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Arduino Mega · SSD1306 OLED

Arduino Mega SSD1306 OLED Simulator: Wiring, Code and a Live Circuit

This is a 0.96 inch SSD1306 OLED, 128 by 64 pixels, wired to an Arduino Mega 2560 over I2C and running live: the board writes two lines of text, draws a box and counts the seconds in it. It uses the Adafruit SSD1306 and GFX libraries, exactly as a tutorial would.

Four wires: power, ground, SDA to pin 20 and SCL to pin 21. The Arduino Mega is a 5 V board and the module takes 5 V directly.

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  • 5 V logic
  • Library: Adafruit_SSD1306.h
  • I2C address 0x3C
  • SDA 20, SCL 21
Arduino Mega · SSD1306 OLEDlive0.000 s 0.00x
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The circuit itself, running on the simulator with the sketch below. Press what can be pressed; click anything else to open it in the editor.

Wiring the SSD1306 OLED to the Arduino Mega

Every connection in the circuit above, with the board’s own pin names.

SSD1306 OLED pinArduino Mega pinWhy
VCC5VThe module has its own regulator and runs from 3.3 or 5 V.
GNDGNDCommon ground.
SDApin 20Arduino Mega’s I2C data line, where Wire.begin() puts it.
SCLpin 21Arduino Mega’s I2C clock line.

On the Mega the I2C bus is pin 20 (SDA) and pin 21 (SCL), not A4 and A5 as on an Uno. The SDA and SCL pins by AREF are the same two nets. The display answers at address 0x3C, the address almost every 0.96 inch module ships with.

Pins to leave alone on the Arduino Mega

Pins 14 to 19 are the three extra serial ports and 20 and 21 are I2C, so keep them free if you will need those. An Uno shield’s I2C on A4 and A5 is not connected to anything on a Mega.

On a real bench

The Mega is long, so put the breadboard beside its double row of pins 22 to 53 and leave the left-hand headers free. The SSD1306 OLED uses only a few of its 54 pins; a Mega earns its size when one sketch drives a keypad, a display and several sensors at once.

The code

The sketch the circuit above runs. Open it in the editor, change a line and press Run: it compiles in your browser.

SSD1306 on Arduino Mega I2C
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

// 128 x 64 pixels, on the I2C bus: SDA pin 20, SCL pin 21
Adafruit_SSD1306 display(128, 64, &Wire, -1);

void setup() {
  Serial.begin(9600);
  if (!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) {
    Serial.println("No SSD1306 at 0x3C");
    for (;;) {}
  }
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(2);
  display.setCursor(0, 0);
  display.println("HELLO");
  display.setTextSize(1);
  display.println("SSD1306 OLED");
  display.drawRect(0, 40, 128, 24, SSD1306_WHITE);
  display.display();
  Serial.println("OLED ready");
}

void loop() {
  // A counter in the box, redrawn twice a second.
  display.fillRect(2, 42, 124, 20, SSD1306_BLACK);
  display.setCursor(8, 48);
  display.print("Up for ");
  display.print(millis() / 1000);
  display.print(" s");
  display.display();
  delay(500);
}

Adafruit_SSD1306 display(128, 64, &Wire, -1) describes a 128 by 64 panel on the Wire bus with no reset pin. display.begin(SSD1306_SWITCHCAPVCC, 0x3C) starts the controller at its address and returns false if nothing answers there, which the sketch reports on the serial monitor rather than drawing into thin air.

Every drawing call (println, drawRect, fillRect) changes a copy of the screen in the board’s RAM. Nothing reaches the glass until display() sends the whole frame, 1024 bytes, over I2C. That is why the loop clears its box, draws the new number and calls display() once.

The frame buffer is 1 KB, which the ATmega2560 has room for many times over. The font in the simulator is a 5 by 7 one with capitals, digits and punctuation, so lower case prints as capitals here.

Common mistakes with the SSD1306 OLED on the Arduino Mega

Forgetting display()

Every call before display() only changes the copy in RAM. A sketch that draws and never calls display() leaves the screen black, and that is the most common "my OLED does not work" there is.

Wiring SDA and SCL to A4 and A5

That is the Uno’s I2C bus. On a Mega, A4 and A5 are ordinary analog pins and the display never answers. Use 20 (SDA) and 21 (SCL), or the SDA and SCL pins by AREF.

Mixing up the 128x32 module

The 128 by 32 OLED uses the same chip and the same address, but the constructor needs 128, 32. Built for 64 rows, everything is drawn half off the panel.

Questions and answers

Can I test an SSD1306 OLED with an Arduino Mega 2560 without the hardware?

Yes. The circuit at the top of this page is an Arduino Mega 2560 with a SSD1306 OLED, running in your browser on a simulated ATmega2560. Every pixel the sketch sends is drawn on the simulated panel, so you can lay out a screen before the module arrives. Press Run it in the editor to change the wiring or the code; it is free and needs no account.

Which pins are SDA and SCL on the Arduino Mega?

On the Mega the I2C bus is pin 20 (SDA) and pin 21 (SCL), not A4 and A5 as on an Uno. The SDA and SCL pins by AREF are the same two nets.

Does the display work at 3.3 V?

Yes, the common modules run from 3.3 to 5 V. On an Arduino Mega 2560 5 V is simpler, because the module’s pull-ups then pull to the same voltage the board uses.

Can I use U8g2 instead of the Adafruit library?

U8g2lib.h compiles here too. The page uses Adafruit_SSD1306 because it is what most tutorials start from, and its calls (setCursor, print, display) carry over to the other Adafruit displays.

Build your own SSD1306 OLED project

Open this circuit in the editor, change the wiring or the code, and keep your version in a free account.

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