General structure of an Arduino program

Programs written in the Arduino IDE are called “sketches”.

Your job is to write the statements that define what those functions do.

Anatomy of a statement

There are many types of statements, but you can get started writing sketches
using only expression-type statements.

Here are some examples:

x = 1000;
Tells the microcontroller to store the value on the right hand side into a
memory location which will henceforth be named x.
x = dt * 2;
Tells the microcontroller to retrieve the value in the memory location named

:dt, multiply it by 2, and then store the result in the location named x.

Syntax note:

So how does a particular area of the microcontroller’s memory get
assigned a “name”? Answer: before a variable can be used, it must be
“declared”, i.e., you must tell arduino what type of variable it is, so
that it will reserve enough memory to store that variable.

For example,

int x;
Reserves 4 bytes of memory, and names that memory location x.
char mychar;
Reserves 1 byte of memory, and names that memory location mychar.

Syntax note:

Anatomy of a function

A function is a block of statements which can be executed as a single
command. A function may have some inputs (arguments) and may have some
outputs (return value).

Here are some examples of using a function:

x = sqrt(y);
Fetches the value stored in the location named y, then
invokes the function sqrt() with the value of y as a parameter, which returns a result, and
stores the result in the location named x.
portlimit = 2 * myfunc(portno, times);
Fetches the value stored in portno, and
fetches the value stored in times, and
invokes the function myfunc() with these parameters, which returns a result, then
multiplies that result by 2, and finally
stores the product at the location named portlimit.
Execute the statements inside initCounter().
(That’s all; do not return any result.)

Here is an example of writing a function:

int addOne(int dt) {
   dt = dt + 1;
   return dt;

As above, this function might be used like this:

newCount = addOne(522);
invoke the function addOne() with the argument 522, and return the result
store the result in the location newCount.

The components of every function are:

Name Input Parameter(s) w/ type Statement(s) Return Type

Syntax notes:

Writing your own functions

To program an Arduino board, you must write the statements that define each of
two functions:

This function is executed only once, immediately after the board is powered on or reset.
It is used, among other thngs, to initialize the state of the I/O pins.
setup() has no input parameters. (This is why the parentheses are empty.)
setup() returns no output value. (So its return “type” is void.)
This function is executed repeatedly until the board is powered off or reset.
loop() has no input parameters. (This is why the parentheses are empty.)
loop() returns no output value. (So its return type is void.)

A bare-bones sketch to turn on the on-board LED for 1 second, and
then turn it off for 1 second is shown below.

void setup() {
    pinMode(13, OUTPUT);    // Call the function "pinMode" with two parameters
                            // pinMode will not return a value.
    digitalWrite(13, LOW);  // Call the function "digitalWrite" with one parameter
                            // digitalWrite will not return a value.

void loop() {
    digitalWrite(13, HIGH); 
    digitalWrite(13, LOW); 


The statements in the setup() function tell the microcontroller to:

The statements in the loop() function tell the microcontroller to::

Enter the above program into the big white programming window in the arduino GUI
(see the figure below at left). Check that it can be understood by the compiler by
clicking the check symbol in in the left of the toolbar (figure below, center).
Then download the (compiled) program to your Arduino by clicking the right arrow
next to the check symbol (figure below, right).

Text Window Compile Button Download Button

Syntax note:

Under the hood

In reality, the sketch above is not, by itself, enough to program the Arduino.

Instead, arduino takes your sketch, and wraps a larger, pre-written program
around it. That larger program calls your setup() function once, and then
calls your loop function forever. When you compile your sketch (by
clicking the check button in the toolbar, you will be actually be
compilng this larger program.

An Improved Version

A simple improvement to the above program makes it much easier to maintain.

Suppose you wanted to use the same code (or similar) on another microcontroller
board where the on-board LED was controlled through digital pin 8. Then you’d
have to modify the above program in 3 places.

To avoid that, write a single statement at the top of the program, where
you specify the pin number of the on-board LED, and assign that value to a
“name”; in this case, the name “LED” is an obvious choice, but “ONBOARDLED”
would work fine also. (It is common practice to use names with all caps for
values that won’t be changed during the course of the program.)

#define LED 13                // This is now the only statement that needs fixing if you
                              // decide to change the pin number where the led is attached.

void setup() {
    pinMode(LED, OUTPUT);     
    digitalWrite(LED, LOW);   

void loop() {
    digitalWrite(LED, HIGH);  
    digitalWrite(LED, LOW);   


Syntax note: