03A: Decisions and Digital I/O

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03A: Decisions and Digital I/O

Contents

1 Questions, Answers and Review
2 Making Decisions
3 Arduino Digital I/O
4 Wrap Up and Reminders

Questions, Answers and Review

Questions from last class?
Questions about homework?

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Making Decisions

Learner Outcomes

At the end of the lesson the student will be able to:

Discuss what is meant by the term flow of control
Make use of relational expressions to make decisions
Implement decisions using if statements
Compare numbers, characters and strings

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Using `if` Statements Review

Flow of control (or control flow) refers to the order in which programs execute instructions
By default, code executes sequentially: one statement after another, top to bottom, left to right
We previously discussed how to change the flow using a selection statement
An if-statement only executes if a test condition evaluates to true
As an example, we have the following code with an if-statement

Example Program With an `if`-Statement

#include <ArduinoSTL.h>
using namespace std;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  Serial.setTimeout(-1);
  cout << "I'm thinking of a number between 1 and 10." << endl;
  cout << "Can you guess it?" << endl;
  cout << "Enter your first guess: " << endl;
}

void loop() {
  if (Serial.available()) {
    int guess = 0;
    cin >> guess;
    while (Serial.available())  {  //clear out any line-end chars
       Serial.read();
    }
    cout << "You entered: " << guess << endl;
    digitalWrite(LED_BUILTIN, LOW);
    if (7 == guess) {
      cout << "*** Correct! ***" << endl;
      digitalWrite(LED_BUILTIN, HIGH);
    }
    cout << "Try again." << endl;
  }
}

Syntax of an `if`-statement

An if statement has two parts: a test and a body
The body can have zero or more statements
The statements in the body execute if and only if the test evaluates to true
If the test condition evaluates to false, the computer skips the code

Syntax:

if (test)
{
   statement1
   statement2
   ...
}

Where:
- test: the test condition to evaluate
- statementX: the statements to execute depending on the test

For example:

    if (7 == guess) {
      cout << "*** Correct! ***" << endl;
    }

In the above, the (7 == guess) is a test that the 7 is equal to the value of guess

putting the variable on the right side is a trick to avoid the error of forgetting to use 2 equal signs for a comparison. (guess = 7) which will compile correctly, but not be what you want.(7 = guess) will generate a compiler error.

For clarity:
- Write the if on a different line than the body
- Indent within the curly braces

Diagram of `if` Statement Operation

About those Curly Braces

Technically, the if statement affects only the single statement that follows
We use curly braces to make that one statement into a block of statements
This allows us to put any number of statements within the body
Curly braces are not always required, but the best practice is to always include them

Check Yourself

True or false: an if-statement requires a test condition.
The code inside the curly braces executes when x is equal to ________.
```
if (x == 3) {
  x = 1;
}
```
The value of x after the following code executes is ________.
```
int x = 7;
if (x == 3) {
  x = 1;
}
```
True or false: an if-statement affects only the single statement following it unless curly braces are used.

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Boolean Variables and Relationships

Notice that relational expressions always evaluate to true or false
Thus we can assign a relational expression to a Boolean variable
```
bool test = 5 != 2;
cout << test; // 1 means true
```
Notice how Boolean values are displayed

false == 0
true == 1

In a Boolean context, zero is false and any other number is interpreted as true

Check Yourself

The value of x after the following code executes is ________.
```
int x = 3;
int y = 4;
if (x < y) {
  x = y;
}
```
The value of y after the following code executes is ________.
```
int x = 42;
bool y = (x == 3);
```

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Using `if-else` Statements

Sometimes we want to choose between two actions
If a condition is true
- then do this
Otherwise it is false
- so do something else
To make this type of selection we use an if...else statement

Syntax:

if (test)
{
   statements1
}
else
{
   statements2
}

Where:
- test: the test condition to evaluate
- statementsX: the statements to execute depending on the test

For example:

if (7 == guess) {
  cout << "*** Correct! ***";
} else {
  cout << "Sorry, that is not correct." << endl;
  cout << "Try again." << endl;
}

Diagram of `if-else` Statement Operation

Notice that there is no test condition for the else clause

if (7 == guess) {
  cout << "*** Correct! ***";
} else {
  cout << "Sorry, that is not correct." << endl;
  cout << "Try again." << endl;
}

The decision on which set of statements to use depends on only one condition

Note that we could write an if-else as a pair of complementary if statements instead, like:

if (7 == guess) {
  cout << "*** Correct! ***";
}
if (7 != guess) {
  cout << "Sorry, that is not correct." << endl;
  cout << "Try again." << endl;
}

However, it is easier and clearer to write an if-else statement instead
For clarity, write the if and else parts on different lines than the other statements
Also, indent the nested statements
We can see an example of an if-else statement in the following example

Example Sketch With an `if-else` Statement

#include <ArduinoSTL.h>
using namespace std;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  Serial.setTimeout(-1);
  cout << "I'm thinking of a number between 1 and 10." << endl;
  cout << "Can you guess it?" << endl;
  cout << "Enter your first guess: " << endl;
}

void loop() {
  int guess = 0;
  if (Serial.available()) {  
    cin >> guess;
    cout << "You entered: " << guess << endl;
    digitalWrite(LED_BUILTIN, LOW);
    if (7 == guess) {
      cout << "*** Correct! ***" << endl;
      digitalWrite(LED_BUILTIN, HIGH);
    } else {
        cout << "Sorry, that is not correct. " << "Try again." << endl;
    }
  }
}

Formatting the `if-else` Statement

It is important to format the if-else statement professionally

    if (7 == guess) {
      cout << "*** Correct! ***" << endl;
      digitalWrite(LED_BUILTIN, HIGH);
    }  else {
      cout << "Try again." << endl;
    }

Note how the conditional code is indented inside both the if and else portions
This lets us easily see which code is conditional and which is not
Also note the placement of curly braces
Different groups have different practices for placing curly braces on if and if-else statements
In practice, you should use the style dictated by your group's policy
- Or your professor's instructions
For the acceptable styles for this course see the instructions at: Curly Braces

Check Yourself

True or false: an if-else statement allows the programmer to select between two alternatives.

The problem with the following if-else statement is ________.

if (7 == guess) {
    cout << "*** Correct! ***" << endl;
}
else (7 != guess) {
    cout << "Sorry, that is not correct." << endl;
}

The value of x after the following code segment executes is ________.

int x = 5;
if (x > 3) {
    x = x - 2;
}
else {
    x = x + 2;
}

True or false: always indent inside the curly braces of an if-else-statement.

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Nested `if` Statements

We can improve our guessing game by providing hints
If the guess is too low or too high, we let user know
For example, if the answer is 7 and the guess is 5, we would display
```
cout << "Your guess is too low, try again." << endl;
```
What test condition would we write to test for a low guess?
What test condition would we write to test for a high guess?

Multiple Test Conditions

We will need multiple test conditions for all the possible cases:
- Guess too low
- Guess too high
- Guess is correct

Novice programmers often use a sequence of if statements for these tests, like:

if (guess < 7) {
  cout << "Your guess is too low." << endl;
}
if (guess > 7) {
  cout << "Your guess is too high."<< endl;
}
if (guess == 7) {
  cout << "*** Correct! ***" << endl;
}

While this works, it is not the recommended way because it is:
- Not clear that only one of the assignment statements will be executed for a given value of guess
- Inefficient since all three conditions are always tested
A better way is to nest if statements

Nested `if-else` Statements

A better way to implement our tests is with nested if statements
We can include if statements within other if statements, nesting in either the if clause or the else clause
When nested in the if clause, the inner if statement is evaluated only if the test condition of the outer if first evaluates to true
When nested in the else clause, the inner if statement is evaluated only if the test condition of the outer if first evaluates to false
The following code shows an example of an if statement nested in the else clause
How would we rewrite the code to nest within the if clause?

Example Showing a Nested `if` Statement

#include <ArduinoSTL.h>
using namespace std;
void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  Serial.setTimeout(-1);
  Serial.print("I'm thinking of a number between");
  Serial.print(" 1 and 10.\nCan you guess it?\n\n");
  Serial.println("Enter your first guess.");
}

void loop() {
  int guess = 0;
  cin >> guess;
  cout << "You entered: " << guess << endl;
  digitalWrite(LED_BUILTIN, LOW);
  if (7 == guess) {
    cout << "*** Correct! ***" << endl;
    digitalWrite(LED_BUILTIN, HIGH);
  } else {
    if (guess < 7) {
      cout << "Your guess is too low, try again." << endl;
    } else {
      cout << "Your guess is too high, try again" << endl;
    }
  }
}

Nesting in the `else` Clause

We can nest in either the if clause or the else clause
When we nest in the else clause, the else clause does not execute if the test evaluates to true
The else part executes only when the if test condition evaluates to false
The if statement inside the else clause works in the same way
By continuing to nest in the else clause, we can build a series of test conditions
The first test condition that evaluates to true executes and the remainder of the clauses are skipped
Let us trace through the logic in the following code extracted from the last example assuming guesses of 5, 8 and 7

Example of Nesting in the `else` Clause

  if (7 == guess) {
    cout << "*** Correct! ***" << endl;
    digitalWrite(LED_BUILTIN, HIGH);
  } else {
    if (guess < 7) {
      cout << "Your guess is too low, try again." << endl;
    } else {
      cout << "Your guess is too high, try again" << endl;
    }
  }

Formatting the `else-if` Statement

if-else-if

if (test1) {
  // do something1
} else {
  if (test2) {
    // do something2
  } else {
    if (test3) {
      // do something3
    ...
    }
  }
}

Because of this tendency to creep to the right, we write nested else-if statement in a special way:

if (test1) {
  // do something1
} else if (test2) {
  //do something2
} else if (test3) {
  //do something3
...
}

We remove the curly-braces for the else clause and align everything to the left
Now the logic is easy to follow:
- The computer starts at the top
- The computer checks each condition, one after the other
- Once the selection is made and processes, the computer skips the rest of the options
We can see the previous example reformatted below

Example Showing a Nested `else-if` Statement

#include <ArduinoSTL.h>
using namespace std

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  Serial.setTimeout(-1);
  Serial.print("I'm thinking of a number between");
  Serial.print(" 1 and 10.\nCan you guess it?\n\n");
  Serial.println("Enter your first guess.");
}

void loop() {
  int guess = 0;
  cin >> guess;
  cout << "You entered: " << guess << endl;
  digitalWrite(LED_BUILTIN, LOW);
  if (guess < 7) {
    cout << "Your guess is too low, try again." << endl;
  } else if (guess > 7) {
    cout << "Your guess is too high, try again." << endl;
  } else {
    cout << "*** Correct! " << endl;
    digitalWrite(LED_BUILTIN, HIGH);
  }
}

Programming Style: Indentation of `else-if` Statements

Format nested else-if statements without the curly braces between the else and if clauses
Then align code to the left as shown below:

 if (guess < 7) {
    cout << "Your guess is too low, try again." << endl;
  } else if (guess > 7) {
    cout << "Your guess is too high, try again." << endl;
  } else {
    cout << "*** Correct! " << endl;
    digitalWrite(LED_BUILTIN, HIGH);
  }

This formatting more clearly shows that we are making a single choice among multiple alternatives
Also, it prevents indentations from cascading to the right as we add more selections

Check Yourself

True or false: you can nest if statements in the if clause, the else clause, or both.

In the following code snippet, when guess = 8 the code displays ________

if (guess != 7) {
    if (guess < 7) {
        cout << "Your guess is too low, try again." << endl;
    } else {
        cout << "Your guess is too high, try again" << endl;
    }
} else {
    cout << "*** Correct! ***" << endl;
}

In the following code snippet, when guess = 7 the code displays ________

if (guess < 7) {
    cout << "Your guess is too low, try again." << endl;
} else if (guess > 7) {
    cout << "Your guess is too high, try again." << endl;
} else {
    cout << "*** Correct! ***" << endl;
}

The following code snippet prints ________

int x = 1;
if (x > 0) {
  x = x + 5;
} else if (x > 1) {
  x = x + 2;
} else {
  x = x + 7;
}
cout << x;

The following code snippet prints ________

int x = 0;
if (x > 0) {
  x = x + 5;
} else if (x > 1) {
  x = x + 2;
} else {
  x = x + 7;
}
cout << x;

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Exercise 1: Changing the Blink Rate (10m)

In this exercise we use if-else-if to change the blinking rate of the built-in LED.

Starter Code

/**
  CS-11M 
  Name: The name of this program
  Purpose: describe the purpose of this program

  @author Your name here
  @version 1.0  Today's date here
*/

#include <ArduinoSTL.h>
using namespace std;

int num = 0;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  cout << "Enter a 1 or 2 or 3 to set the LED blink rate." << endl;
}

void loop() {
  if (Serial.available()) {
    cin >> num;
    while (Serial.available())  {  //put this after your cin statement
      Serial.read();
    }
    cout << "You entered: " << num << endl;
  }
}

Specifications

Start the Arduino IDE with a new sketch and save the project as if-else-if.
Copy the starter code above and paste it into the if-else-if sketch.
Compile the sketch to verify you copied the starter code correctly.
After the first if-statement, add another if-statement that tests for num equal to one and that blinks the LED at 1000 ms if true, like:
```
if (1 == num) {
  digitalWrite(13, HIGH);
  delay(1000);
  digitalWrite(13, LOW);
  delay(1000);
}
```
Compile and upload your code to verify the changes, opening the serial monitor to test the program.
Make sure the LED blinks when you enter the number 1 into the serial monitor. If you have problems, ask a classmate or the instructor for help.
Add an else-if statement that tests for num equal to two and that blinks the LED at 2 seconds.
Add an else statement that blinks the LED at 3 seconds if the number is 3 or anything other than 1 or 2.
Compile and upload your code to verify it works correctly.
You should now be able to toggle between the three rates by entering a one or a two or a three into the serial monitor. If you have problems, ask a classmate or the instructor for help.
Submit your if-else-if.ino file to Canvas.

When finished, please help those around you.

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Summary

Flow of control (or control flow) refers to the order in which programs execute instructions
By default, code executes sequentially: one statement after another, top to bottom, left to right
An if-statement only executes if a test condition evaluates to true
An if statement has two parts: a test and a body
The body can have zero or more statements
The statements in the body execute if and only if the test evaluates to true
If the test condition evaluates to false, the computer skips the code

Syntax:

if (test) {
   statement1
   statement2
   ...
}

Where:
- test: the test condition to evaluate
- statementX: the statements to execute depending on the test
One way to make a test condition is with relational operators
Relational operators compare two values and evaluate to either true or false
The basic C/C++ relational operators are: ==, !=, <, <=, >, >=
To choose between two actions we use an if-else statement

Syntax:

if (test) {
   statements1
} else {
   statements2
}

Where:
- test: the test condition to evaluate
- statementsX: the statements to execute depending on the test
When we need to evaluate multiple test conditions, we may nest if statements
We can nest if statements either in the if clause or the else clause
Each has benefits depending on the logic we need to implement
Nesting in the else clause we can build a series of logically related test conditions

As an example:

if (test1) {
  // do something1
} else if (test2) {
  //do something2
} else if (test3) {
  //do something3
...
}

Notice the formatting that removes the curly braces for the else clause

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Arduino Digital I/O

Learner Outcomes

At the end of the lesson the student will be able to:

Discuss what is meant by digital I/O
Write code to control digital input and output pins

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Digital I/O

Arduino is intended for physical computing and thus has many I/O possibilities
Much of the I/O is through attaching electronics to the Arduino pins
These attachments can be either digital (on or off) or analog with various voltage levels
We will discuss Digital I/O today

Representing Two States

Digital means there are two states
In electronics, we represent these two states as low and high voltages
Recall that voltage is the difference in electric charge between two points
Typically, the Arduino uses 0 volts (ground) as low and +5 volts as high
Arduino designates these states as LOW and HIGH
The LOW and HIGH states can be input and output using Arduino's digital pins

Arduino's `HIGH` and `LOW` Voltage States

Source: arduinomatic.com

Check Yourself

A digital signal has ________ states.
1. 0
2. 1
3. 2
4. 3 or more
In electronics, digital states are represented using the electromotive potential difference between two points expressed in ________.
For the Arduino, a LOW state is ground and a HIGH state is ________.

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Digital Pins and I/O Functions

The Arduino has many digital pins available
For example, the Arduino Uno from our kit has 14 digital pins numbered 0 to 13
These pins can be used as either input or output using the functions described below
Some of the pins have specialized functions in addition
For example, pins 0 and 1 receive (RX) and transmit (TX) serial data

Source: arduinomatic.com

Digital I/O Functions

Name Description

pinMode() Calling pinMode(pin, mode) configures the specified pin to behave either as an INPUT, INPUT_PULLUP or OUTPUT.

digitalWrite() Calling digitalWrite(pin, value) writes a HIGH or a LOW value to the specified digital pin.

digitalRead() Calling digitalRead(pin) returns a HIGH or a LOW value from the specified digital pin.

Check Yourself

The Arduino Uno has 14 digital pins, numbered ________ to ________.
True or false: in addition to providing digital I/O, some digital pins have specialized functions.
Digital pins can be put into this many modes: ________.
1. 1
2. 2
3. 3
4. 4
The function to write a digital HIGH or LOW value is named ________.
The function to read a digital HIGH or LOW value is named ________.

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Coding Digital I/O

We can write Arduino commands to control the digital pins
To set the pins to input or output, we call the pinMode(pin, mode) function
Where:
- pin: the digital pin to set (0 - 13)
- mode: one of INPUT, INPUT_PULLUP or OUTPUT
We will look at basic examples of input and output in this section

Digital Output

We have made use of an LED attached to digital pins for output, such as blinking with pin 13
With digital output set to a HIGH state, the pin provides up to 40 milliamps (mA) of current at 5 volts
The pins are useful for powering LEDs which typically use less than 40 mA
Loads greater than 40 mA, like motors, require a transistor or other interface circuitry
Pins configured as outputs can be damaged or destroyed if they are connected to either the ground or positive power rails
We must protect the output using component such as a resistor
Digital output is coded using the digitalWrite(pin, value) function
Where:
- pin: the digital pin to set (0 - 13)
- value: either HIGH or LOW

For example, from the blink sketch:

void loop() {
  digitalWrite(13, HIGH);
  delay(1000);
  digitalWrite(13, LOW);
  delay(1000);
}

Digital Input

When set to INPUT the digital pins can read a voltage as HIGH or LOW
HIGH is when a voltage greater than 3 volts is present at the pin (5V boards)
LOW is when a voltage at the pin is less than 3 volts
When set to INPUT, the pin is in a high impedance state equivalent to a series resistor of 100 MΩ in front of the pin
Since very little current gets through, the pin has a very low effect on the circuit it is sampling
Digital input is coded using the digitalRead(pin) function
Where pin is the digital pin to read (0 - 13)
The function returns a value of HIGH or LOW that we can assign to a variable

For example:

int buttonState = digitalRead(pushButton);

The following code uses digitalRead() to measure a pin on an Arduino
The Arduino is connected to a pushbutton circuit, which we explore in the next section

Example Code Calling `digitalRead()`

int pushButton = 2;

void setup() {
  Serial.begin(9600);
  pinMode(pushButton, INPUT);
}

void loop() {
  int buttonState = digitalRead(pushButton); // read pin
  cout << buttonState << endl; // display state
  delay(100);
}

Check Yourself

A digital pin can source (provide) current up to ________ milliamps.
To read from a pin, set the pin mode to ________.
Which of the following is a correct way to read a digital value from a pin and save the value read in a variable named foo?
1. digitalRead(pin, foo)
2. digitalRead(foo, pin)
3. foo = digitalRead(pin)
4. pin = digitalRead(foo)

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Digital Input from Pushbuttons

One use for digital read is to test if a button is pressed
Buttons, or switches, connect two points in a circuit when pressed
When not pressed, there is no connection between the two sides of the button
The following image shows a pushbutton and its schematic symbol

Breadboard Pushbutton and Schematic Symbol

Source: arduinoclassroom.com

Floating Input and Pull-Up Resistors

Reading a switch that is open will result in a pin that is "floating"
Digital input pins are very sensitive and will pick up stray capacitance from nearby sources like breadboards, human fingers and wires
Any wire connected to an input pin will act like a little antenna and cause the input state to change
To solve the problem, we use a pull-up or pull-down resistor
The resistor pulls the pin to a known state when the switch is open
A 10kΩ resistor is often chosen as a pull-up or pull-down resistor
A kΩ is electrical resistance equal to one thousand ohms
So our resistor is equal to 10,000 Ohms

Example of Digital Read with a Pushbutton

Let us implement a pushbutton circuit with Arduino
Here is the Fritzing file for the circuit: button.fzz
We start with the following circuit breadboard

One pole of the button is connected to pin 2 and then to 5 volts through a 10kΩ resistor
The resistor is known as a pull-up resistor because it connects to 5 volts, forcing the pin high by default
The other side of the button is connected to ground
When the button is pressed, the metal contacts close and force the pin to ground
The code to test for button presses is shown below

Example Sketch for Detecting Pushbuttons

#include <ArduinoSTL.h>
using namespace std;

int pushButton = 2;

void setup() {
  Serial.begin(9600);
  pinMode(pushButton, INPUT);
}

void loop() {
  int buttonState = digitalRead(pushButton); // read pin
  cout << "buttonState is " << buttonState << endl; // display state
  delay(1000);
}

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Internal Pull-up Resistors

Arduino has internal pull-up resistors which are controlled by software
By default the internal pull-up resistors are turned off
We turn on the pull-up resistors by setting the pin mode to INPUT_PULLUP
For example:
```
pinMode(2, INPUT_PULLUP);
```
On the Arduino Uno, the value of the pull-up is guaranteed to be between 20kΩ and 50kΩ
By changing the pin mode we can eliminate the pull-up resistor from our circuit
The new code and breadboard circuit is shown below

Example Sketch with INPUT_PULLUP

int pushButton = 2;

void setup() {
  Serial.begin(9600);
  pinMode(pushButton, INPUT_PULLUP);
}

void loop() {
  int buttonState = digitalRead(pushButton); // read pin
  cout << "buttonState is " << buttonState << endl; // display state

Breadboard Without Pull-up Resistor

Push button circuit without pull-up resistor

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Exercise 2: Counting Button Presses (10m)

In this exercise we write code to count button presses.

Parts

Solderless breadboard
Pushbutton
Jumper Wires

Breadboard Layout

Start with the Arduino unplugged.
Make the circuit as shown in the image.

Starter Code

int pushButton = 2;

void setup() {
  Serial.begin(9600);
}

void loop() {
  int buttonState = digitalRead(pushButton); // read pin
  cout << buttonState << endl; // display state
 }

Specifications

Start the Arduino IDE with a new sketch and save the project as button_counter.
Copy the starter code above and paste it into the button_counter sketch.
in Setup() add a statement to configure pin pushButton to be INPUT_PULLUP.
Compile the sketch to verify you copied the starter code correctly.

Add two new variables at the start of the code as follows:

int counter = 0;
int priorButtonState = LOW;    //this will store the last value of the ButtonState

In the loop() function, replace the line displaying the button state with the following code:

delay(1); // debounce - give button time to transition
if (buttonState != priorButtonState) {
  priorButtonState = buttonState;
}

Inside the if statement Curly braces, add another if statement that checks to see if buttonState is LOW. If buttonState is LOW, then add one to counter and print out the current count of button presses.
Compile and upload your code to verify it works correctly.
When you open the serial monitor, you should see a report of the button count increasing every time you press the breadboarded button.
Your output should look something like this:
Count of button presses: 1
Count of button presses: 2
Count of button presses: 3
Count of button presses: 4
If you have problems, ask a classmate or the instructor for help.
Save your button_counter.ino file to submit to Canvas with the next homework.

When finished, please help those around you.

^ top

Summary

The Arduino has many digital pins we can use to read or write digital data as shown below

Source: arduinomatic.com

The commonly used functions for reading and writing digital data are shown below

Name Description

pinMode() Calling pinMode(pin, mode) configures the specified pin to behave either as an INPUT, INPUT_PULLUP or OUTPUT.

digitalWrite() Calling digitalWrite(pin, value) writes a HIGH or a LOW value to the specified digital pin.

digitalRead() Calling digitalRead(pin) returns a HIGH or a LOW value from the specified digital pin.

We have used digitalWrite() many times, like with the blink sketch

void loop() {
  digitalWrite(13, HIGH);
  delay(1000);
  digitalWrite(13, LOW);
  delay(1000);
}

For digital input we use code like:

int buttonState = digitalRead(pushButton);

One use for digitalRead() is to test if a button is pressed
However, reading a switch that is open will result in a pin that is "floating"
Thus we add pull-up or pull-down resistors to "pull" to a default state
Rather than adding a pull-up resistor to our circuits, Arduino has an internal pull-up resistor we can enable
```
pinMode(2, INPUT_PULLUP);
```

Further Study

constants: Arduino documentation describing the values of the digital pin modes: INPUT, INPUT_PULLUP, and OUTPUT.
Digital Pins: Arduino tutorial describing the properties of pins configured as INPUT, INPUT_PULLUP or OUTPUT.

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Wrap Up and Reminders

For next homework, see the schedule
When class is over, please shut down your computer.
Complete unfinished exercises from today before the next class meeting

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