Input and Output

By Prof. Seungchul Lee
http://iai.postech.ac.kr/
Industrial AI Lab at POSTECH

Table of Contents

1. Pins¶

Pins are wires connected to the microcontroller
Pins are the interface of the microcontroller
Pin voltages are controlled by a sketch
Pin voltages can be read by a sketch

Input pins

Input pins are controlled by other components
Arduino reads the voltage on the pins
Allows it to respond to events and data

Output pins

Output pins are controlled by the Arduino
Voltage is determined by your sketch
Other components can be controlled through outputs

Digital vs Analog

Some pins are digital-only
- Read digital input, write digital output
- 0 volts or 5 volts
Some pins can be analog inputs
- Can read analog voltages on the pin
- Useful for analog sensors
Analog-only pins are clearly labeled
No pins can generate a true analog output

Input/Ouput (I/O) in Arduino¶

These functions allow access to the pins

void pinMode(pin, Mode)

Sets a pin to act as either an input or an output
pin is the number of the pin
- 1 -13 for the digital pins
- A0 - A5 for the analog input pins
Mode is the I/O mode the pin is set to
- INPUT, OUTPUT

Digital Input

int digitalRead(pin)

Returns the state of an input pin
Returns either LOW (0 volts) or HIGH (5 volts)

int pinval;
pinval = digitalRead(3);

pinval is set to the state of digital pin3

Digital Output

void digitalWrite(pin, value)

Assigns the state of an output pin
Assigns either LOW (0 volts) or HIGH (5 volts)

digitalWrite(3, HIGH);

Digital pin 3 is set HIGH (5 volts)

Analog Input

int analogRead(pin)

Returns the state of an analog input pin
Returns an integer from 0 to 1023
0 for 0 volts, 1023 for 5 volts

int pinval;
pinval = analogRead(A3);

Pin must be an analog pin

Analog Output

No pins can generate a true analog output
But PWM can be used

2. Digital Input and Output¶

http://www.allaboutcircuits.com/projects/learn-how-to-use-the-arduinos-digital-i-o/

2.1. Lab 01: Blinking LED using delay¶

the LED connects to digital I/O pin 8 of the Arduino through the 220 ohm resistor.
The resistor controls the current through the LED.
sets the digital I/O pin to HIGH for 1000 ms, then to LOW for another 1000 ms.

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const int led  =  8;        // use digital I/O pin 8

void setup() {
  pinMode(led, OUTPUT);     // set pin 8 to be an output output
}

void loop() {
  delay(1000);              // delay 1000 milliseconds
  digitalWrite(led, HIGH);  // set pin 8 HIGH, turning on LED
  delay(1000);              // delay 1000 milliseconds
  digitalWrite(led, LOW);   // set pin 8 LOW, turning off LED
}

2.2. Lab 02: Blinking LED using Button¶

An LED is connected to pin 8, which is configured as an OUTPUT.
A pushbutton is connected to pin 9, which is configured as an INPUT.
When someone presses the pushbutton switch, pin 9 is set to HIGH, and the program will then set the output of pin 8 to HIGH and turning on the LED.
Releasing the pushbutton resets pin 9 to LOW, then sets pin 8 to LOW, which turns off the LED.
debouncing will be discussed later

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const int led    =   8;       //name pin 8 as led
const int button =   9;       //name pin 9 as button

void setup() {
  pinMode(led, OUTPUT);       //set pin 8 as OUTPUT
  pinMode(button, INPUT)  ;   //set pin 9 as INPUT
}

void loop() {
  int reads = digitalRead(button);  //read the digital value on pin 9
  digitalWrite(led, reads);         //set the digital output value of pin 8 to that value
}

3. Analog Inputs and Output (or Pulse Width Modulation)¶

Analog Inputs

int analogRead(pin)

Returns the state of an analog input pin
Returns an integer from 0 to 1023 ($2^{10}$)
0 for 0 volts, 1023 for 5 volts

Analog Outputs

No pins can generate a true analog output

Pulse Width Modulation

Can be used for analog output
Duty cycle is the percent of time the pulse is high
Increasing duty cycle increases perceived voltage

analogWrite()

Generates a square wave on a pin, 490 Hz
First argument is the pin number
Second argument is the pulse width
- 0 is 0% duty cycle
- 255 is 100% duty cycle ($2^8$)
Pin number must be a PWM pin
- Marked on the Arduino with the ~ symbol
For example: analogWrite(9, 128)

3.1. Lab 03: Fade LED In and Out (PWM)¶

220 ohm resistor
Note that pin 9 is used

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int i = 0;
const int LED = 9;     //define the pin we use for LED

void setup() {
  pinMode(LED, OUTPUT); //set pin 9 as OUTPUT
}

void loop() {
  for (int i = 0; i < 255; i++) { //if i is less than 255 then increase i with 1
    analogWrite(LED, i);          //write the i value to pin 11
    delay(5);                     //wait 5 ms then do the for loop again
  }

  for (int i = 255; i > 0; i--) { //descrease i with 1
    analogWrite(LED, i);
    delay(5);
  }
}

4. Serial Communication¶

UART protocol used over the USB cable
One bit at a time
Initialize by using Serial.begin()
Serial.begin(speed)
- speed is the baud rate
- Serial.begin(9600)
Usually call Serial.begin() in the setup function

Serial.available()

Get the number of bytes (characters) available for reading from the serial port. This is data that's already arrived and stored in the serial receive buffer (which holds 64 bytes).

Serial.read()

Reads incoming serial data.
Returns the first byte of incoming serial data available (or -1 if no data is available)

Serial.print()

https://www.arduino.cc/en/Serial/Print

Serial.println(): new line

int incomingByte = 0;   // for incoming serial data

void setup() {
  Serial.begin(9600);     // opens serial port, sets data rate to 9600 bps
}

void loop() {
  // send data only when you receive data:
  if (Serial.available() > 0) {
    // read the incoming byte:
    incomingByte = Serial.read();

    // say what you got:
    Serial.print("I received: ");
    Serial.println(incomingByte);
  }
}

Serial.write()

Writes binary data (ASCII) to the serial port. This data is sent as a byte or series of bytes
to send the characters representing the digits of a number use the Serial.print() function instead.
Serial.write(val)
- val: a value to send as a single byte

Serial.write(str)
- str: a string to send as a series of bytes

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

void loop() {
  Serial.write(45); // send a byte with the value 45

  int bytesSent = Serial.write("hello"); //send the string “hello” and return the length of the string.
}

Debug via Serial Communication¶

Serial.print()

int x = 0;

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

void loop() {
  Serial.println(x);
  if (x > 10) {
    x = 0;
  }
  else {
    x++;
  }
  delay(1000);
}

Serial.write()

int x = 48;

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

void loop() {
  Serial.write(x);
  Serial.println(' ');
  if (x > 56) {
    x = 48;
  }
  else {
    x++;
  }
  delay(1000);
}

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4.1. Lab 04: Serial Communication for Analog Inputs¶

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void setup() {
  // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
}

// the loop routine runs over and over again forever:
void loop() {
  // read the input on analog pin 0:
  int sensorValue = analogRead(A0);

  // Convert the analog reading (which goes from 0 - 1023) to a voltage (0 - 5V):
  float voltage = sensorValue * (5.0 / 1023.0);

  // print out the value you read:
  Serial.println(voltage);
}

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