7

Arduino programming: Stacks, classes, and scrolling displays

SCHOOL OF MAKING

we want to include the two new library header files

alongside dht.h for the sensor:

#include <dht.h>

#include <Adafruit_SSD1306.h>

#include <Adafruit_GFX.h>

Beneath these lines, we’re going to use three

#define statements to bake-in system-wide values

that save us from changing the actual code to

accommodate hardware differences:

#define DHT11_PIN 2

#define SCREENADR 0x3C

#define MAXSTACK 128

The first line sets the pin connected to the

temperature and humidity sensor, the same as in the

previous tutorial. The second line is the I^2 C address

of the screen. The screen and the Arduino talk to

each other using the I^2 C protocol, and because

you can connect multiple devices over I^2 C, each is

differentiated with an address. Ours is 0x3C. This

should be included in your screen documentation,

or even burnt into the PCB, but you can also run

a script to probe any connected I^2 C devices and

return the address of each device if you need to

(hsmag.cc/kigPeT).

The third statement in the above code is a

precursor to a new and important concept we’re

going to introduce in this tutorial, and that’s

something called a ‘stack’. We’re going to use a

simplified stack to hold 128 separate temperature

measurements, so that we can draw a histogram

of changes in temperature over time. You might

wonder why we don’t use a simple array to hold

these values, but this is because we want the

histogram to scroll in real time as temperatures are

added. If we were to simply update the values in an

array sequentially, the histogram would draw itself

across the screen, left to right, and then simply

reset to the left border of the screen again, as you

see in many such implementations. But a stack

allows us to have a sliding window of values that

follow a leading edge, effectively creating a scrolling

histogram of temperature data. This all sounds

more complicated than the actual code, so let’s take

a look:

class Stack

{

private:

int ourList[MAXSTACK];

int top;

public:

Stack() {

top = 0;

for (int i = 0; i <= MAXSTACK; i++)

ourList[i] = 0;

}

Using the text

function requires

a foreground

and background

colour. Without a

background colour,

when the text

updates it will look

corrupted, but it’s

because old text

pixels are still there in

the background.

QUICK TIP

Right

The display we’re

using is less than an

inch across, which

is ideal for tiny IoT

installations and self-

contained devices