Programming Arduino Getting Started with Sketches

setup
and 
loop
, you do not
actually have to put any lines of code in them. However, failing to add code will make
your sketch a little dull.
Blink—Again!
The reason that Arduino has the two functions 
setup
and 
loop
is to separate the things that
only need to be done once, when the Arduino starts running its sketch, from the things that
have to keep happening continuously.
The function 
setup
will just be run once when the sketch starts. Let’s add some code to
it that will blink the LED built onto the board. Add the lines to your sketch so that it
appears as follows and then upload them to your board:
The 
setup
function itself calls two built-in functions, 
pinMode
and 
digitalWrite
. You
already know about 
digitalWrite
, but 
pinMode
is new. The function 
pinMode
sets a
particular pin to be either an input or an output. So, turning the LED on is actually a two-
stage process. First, you have to set pin 13 to be an output, and second, you need to set
that output to be high (5V).
When you run this sketch, on your board you will see that the LED comes on and stays
on. This is not very exciting, so let’s at least try to make it flash by turning it on and off in
the 
loop
function rather than in the 
setup
function.

---

You can leave the 
pinMode
call in the 
setup
function because you only need to call it
once. The project would still work if you moved it into the loop, but there is no need and it
is a good programming habit to do things only once if you only need to do them once. So
modify your sketch so that it looks like this:
Run this sketch and see what happens. It may not be quite what you were expecting.
The LED is basically on all the time. Hmm, why should this be?
Try stepping through the sketch a line at a time in your head:
1. Run