a certain amount of its stored
charge as it supplies current to a resistor. By measuring the time it takes for the capacitor
to discharge with different size resistors and capacitors, you will become more familiar
with RC-time. In this activity, you will program the BASIC Stamp to charge a capacitor
and then measure the time it takes the capacitor to discharge through a resistor.
Introducing the Capacitor
Figure 5-7 shows the schematic symbol and part drawing for the type of capacitor used in
this activity. Capacitance value is measured in microfarads (µF), and the measurement is
typically printed on the capacitors.
The cylindrical case of this particular capacitor is called a canister. This type of
capacitor, called an electrolytic capacitor, must be handled carefully.
9 Read the CAUTION box on the next page.
Page 166 · What’s a Microcontroller?
Solutions
Q1. A potentiometer.
Q2. No, it’s fixed. The variable resistance is between either outer terminal and the
wiper (middle) terminal.
Q3. A capacitor is like a rechargeable battery in that it can be charged up to hold
voltage. The difference is that it only holds a charge for a very small amount of
time.
Q4. You can measure the time it takes for the capacitor to discharge (or charge).
This time is related to the resistance and capacitance. If the capacitance is
known and the resistance is variable, then the discharge time gives an indication
of the resistance.
Q5. As R gets larger, the RC discharge time increases in direct proportion to the
increase in R. As R gets smaller, the RC discharge time decreases in direct
proportion to the decrease in R.
Q6. The CON directive substitutes a name for a number.
E1. New cap = (10 x old cap value) = (10 x 0.5µF) = 5 µF
P1. Activity #4 with bicolor LED added.
P13
1
2
P12
470 Ω
'
'
'
'
'
'
'
Potentiometer schematic from Figure 5-11
on page 151, servo from Chapter 4,
Activity #1, and bicolor LED from Figure
2-19 on page 53 with P15 and P14 changed
to P13 and P12 as shown.
What's a Microcontroller - Ch5Prj01_ControlServoWithPot.bs2
Read potentiometer in RC-time circuit using RCTIME command.
Use RCTIME result in time variable to control servo position.
Bicolor LED on P12, P13 tells direction of servo rotation:
green for CW, red for CCW, off when servo is holding position.
{$STAMP BS2}
{$PBASIC 2.5}
PAUSE 1000
DEBUG "Program Running!"
time
prevTime
VAR
VAR
Word
Word
' time reading from pot
' previous reading
Measuring Rotation · Page 167
DO
prevTime = time
HIGH 7
PAUSE 10
RCTIME 7, 1, time
time = time */185
time = time + 500
IF ( time > prevTime + 2) THEN
HIGH 13
LOW 12
ELSEIF ( time < prevTime - 2) THEN
LOW 13
HIGH 12
ELSE
LOW 13
LOW 12
ENDIF
' Store previous time reading
' Read pot using RCTIME
'
'
'
'
Scale pot, match servo range
Scale pot, match servo range
increased, pot turned CCW
Bicolor LED red
' value decreased, pot turned CW
' Bicolor LED green
' Servo holding position
' LED off
PULSOUT 14, time
LOOP
P2. The key is to add IF...THEN blocks; an example is shown below. CLREOL is a
handy DEBUG control character meaning “clear to end of line.”
'
'
'
'
'
What's a Microcontroller - Ch5Prj02_ControlServoWithPot.bs2
Read potentiometer in RC-time circuit using RCTIME command.
Modify with IF…THEN so the servo only rotates from 650 to 850.
The time variable ranges from 1 to 691, so an offset of at least
649 is needed.
' {$STAMP BS2}
' {$PBASIC 2.5}
PAUSE 1000
DEBUG "Program Running!"
time VAR Word
DO
HIGH 7
PAUSE 10
RCTIME 7, 1, time
time = time + 649
IF (time < 650) THEN
time = 650
ENDIF
' Read pot with RCTIME
' Scale time to servo range
' Constrain range from 650 to 850
Page 204 · What’s a Microcontroller?
highlighted in blue, and only 35 bytes out of the 2048 byte EEPROM are used for the
program. The remaining 2013 bytes are free to store data.
Figure 7-6
Memory Map
To view this
window, click
Run, and select
Memory Map.
The EEPROM Map shows the addresses as hexadecimal values, which were discussed
briefly in the Decimal vs. Hexadecimal box on page 183. The values along the left side
show the starting address of each row of bytes. The numbers along the top show the byte
number within that row, from 0 to F in hexadecimal, which is 0 to 15 in decimal. For
example, in Figure 7-6, the hexadecimal value C1 is stored at address 7E0. CC is stored
at address 7E1, 6D is stored at address 7E2, and so on, up through E8, which is stored at
address 7EF. If you scroll up and down with the scroll bar, you’ll see that the largest
memory addresses are at the bottom of the EEPROM Map, and the smallest addresses are
at the top, with the very top row starting at 000.
PBASIC programs are always stored at the largest addresses in EEPROM, which are
shown at the bottom of the EEPROM Map. So, if your program is going to store data in
EEPROM, it should start with the smallest addresses, starting with address 0. This helps
ensure that your stored data won’t overwrite your PBASIC program, which will usually
result in a program crash. In the case of the EEPROM Map shown in Figure 7-6, the
PBASIC program resides in addresses 7FF through 7DD, starting at the largest address
and building to smaller addresses. So your application can store data from address 000
through 7DC, building from the smallest to the largest. In decimal, that’s addresses 0
through 2012.
If you plan on storing data to EEPROM, it is important to be able convert from
hexadecimal to decimal in order to calculate the largest writable address. Below is the