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can go on during the delay function, so in effect, it brings
most other activity to a halt. For alternative approaches
to controlling timing, see the millis() function [8] used
on line 49 in the sketch below. Arduino veterans usually
avoid the use of delay() for timing of events longer than
10’s of milliseconds unless the Arduino sketch is very
simple.
However, certain things do go on while the delay() function
is controlling the Atmega chip because the delay function
does not disable interrupts. Serial communication that
appears at the RX pin is recorded, PWM (analogWrite)
values and pin states are maintained, and interrupts will
work as they should.
Author’s Comment
From what we have seen above, the delay()
implementation behaves like a busy loop with interrupts
enabled.
Variables
A variable is a place for storing a piece of data. It has
a name, a type, and a value. For example, the line 26
from the sketch above declares a variable with the name
ledPin, the type int, and an initial value of 13. It’s being
used to indicate which Arduino pin the LED is connected
to and since it never changes, it’s declared as const,
which could give the linker a hint to place it in a section
other then precious RAM. Every time the name ledPin
appears in the code, its value will be retrieved. In this
case, the person writing the program could have chosen
not to bother creating the ledPin variable and could have
simply written 13 everywhere they needed to specify a
pin number instead. The advantage of using a variable
is that it’s easier to move the LED to a different pin—you
only need to edit the one line that assigns the initial value
to the variable.
Often, however, the value of a variable will change while
the sketch runs (lines 29, 34, 49). Due to the lack of
insightful comments, it’s not quite obvious to me why the
variable in line 30 is not defined as const as well since it
does not change.
Functions
A function (otherwise known as a procedure or subroutine)
is a named piece of code that can be used from
elsewhere in a sketch. The setup() function [9] as seen in
Listing 2: delay-example.ino
int ledPin = 13; // LED connected to digital pin 13
1
2
3
4
5
6
7
8
9
10
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14
void setup()
{
pinMode(ledPin, OUTPUT); // sets the digital pin as output
}
void loop()
{
digitalWrite(ledPin, HIGH); // sets the LED on
delay(500); // waits 500 msec
digitalWrite(ledPin, LOW); // sets the LED off
delay(500); // waits 500 msec
}
Figure 3
Listing 3: BlinkWithoutDelay.ino continued
24 // constants won’t change. Used here to
25 // set pin numbers:
26
27
const int ledPin = 13; // the number of the LED pin
28 // Variables will change:
29 int ledState = LOW; // ledState used to set the LED
30
31
long previousMillis = 0; // will store last time LED was updated
32 // the following variable is a long because the time, measured in
milliseconds,
33 // will quickly become a bigger number than can be stored in an int.
34
35
long interval = 1000; // interval at which to blink (milliseconds)
36 void setup() {
37 // set the digital pin as output;
38 pinMode(ledPin, OUTPUT)
39 }
Figure 4
Listing 4: BlinkWithoutDelay.ino continued
41 void loop()
42 {
43
44
// here is where you’d put code that needs to be running all the time.
45 // check to see if it’s time to blink the LED; that is, if the
46 // difference between the current time and last time you blinked
47 // the LED is bigger than the interval at which you want to
48 // blink the LED.
49
50
unsigned long currentMillis = millis();
51 if(currentMillis – previousMillis > interval) {
52 // save the last time you blinked the LED
53
54
previousMillis = currentMillis;
55 // if the LED is off turn it on and vice-versa:
56 if (ledState = LOW)
57 ledState = HIGH;
58 else
59
60
ledState = LOW;
61 // set the LED with the ledState of the variable:
62 digitalWrite(ledPin, ledState);
63 }
64 }
Figure 5
line 36 is called once, when the sketch starts. It’s a good
place to configure pin modes and to initialize libraries.
The pinMode() function [10] line 38 configures a digital
pin as either an input or an output. To use it, you pass the
number of the pin to configure and the constant INPUT
or OUTPUT. When configured as an input, a pin can
detect the state of a sensor like a “Push” button. As an
output, it can drive an actuator like an LED, which, in
our case, utilizes the digitalWrite() function [11] line 62 to
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