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Smooth Scrolling on a SSD1306

A project log for Nano TTY

Small serial terminal for debugging your robots

deshipudeshipu 03/25/2016 at 13:443 Comments

The OLED display that I'm using doesn't have much room in it -- only 4 lines with 21 characters in each. That means that to be useful and convenient, it has to have good scrolling. For the scrolling to be fast and smooth, it's best to implement it in hardware.

Luckily, the SSD1306 supports vertical scrolling by using the SSD1306_SETSTARTLINE command, which basically tells it where it should start reading its image buffer. Since the buffer wraps around, that lets us scroll it pixel-by-pixel. Neat.

The second piece of the puzzle is that the display I have is 128×32 pixels, and that chip was made for 128×64 pixels display -- so I have twice the amount of buffer space than the screen. I can do double buffering and other tricks!

First I tried to use the display with the Adafruit SSD1306 library library in I²C mode, but I couldn't quite get it to work. Turns out that the breakout I have uses SPI not I²C, even though the pins are labeled SCL and SDA... Turns out that SCL is simply just SPI SCK, and SDA is MOSI. There are also DC and RST pins, which actually gave me a hint that this is not I²C. Madness.

Anyways, after playing with that library a little bit, I started to look for something more efficient, and found the SSD1306Ascii library, which only does text output, but allows you to use custom fonts. Exactly what I need. It also has relatively small memory footprint, which leaves more room for my serial buffer.

I only needed one small hack. In order to get access to the whole image buffer, I needed to trick the library into thinking that I actually have 64 rows of pixels, not 32. Fortunately, you pass a screen definition to it at initialization time, so that was trivial. Several hours of playing with the commands gave me this simple scrolling demo:

With this simple code:

#define OLED_RST 8
#define OLED_DC 9
#define OLED_CS -1

#include <SPI.h>
#include "SSD1306Ascii.h"
#include "SSD1306AsciiSpi.h"

SSD1306AsciiSpi oled;

// A version of the Adafruit128x32 display with 64 lines of buffer.
static const DevType MEM_TYPE Adafruit128x32x64 = {
    Adafruit128x32init, sizeof(Adafruit128x32init), 128, 64, 0
};


void setup() {
    int scroll = 0;
    oled.begin(&Adafruit128x32x64, OLED_CS, OLED_DC, OLED_RST);
    oled.setFont(Stang5x7);

    for (int line = 0; line < 20; ++line) {
        oled.clearToEOL();
        oled.print("Line ");
        oled.println(line);
        if (line >= 4) {
            for (int i = 0; i < 8; ++i) {
                delay(50);
                oled.ssd1306WriteCmd(SSD1306_SETSTARTLINE | scroll % 64);
                ++scroll;
            }
        } else {
            delay(400);
        }
        if (oled.row() >= 7 && scroll >= 32) {
            oled.home();
        }
        if (scroll >= 64) {
            scroll = 0;
        }
    }
}


void loop() {}
Next, I will try to get the encoder to work, so that I can do the scrolling with a convenient scroll wheel. Unfortunately, I'm still waiting for the encoder to arrive, so that will be some time later.

Discussions

davedarko wrote 12/22/2017 at 22:54 point

just saw this page here :) https://www.youtube.com/watch?v=QFubFhrEfmQ

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deshipu wrote 12/23/2017 at 14:09 point

Awesome, now I'm famous! I should really get back to this project and finish it one of these days...

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davedarko wrote 12/23/2017 at 16:56 point

I hope that wasn't too much pressure :D

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