Arduino Programming Fundamentals

Functions

In computer science, a subroutine or subprogram (also called procedure, method, function, or routine) is a portion of code within a larger program, which performs a specific task and is relatively independent of the remaining code.
A function is a way for programmers to reuse code without having to rewrite it, this is time saving, and often makes code more readable.
what we need to have a function:
1) each function must have a unique name
2) the function name is followed by parentheses()
3) functions have a return type, e.g. void
4) the body of a function is enclosed in opening and closing braces {}

Lets do a very simple example to see how it works

void setup() {
  Serial.begin(9600);
  
  DashedLine(); //here we are calling the function
  Serial.println("| BONJOUR |");
  DashedLine(); //here we call it again
}

void loop() {
}

void DashedLine() //here is where we create the function
{
  Serial.println("----------------");
}

Passing a value to a function

and a bit of for loop

void setup() {

  Serial.begin(9600);
  
  // draw the menu box
  DashedLine(24);
  Serial.println("| Program Options Menu |");
  DashedLine(24);
}

void loop() {
}
//a function needs to be able to accept an integer value that is passed to it, 
//the variable type and the name of the variable 
//are inserted between the opening an closing parentheses after the function name

void DashedLine(int len) 
{
  int i;
  
  // draw the line
  //The body of the sketch uses the len variable in a for loop 
  //to print out the correct number of dashes that make up the dashed line of the menu box
  for (i = 0; i < len; i++) {
    Serial.print("-");
  }
  // move the cursor to the next line
  Serial.println("");
}

let's try it with our usual Hello World blink

  pinMode(13, OUTPUT);
}
 
// the loop function runs over and over again forever
void loop() {
  blinkLed();
}
 
void blinkLed() {
  digitalWrite(13, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(1000);              // wait for a second
  digitalWrite(13, LOW);    // turn the LED off by making the voltage LOW
  delay(1000);              // wait for a second
}

now we can reuse the function multiple times

In this example, we’ll add a parameter to the function to allow the function to vary the speed of the blink

void setup() {
  // initialize digital pin 13 as an output.
  pinMode(13, OUTPUT);
}
 
// the loop function runs over and over again forever
void loop() {
  blinkLed(200);
}
 
void blinkLed(int delayTime) {
  digitalWrite(13, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(delayTime);              // wait for a second
  digitalWrite(13, LOW);    // turn the LED off by making the voltage LOW
  delay(delayTime);              // wait for a second
}

Beyond the delay()

A Scheduler

A scheduler is like writing down a list of when you are going to do something and a state machine is like keeping a record of how far through a task you have got.

If you use the function delay(1000) then the processor goes away and does nothing for one second. We say this delay() is a blocking function, that is it blocks further activity in the program. So in order for our code to do something whilst waiting for the delay time to expire, what we need is a scheduler, something that will call a specific task at a specific time. There are several ways to do this but this is perhaps the simplest. To make a scheduler we need to call on the aide of the system clock. This keeps on ticking every millisecond and we can access the current tick count with the millis() function.
What we need to do is to set up a variable called say goTime that contains the time in milliseconds when we need to do the next operation. That is made from simply the time now plus how long from now you want to do the next operation. Then all we do in the main loop is check if it is time to do our operation and if it is do it. Finally before we exit that routine we need to set our variable goTime to reflect when we want to do it again.

A State Machine

A ‘state’ is the condition of a thing at a specific time. Something that can accomplish tasks and that utilizes states at its core is a state machine. They are also known as Finite State Machines (FSM), meaning that we know all possible states of the thing. The key to the state machine is the concept of time and history. The state of the machine is evaluated periodically. Each time it is evaluated, a new state is chosen (which could be the same state again), and the output is presented.

int ledPin =  13;      // the number of the LED pin
int ledState = LOW;             // ledState used to set the LED
unsigned long previousMillis = 0;        // will store last time LED was updated
long OnTime = 500;           // milliseconds of on-time
long OffTime = 500;          // milliseconds of off-time

void setup() 
{
  // set the digital pin as output:
  pinMode(ledPin, OUTPUT);      
}

void loop()
{
  // check to see if it's time to change the state of the LED
  unsigned long currentMillis = millis();
  unsigned long elapsed = currentMillis - previousMillis;  // elapsed is the time since the end of the last cycle

  if((ledState == HIGH) && (elapsed >= OnTime))
  {
    ledState = LOW;  // Turn it off
    previousMillis = currentMillis;  // Remember the time
    digitalWrite(ledPin, ledState);  // Update the actual LED
  }
  else if ((ledState == LOW) && (elapsed >= OffTime))
  {
    ledState = HIGH;  // turn it on
    previousMillis = currentMillis;   // Remember the time
    digitalWrite(ledPin, ledState);    // Update the actual LED
  }
}