CSCI 255 — Using an Arduino

A bit about the Arduino

Let’s talk about the Arduino for a while.

• Arduino Uno
• Uno schematic
• ATmega328P — see page 5
• Arduino = Processing + Wiring
• Arduino Language Reference — setup() and loop()

Making sure it all works

There are a few things we must verify before we start this lab.

Getting in the right group

Making sure the Arduino is recognized

Making sure the Arduino device file is present

Having up-to-date software

An exploration of the Arduino, ATmega328, and C++

In this part of the lab, you are going to turn an LED on and off as you are introduced to many things that will appear later in this course.

Connecting the IDE to the board

Make the following choices in the Arduino IDE to direct the IDE’s attention to your board.

• Tools → Board → Arduino/Genuino Uno
• Tools → Serial Port → /dev/ttyACMN

Blinking the LED

Open Blink tutorial, However, don’t use a breadboard! On the Arduinos we have, pin 13 is already connected to a built-in LED and current-limiting resistor. Also, don’t connect an LED to your Arduino! Cut-and-paste the blink program into a new sketch and run it. You should see the LED blink.

Blinking the LED with memory

In Chapter 8 of the textbook, you will read about how “special function registers” are used to control I/O in embedded processors. The special function register looks like a memory location to a C program. In this secdtion, you’ll skip the digitalWrite and write directly to the pin.

Printing in the serial monitor

You will use two very important features of the Arduino language in this part of the lab. Take a look at the following references so that you can do the next part of the lab.

• Serial
• Serial.print()

Add a call to Serial.begin(9600) in your setup function to initialize the serial port. Now add a statement to print the variable PORTB in binary in your loop function after each call to digitalWrite.

Build and run your program. You will need to do Tools → Serial Monitor to see the output of your program.

Special function registers of the ATmega328

The variable PORTB is a special memory location within the ATmega328 chip. Writing to this location sets the output of pins 8 to 13 of the Arudino to either 0 (low) or 1 (high). In embedded microcontrollers these pins are called GPIO, general-purpose input/output, pins.

Look at the Arduino page for port manipulation to see how to change PORTB without calling DigialWrite, along with a reason why you might want to do this.

C++ implementation of Arduino classes

Serial is an object within the C++ class HardwareSerial and print is a method of HardwareSerial. The HardwareSerial class extends the Stream class, and The Stream class extends the Print class.

First connect to the Arduino GitHub in a new tab and navigate to the directory hardware/arduino/avr/cores/arduino. Take a look at the following files:

• HardwareSerial.h — Class definition for HardwareSerial
• HardwareSerial.cpp — Class implementation for HardwareSerial
• Stream.h — Class definition for Stream
• Stream.cpp — Class implementation for Stream
• Print.h — Class definition for Print
• Print.cpp — Class implementation for Print

Next take a look at how the C code implements GPIO in wiring_digital.c where you will find the C function Digital Write (around line 137). Setting an individual bit is accomplished by use of masking operations inside this if statement:

    if (val == LOW) {
        *out &= ~bit;
    } else {
        *out |= bit;
    }

The * in the above code segment is the C and C++ dereference operator. You will meet it in Appendix C.

Most likely, the instructor will guide you through this.

Input to the Arudio

Right now, your Arduino has only one output. In this part of the lab, you will also process input.

Start by taking a look at an Arduino lesson, from Adafruit, on Digital inputs. Best to open it in a second browser window.

Building a breadboard

Stop the Arduino IDE and unplug your Arduino.

Get a breadboard (the smaller ones will work fine) and create a circuit similar to that shown in lesson but include a second LED connected to the Arduino output pin of your choice. (I suggest pin 2.)

You must use a current limiting resistor for each LED, because only pin 13 is connected through a resistor on the Arduino board. You do not need a pullup resistor for your switches, because there is an internal pullup of 20 to 50 kΩ in the ATmega328 chip. This is common for microcontroller chips.

Also because the USB standard limits current to 500 mA, we are not going to bother with the resettable fuse.

Loading the initial program

Start up the Arduino IDE and cut-and-paste the program from the lesson into a sketch.

Improving the program

Meaningful variable names

Change the names of the variables to something more useful than buttonApin and ledPin. I suggest you use leftSwitch, rightSwitch, leftLight and rightLight to hold the pin numbers.

Meaningful function names

Think a little about how your program reacts to the switches and LED’s. What value is read when then switch is pressed: HIGH or LOW? What value is written to turn on the LED: HIGH or LOW? You might recall from the lab with transistors, it could be confusing to remember if the “output” of the switch was 0 or 1 when it was pressed. Let’s write two functions to help us out here.

• switchRead, like digitalRead, has a single parameter. It returns 1, if the switch is pressed, and 0, if the switch is not pressed.
• ledWrite has the same parameters as digitalWrite. It turns the LED on, if its second parameter is non-zero, and off, if its second parameter is zero.

These two functions are very short, probably just a single if—else statement.

The combinational gates

The SR flip-flop

Now create an SR flip-flop with your Arduino. Assume your switches are S and R and your LED’s are Q and Q'. You may assume that no one ever presses both S and R at the same time. If they do, your flip-flop is allowed to self destruct.

You may want to ad an extra variable to your program that records the present value of the flip-flop, but this really isn’t necessary.

The evil of delay

It’s time for the instructor to talk about one of his favorite topic: the evils of delay. However, it will take him a while to get to it.

The T flip-flop

The T flip-flop will toggle its output whenever the left switch is pressed. (You may ignore the right switch.) The initial state of the flip-flop should be 0 for the T.

Switch (de)bounce

Generally when you press a switch it will bounce on and off a few times. The bounces are very quick. You won’t see them when turning on a desk lamp. However, computers are so fast that the Arduino may “think” you are really pressing that switch several times a second.

You can fix switch bounce mechanically by using mercury in the switch, but too much mercury can make you mad. You could also use a low pass filter, but that requires a capacitor and a resistor and clearly costs more.

Alternatively, you can add a call to delay() to your program to make it wait for the bounces to subside. You can find a few tutorials for switch debounce with delay on the internet.

The problem with delay

Avoid delay in embedded systems. When your Arduino sketch is executing delay() it does nothing else. That means it may miss other important events.

Once you start using delay(), it becomes hard to break the habit. The designers of Processing tried to remove delay() between versions 1 and 2 of Processing but the delay() addicts demanded its return. It is back in Processing, but is no longer mentioned in the Processing reference page. By the way, JavaScript does not have a delay operator, but jQuery does.

Also never use Arduino interrupts to debounce a switch. That is simply silly.

Using timers to avoid using delay

Fortunately there are many on-line tutorials on how to use timers to debounce a switch. Read the Debounce tutorial on the Arduino web site to see how this is done.