Page 101 - Programming the Photon Getting Started With the Internet of Things

In the previous code examples the variables were declared outside both the setup and
loop functions. Those variables are always referred to as global variables because they

can easily be accessed and changed from either the setup or loop functions. In our
example code we declared a new variable within the loop function:

int state = digitalRead(pushbutton);

When you call a variable within a block of code, it can only be accessed within that
block. This is known as a local variable because it sits within a function. When the
program is running on the Photon and it finishes executing a particular block of code, any
local variables within that block are automatically freed up in memory so they can be used

for other variables in the next block of code.

Debouncing

When you press down on a push button, you would expect to just get a single change from

1 to 0. However, in reality, that does not always occur. Sometimes when the contacts in the
push button come together, they bounce when you release the button and create static
signals. So now a single button press has now become two or more presses, depending on

the push-button switch. All of this happens in a split second—the total amount of time the
button registers its press is less than 200 milliseconds. Most new tactile-type switches may
not bounce at all; however, a very old switch may have a lot of bounce. Sometimes the
bouncing does not have any effect on the outcome of our sketch. For example, in our
previous sketch we detected a push-button press, which then turned on an LED.

Debouncing makes no difference to the outcome because when we release the switch, it
will stabilize itself and the LED will switch off; this may take only a few milliseconds so
we do not notice the debouncing effect.

One situation where debouncing may cause your outcome to be different from what is

expected is if we use a push-button switch to turn an LED on or off every time the switch
is pressed. When you press the button, the LED comes on and stays on; when you press
the button again, the LED turns off and stays off. If you had a button that bounced every
time you pressed it, then the LED would be on or off based on whether you had an odd or

even number of bounces. Using the same circuit as our previous sketch, try flashing the
following program to your Photon:

int ledpin = D7;
int ledValue = LOW;

void setup() {

pinMode (D0, INPUT);
pinMode (ledpin, OUTPUT);
}