Manage the 128x32 monochrome SSD1306 OLED display

published: 10 October 2020 / updated 12 October 2020

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Structure of the SSD1306 128x32 display screen

The SSD1306 128x32 display screen uses the same internal component as SSD1306 128x64 display

The internal memory is common to both models, the 128x32 display screen does not use that part of this memory.

Organization of memory

The internal memory of the display has 1KB RAM.

In this diagram, here is the organization of the screen for a definition of 128x64 pixels:

Each column contains 8 bits. A line is designated by page :

the 128x64 display: contains 8 pages, numbered from 0 to 7
the 128x32 display: contains 4 pages, numbered from 0 to 3

In the rest of the article, we will not only focus on the display 128x32 pixels. But most of the explanations still apply to the 128x64 display.

Each page is divided into segments:

Here, in blue in the figure, a segment represents a byte. The least significant bit is at the top.

Transmission of a graphic icon

From the organization of pages and segments, it becomes easy to create a graphic pattern:

. X X X X X X .
. X . . . . X .
X X . . . . X X
X . X X X X . X
X . X X X X . X
X . X X X X . X
X . . . . . . X
X . X X X X . X
X . X X X X . X
X . X X X X . X
X . . . . . . X
X . X X X X . X
X . X X X X . X
X . X X X X . X
X . . . . . . X
X . X X X X . X
X . X X X X . X
X . X X X X . X
X . . . . . . X
X X X X X X X X

Tilt your head to the right. You will guess the logo of a battery. We will code this logo in binary:

flash 
stream: BATT 
    CTRL_DATAS c, 
    %01111110 c, 
    %01000010 c, 
    %11000011 c, 
    %10111101 c, 
    %10111101 c, 
    %10111101 c, 
    %10000001 c, 
    %10111101 c, 
    %10111101 c, 
    %10111101 c, 
    %10000001 c, 
    %10111101 c, 
    %10111101 c, 
    %10111101 c, 
    %10000001 c, 
    %10111101 c, 
    %10111101 c, 
    %10111101 c, 
    %10000001 c, 
    %11111111 c, 
    ;stream 
ram

In this code, each bit set to 1 represents a lit pixel. Once compiled, we can run BATT from the Tera Term terminal. And here's what we get on display:

Here, this logo is displayed at the beginning of page 0. Question: How to display our logo elsewhere?

Set display position

Here, to define the display position of our battery logo, we will generate a positioning command:

flash 
stream: xy.batt 
    CTRL_COMMANDS c, 
    $21 c,  \ COL START_END 
    $6b c,  \ start 
    $7f c,  \ end 
    $22 c,  \ PAGE START_END 
    $00 c,  \ start 
    $00 c,  \ end 
    ;stream 
ram

The $21 command is used to define the start and end display segment: $6b (107 in decimal) will be the starting segment, $7f (127 in decimal) will be the segment end.

The $22 command is used to define the start and end display page: $00 will be the start and end page.

We run xy.bat BATT and we have on display:

Define a dynamic logo

It's fine to display a battery logo. but to be really useful, it would be nice if this logo indicates the real state of charge of a real battery.

If we analyze the design of the logo, we can see that the internal motif is repeated. It's about obviously four internal pavers. We will therefore define a word BATTfull for this only part:

flash 
stream: BATTfull 
    CTRL_DATAS c, 
    %10111101 c, 
    %10111101 c, 
    %10111101 c, 
    %10000001 c, 
    ;stream 
ram

Then we define the same drum section, but empty:

flash 
stream: BATTempty 
    CTRL_DATAS c, 
    %10000001 c, 
    %10000001 c, 
    %10000001 c, 
    %10000001 c, 
    ;stream 
ram

And finally, we define the end and the beginning of the battery:

flash 
stream: BATTend 
    CTRL_DATAS c, 
    %11111111 c, 
    ;stream 
 
stream: BATTstart 
    CTRL_DATAS c, 
    %01111110 c, 
    %01000010 c, 
    %11000011 c, 
    ;stream     
ram

And now it's enough to put the pieces back together:

stream: 4SPACES 
    CTRL_DATAS c, 
    %00000000 c, 
    %00000000 c, 
    %00000000 c, 
    %00000000 c, 
    ;stream 
ram 
 
: bat04 ( ---) 
    BATTstart  
    BATTempty  BATTempty  BATTempty  BATTempty 
    BATTend ; 
 
: bat14 ( ---) 
    BATTstart  
    BATTempty  BATTempty  BATTempty  BATTfull 
    BATTend ; 
 
: bat24 ( ---) 
    BATTstart  
    BATTempty  BATTempty  BATTfull   BATTfull 
    BATTend ; 
 
: bat34 ( ---) 
    BATTstart  
    BATTempty  BATTfull   BATTfull   BATTfull 
    BATTend ; 
 
: bat44 ( ---) 
    BATTstart  
    BATTfull   BATTfull   BATTfull   BATTfull 
    BATTend ; 
 
: testBATTS ( ---) 
    disp.reset 
    bat04  4SPACES 
    bat14  4SPACES 
    bat24  4SPACES 
    bat34  4SPACES 
    bat44  ;

Running testBATTS displays this:

So to display text just set as many logos for each character to display. This is what we will see in the next article.