SSD1331-Based OLED Display - DONE.LAND

Most OLED displays are monochrome. OLED displays using the SSD1331 driver can display 65K colors.

Resolution

Since full color OLED displays are still quite expensive, most displays are very small with limited resolution: for example 0.95inch at a resolution of 96x64.

Regardless, the added color capabilities help these small displays carry over a lot more content than monochrome alternatives.

Interface

Due to the higher data volume that is required for color displays, these OLED displays use the faster SPI interface.

Libraries

There is a number of libraries to select from, for example Adafruit SSD1331 OLED Driver Library for Arduino.

Wiring

Pin labels on this OLED display breakout board are confusing as they often do not use the commonly used SPI pin names.

Some pins are labeled SCL and SDA and suggest an I2C interface.

That is not the case. The display supports SPI only. Here is what the pins do:

The display has seven pins:

Pin	Label	Description
1	GND	Ground
2	VCC	3.3-5.0V
3	SCL	SPI: Clock (SCLK, CLK)
4	SDA	SPI: MOSI
5	RES	Reset
6	DC	Data/Command
7	CS	SPI: Chip Select

The SPI interface is implemented as a typical three-wire design using the pins SDA (MOSI), SCL (CLK), and CS.

Pin DC is not common and used with components that can receive large amounts of data (such as displays): when this pin is pulled low, it indicates that the data received is a command, else display data.

Connecting To ESP32 S2 Mini

In my tests, I am connecting the display to a ESP32 S2 Mini like so:

Pin Display	Pin Wemos/Lolin S2 Mini	Description
Gnd	Gnd	Ground
VCC	3V3	3.3V
SCL	7	SPI Clock
CS	12	SPI Chip Select
SDA	11	SPI MOSI
RES	33	Reset
DC	35	Data/Command

Example #1

Here is example code taken from the Adafruit SSD1306 library that I used to create the pictures on this page. The code is already adjusted and runs fine in platformio:

I was unable to run this code with hardware SPI on a S2 Mini (despite using the hardware SPI pins). Only the (much slower) software SPI constructor was able to control the display. See the following examples for more.

When the display is connected in the wrong manner or when hardware SPI was chosen in code but the connected pins do not provide hardware SPI, the display typically shows random pixels:

/***************************************************
  This is a example sketch demonstrating the graphics
  capabilities of the SSD1331 library  for the 0.96"
  16-bit Color OLED with SSD1331 driver chip

  Pick one up today in the adafruit shop!
  ------> http://www.adafruit.com/products/684

  These displays use SPI to communicate, 4 or 5 pins are required to
  interface
  Adafruit invests time and resources providing this open source code,
  please support Adafruit and open-source hardware by purchasing
  products from Adafruit!

  Written by Limor Fried/Ladyada for Adafruit Industries.
  BSD license, all text above must be included in any redistribution
 ****************************************************/
#include <Arduino.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1331.h>
#include <SPI.h>


// You can use any (4 or) 5 pins
#define sclk 7
#define mosi 11
#define cs   12
#define rst  33
#define dc   35


// Color definitions
#define	BLACK           0x0000
#define	BLUE            0x001F
#define	RED             0xF800
#define	GREEN           0x07E0
#define CYAN            0x07FF
#define MAGENTA         0xF81F
#define YELLOW          0xFFE0
#define WHITE           0xFFFF

// Option 1: use any pins but a little slower
// this was the ONLY option that worked for me on a S2 Mini microcontroller:
Adafruit_SSD1331 display = Adafruit_SSD1331(cs, dc, mosi, sclk, rst);

// Option 2: must use the hardware SPI pins
// this constructor did not work and seemed to not change the display content at all
//Adafruit_SSD1331 display = Adafruit_SSD1331(&SPI, cs, dc, rst);

float p = 3.1415926;


void testlines(uint16_t color) {
   display.fillScreen(BLACK);
   for (int16_t x=0; x < display.width()-1; x+=6) {
     display.drawLine(0, 0, x, display.height()-1, color);
   }
   for (int16_t y=0; y < display.height()-1; y+=6) {
     display.drawLine(0, 0, display.width()-1, y, color);
   }

   display.fillScreen(BLACK);
   for (int16_t x=0; x < display.width()-1; x+=6) {
     display.drawLine(display.width()-1, 0, x, display.height()-1, color);
   }
   for (int16_t y=0; y < display.height()-1; y+=6) {
     display.drawLine(display.width()-1, 0, 0, y, color);
   }

   // To avoid ESP8266 watchdog timer resets when not using the hardware SPI pins
   delay(0);

   display.fillScreen(BLACK);
   for (int16_t x=0; x < display.width()-1; x+=6) {
     display.drawLine(0, display.height()-1, x, 0, color);
   }
   for (int16_t y=0; y < display.height()-1; y+=6) {
     display.drawLine(0, display.height()-1, display.width()-1, y, color);
   }

   display.fillScreen(BLACK);
   for (int16_t x=0; x < display.width()-1; x+=6) {
     display.drawLine(display.width()-1, display.height()-1, x, 0, color);
   }
   for (int16_t y=0; y < display.height()-1; y+=6) {
     display.drawLine(display.width()-1, display.height()-1, 0, y, color);
   }

}

void testdrawtext(char *text, uint16_t color) {
  display.setTextSize(1);
  display.setTextColor(WHITE);
  display.setCursor(0,0);

  for (uint8_t i=0; i < 168; i++) {
    if (i == '\n') continue;
    display.write(i);
    if ((i > 0) && (i % 21 == 0))
      display.println();
  }
}

void testfastlines(uint16_t color1, uint16_t color2) {
   display.fillScreen(BLACK);
   for (int16_t y=0; y < display.height()-1; y+=5) {
     display.drawFastHLine(0, y, display.width()-1, color1);
   }
   for (int16_t x=0; x < display.width()-1; x+=5) {
     display.drawFastVLine(x, 0, display.height()-1, color2);
   }
}

void testdrawrects(uint16_t color) {
 display.fillScreen(BLACK);
 for (int16_t x=0; x < display.height()-1; x+=6) {
   display.drawRect((display.width()-1)/2 -x/2, (display.height()-1)/2 -x/2 , x, x, color);
 }
}

void testfillrects(uint16_t color1, uint16_t color2) {
 display.fillScreen(BLACK);
 for (int16_t x=display.height()-1; x > 6; x-=6) {
   display.fillRect((display.width()-1)/2 -x/2, (display.height()-1)/2 -x/2 , x, x, color1);
   display.drawRect((display.width()-1)/2 -x/2, (display.height()-1)/2 -x/2 , x, x, color2);
 }
}

void testfillcircles(uint8_t radius, uint16_t color) {
  for (uint8_t x=radius; x < display.width()-1; x+=radius*2) {
    for (uint8_t y=radius; y < display.height()-1; y+=radius*2) {
      display.fillCircle(x, y, radius, color);
    }
  }
}

void testdrawcircles(uint8_t radius, uint16_t color) {
  for (int16_t x=0; x < display.width()-1+radius; x+=radius*2) {
    for (int16_t y=0; y < display.height()-1+radius; y+=radius*2) {
      display.drawCircle(x, y, radius, color);
    }
  }
}

void testtriangles() {
  display.fillScreen(BLACK);
  int color = 0xF800;
  int t;
  int w = display.width()/2;
  int x = display.height();
  int y = 0;
  int z = display.width();
  for (t = 0 ; t <= 15; t+=1) {
    display.drawTriangle(w, y, y, x, z, x, color);
    x-=4;
    y+=4;
    z-=4;
    color+=100;
  }
}

void testroundrects() {
  display.fillScreen(BLACK);
  int color = 100;
  int i;
  int t;
  for(t = 0 ; t <= 4; t+=1) {
    int x = 0;
    int y = 0;
    int w = display.width();
    int h = display.height();
    for(i = 0 ; i <= 8; i+=1) {
      display.drawRoundRect(x, y, w, h, 5, color);
      x+=2;
      y+=3;
      w-=4;
      h-=6;
      color+=1100;
    }
    color+=100;
  }
}

void tftPrintTest() {
  display.fillScreen(BLACK);
  display.setCursor(0, 5);
  display.setTextColor(RED);
  display.setTextSize(1);
  display.println("Hello World!");
  display.setTextColor(YELLOW, GREEN);
  display.setTextSize(2);
  display.print("Hello Wo");
  display.setTextColor(BLUE);
  display.setTextSize(3);
  display.print(1234.567);
  delay(10000);
  display.setCursor(0, 5);
  display.fillScreen(BLACK);
  display.setTextColor(WHITE);
  display.setTextSize(0);
  display.println("Hello World!");
  display.setTextSize(1);
  display.setTextColor(GREEN);
  display.print(p, 5);
  display.println(" Want pi?");
  display.print(8675309, HEX); // print 8,675,309 out in HEX!
  display.print(" Print HEX");
  display.setTextColor(WHITE);
  display.println("Sketch has been");
  display.println("running for: ");
  display.setTextColor(MAGENTA);
  display.print(millis() / 1000);
  display.setTextColor(WHITE);
  display.print(" seconds.");
}

void mediabuttons() {
 // play
  display.fillScreen(BLACK);
  display.fillRoundRect(25, 10, 78, 60, 8, WHITE);
  display.fillTriangle(42, 20, 42, 60, 90, 40, RED);
  delay(500);
  // pause
  display.fillRoundRect(25, 90, 78, 60, 8, WHITE);
  display.fillRoundRect(39, 98, 20, 45, 5, GREEN);
  display.fillRoundRect(69, 98, 20, 45, 5, GREEN);
  delay(500);
  // play color
  display.fillTriangle(42, 20, 42, 60, 90, 40, BLUE);
  delay(50);
  // pause color
  display.fillRoundRect(39, 98, 20, 45, 5, RED);
  display.fillRoundRect(69, 98, 20, 45, 5, RED);
  // play color
  display.fillTriangle(42, 20, 42, 60, 90, 40, GREEN);
}

/**************************************************************************/
/*!
    @brief  Renders a simple test pattern on the LCD
*/
/**************************************************************************/
void lcdTestPattern(void)
{
  uint8_t w,h;
  display.setAddrWindow(0, 0, 96, 64);

  for (h = 0; h < 64; h++) {
    for (w = 0; w < 96; w++) {
      if (w > 83) {
        display.writePixel(w, h, WHITE);
      } else if (w > 71) {
        display.writePixel(w, h, BLUE);
      } else if (w > 59) {
        display.writePixel(w, h, GREEN);
      } else if (w > 47) {
        display.writePixel(w, h, CYAN);
      } else if (w > 35) {
        display.writePixel(w, h, RED);
      } else if (w > 23) {
        display.writePixel(w, h, MAGENTA);
      } else if (w > 11) {
        display.writePixel(w, h, YELLOW);
      } else {
        display.writePixel(w, h, BLACK);
      }
    }
  }
  display.endWrite();
}

void setup(void) {
  Serial.begin(9600);
  Serial.print("hello!");
  display.begin();

  Serial.println("init");
  uint16_t time = millis();
  display.fillScreen(BLACK);
  time = millis() - time;

  Serial.println(time, DEC);
  delay(10000);

  lcdTestPattern();
  delay(10000);

  display.fillScreen(BLACK);
  display.setCursor(0,0);
  display.print("Lorem ipsum dolor sit amet, consectetur adipiscing elit. Curabitur adipiscing ante sed nibh tincidunt feugiat. Maecenas enim massa");
  delay(10000);

  // tft print function!
  tftPrintTest();
  delay(10000);

  //a single pixel
  display.drawPixel(display.width()/2, display.height()/2, GREEN);
  delay(10000);

  // line draw test
  testlines(YELLOW);
  delay(10000);

  // optimized lines
  testfastlines(RED, BLUE);
  delay(10000);

  testdrawrects(GREEN);
  delay(10000);

  testfillrects(YELLOW, MAGENTA);
  delay(10000);

  display.fillScreen(BLACK);
  testfillcircles(10, BLUE);
  testdrawcircles(10, WHITE);
  delay(10000);

  testroundrects();
  delay(10000);

  testtriangles();
  delay(10000);

  display.fillScreen(BLACK);
  Serial.println("done");
}

void loop() {
}

This is the platformio.ini I used to compile the sketch in platform.io and run it on a ESP32 S2 Mini:

[env:lolin_s2_mini]
platform = espressif32
board = lolin_s2_mini
framework = arduino
lib_deps = adafruit/Adafruit SSD1331 OLED Driver Library for Arduino@^1.2.0

Example #2

Due to the issues with hardware SPI, I decided to use one of the other popular SSD1331 libraries: namely usglib. This library is similar to u8g2 in that it supports a multitude of hardware. While u8g2 targets monochrome displays, usglib does the same for color displays.

Like usual with these libraries, the connected device is chosen by commenting in the appropriate constructor line. Here are the two constructors I used for the *96x64 color OLED display:

// Software-defined SPI pins:    
Ucglib_SSD1331_18x96x64_UNIVISION_SWSPI ucg(/*sclk=*/ 7, /*data=*/ 11, /*cd=*/ 35, /*cs=*/ 12, /*reset=*/ 33);

// Hardware SPI pins:
//Ucglib_SSD1331_18x96x64_UNIVISION_HWSPI ucg(/*cd=*/ 35, /*cs=*/ 12, /*reset=*/ 33);

The library comes with many great example sketches. I picked the FPS example which performs speed tests, figuring this would be a nice added value when comparing the performance of software SPI versus hardware SPI.

Just make sure you pick one of the two constructors, and comment out the other one. The code below starts by using the software SPI pins:

#include <SPI.h>
#include "Ucglib.h"

    
// Software-defined SPI pins:    
Ucglib_SSD1331_18x96x64_UNIVISION_SWSPI ucg(/*sclk=*/ 7, /*data=*/ 11, /*cd=*/ 35, /*cs=*/ 12, /*reset=*/ 33);

// Hardware SPI pins:
//Ucglib_SSD1331_18x96x64_UNIVISION_HWSPI ucg(/*cd=*/ 35, /*cs=*/ 12, /*reset=*/ 33);

void setup(void) {
  delay(1000);
  ucg.begin(UCG_FONT_MODE_TRANSPARENT);
  ucg.setColor(0, 0,0,0);
  ucg.setColor(1, 0,0,0);
  ucg.setColor(2, 0,0,0);
  ucg.setColor(3, 0,0,0);
}

/*
  Linear Congruential Generator (LCG)
  z = (a*z + c) % m;  
  m = 256 (8 Bit)
  
  for period:
  a-1: dividable by 2
  a-1: multiple of 4
  c: not dividable by 2
  
  c = 17
  a-1 = 64 --> a = 65
*/
uint8_t z = 127;	// start value
uint8_t lcg_rnd(void) {
  z = (uint8_t)((uint16_t)65*(uint16_t)z + (uint16_t)17);
  return z;
}

void draw_text(void) {
  ucg.setFont(ucg_font_ncenR14_tr);
  //ucg.setColor(255, 255, 255);
  ucg.setColor(lcg_rnd(),lcg_rnd(),lcg_rnd());
  ucg.setPrintPos(0,20);
  ucg.print("The quick brown");
  ucg.setPrintPos(0,40);
  ucg.print("fox jumps over");
  ucg.setPrintPos(0,60);
  ucg.print("the lazy dog");
}

void draw_box(void) {
  ucg_int_t x, y, w, h;
  ucg.setColor(lcg_rnd(),lcg_rnd(),lcg_rnd());
  x = lcg_rnd() & 31;
  y = lcg_rnd() & 31;
  w = 63;
  w += lcg_rnd() & 31;
  h = 63;
  h += lcg_rnd() & 31;
  ucg.drawBox(x,y,w, h);
}

void draw_gradient_box(void) {
  ucg_int_t x, y, w, h;
  static uint8_t idx = 0;
  switch(idx & 3)
  {
    case 0: ucg.setColor(0, lcg_rnd(),lcg_rnd(),lcg_rnd()); break;
    case 1: ucg.setColor(1, lcg_rnd(),lcg_rnd(),lcg_rnd()); break;
    case 2: ucg.setColor(2, lcg_rnd(),lcg_rnd(),lcg_rnd()); break;
    case 3: ucg.setColor(3, lcg_rnd(),lcg_rnd(),lcg_rnd()); break;
  }
  idx++;
  x = lcg_rnd() & 31;
  y = lcg_rnd() & 31;
  w = 63;
  w += lcg_rnd() & 31;
  h = 63;
  h += lcg_rnd() & 31;
  ucg.drawGradientBox(x,y,w, h);
}


// returns FPS*10
uint16_t measure(void (*draw_fn)(void)) {
  uint16_t FPS10 = 0;
  uint32_t time;

  ucg.clearScreen();

  time = millis() + 10*1000;
  do {
    draw_fn();
    FPS10++;
  } while( millis() < time );
  
  return FPS10;  
}


static const unsigned char u8d_tab[3]  = { 100, 10, 1 } ;
const char *u8dp(char * dest, uint8_t v)
{
  uint8_t pos;
  uint8_t d;
  uint8_t c;
  for( pos = 0; pos < 3; pos++ )
  {
      d = '0';
      c = *(u8d_tab+pos);
      while( v >= c )
      {
	v -= c;
	d++;
      }
      dest[pos] = d;
  }  
  dest[3] = '\0';
  return dest;
}

/* v = value, d = number of digits */
const char *u8d(uint8_t v, uint8_t d)
{
  static char buf[8];
  d = 3-d;
  return u8dp(buf, v) + d;
}

const char *convert_FPS(uint16_t fps) {
  static char buf[6];
  strcpy(buf, u8d( (uint8_t)(fps/10), 3));
  buf[3] =  '.';
  buf[4] = (fps % 10) + '0';
  buf[5] = '\0';
  return buf;
}

void show_result(const char *s, uint16_t fps)  {
  ucg.clearScreen();
  ucg.setFont(ucg_font_helvR18_tr);
  ucg.setColor(255, 255, 255);
  ucg.setPrintPos(0,25);
  ucg.print(s);
  ucg.setPrintPos(0,50);
  ucg.print(convert_FPS(fps));  
  delay(2000);
}

void loop(void)
{
  show_result("Text", measure(draw_text));
  show_result("Box", measure(draw_box));
  show_result("Gradient", measure(draw_gradient_box));
  delay(500); 
}

Hardware SPI 70x Faster

The code ran flawlessly both with software SPI and with hardware SPI . It runs three speed drawing tests: Text, Boxes, and Gradients. At the end of each test, it reports back the achieved frame rate. When all three tests are completed, the code starts over again.

Obviously, the speed of software SPI depends on many factors including the microcontroller. Generally, though, software SPI is always dramatically slower than the optimized hardware SPI. This may not become evident when you just output a few lines of text. When doing animations or displaying video, the speed differences are dramatically.

The test above exposed that with software SPI, the average framerate was around 1-2fps. With hardware SPI, it was typically around 70fps and better.

Data Sheets

SSD1331

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