OLED Expansion C Library
The Onion OLED Expansion library, libonionoledexp is a dynamic C library that provides functions to setup and perform various actions on the OLED display: writing text, displaying images, and adjusting various settings.
The library can be used in C and C++ programs.
This library is also available as a module for use in Python. The module is called oledExp and is part of the OmegaExpansion package.
Programming Flow
After each power-cycle, the chip that controls the OLED Expansion must be programmed with an initialization sequence to setup the display and enable it to receive additional commands.
After the initialization, the other functions can be used to ajdust various screen settings or display text or images.
Understanding the Display
The screen has a resolution of 128x64 pixels. It is addressable by 128 vertical columns and 8 horizontal pages:
Each page consists of 8 horizontal pixel rows. When a byte is written to the display, the Least Significant Byte (LSB) corresponds to the top-most pixel of the page in the current column. The Most Significant Byte (MSB) corresponds to the bottom-most pixel of the page in the current column.
So writing 0x0f would produce the top 4 pixels being coloured in, and the bottom 4 being left blank.
The display keeps a cursor pointer in memory that indicates the current page and column being addressed. The cursor will automatically be incremented after each byte is written, the cursor can also be moved by the user through functions discussed below.
The C Library
The libonionoledexp C library is a series of functions that perform all of the actions specified in the Programming Flow section.
Source Code
The source code can be found in the Onion i2c-exp-driver GitHub Repo.
Header File
To add the Onion OLED Expansion Library to your program, include the header file in your code:
Library for Linker
In your project’s makefile, you will need to add the following dynamic libraries to the linker command:
The dynamic libraries are stored in /usr/lib on the Omega.
Example Code
The Onion OLED Expansion C library is used in the implementation of the oled-exp command line tool..
The source code can be found here, on the i2c-exp-driver Onion GitHub Repo.
Return Values
All functions follow the same pattern with return values:
If the function operation is successful, the return value will be EXIT_SUCCESS which is a macro defined as 0 in cstdlib.h.
If the function operation is not successful, the function will return EXIT_FAILURE which is defined as 1.
A few reasons why the function might not be successful: * The specified device address cannot be found on the I2C bus (the Expansion is not plugged in) * The system I2C interface is currently in use elsewhere
An error message will be printed that will give more information on the reason behind the failure.
Functions
| Function | Prototype |
|---|---|
| Initialization Function | int oledDriverInit () |
| Turn the Screen On and Off | int oledSetDisplayPower (int bPowerOn) |
| Invert Display Colours | int oledSetDisplayMode (int bInvert) |
| Set the Display Brightness | int oledSetBrightness (int brightness) |
| Dim the Display | int oledSetDim (int dim) |
| Set Memory Mode | int oledSetMemoryMode (int mode) |
| Set Column Addressing | int oledSetColumnAddressing (int startPixel, int endPixel) |
| Set Columns for Text | int oledSetTextColumns () |
| Set Columns for Images | int oledSetImageColumns () |
| Set Cursor Position by Character Column | int oledSetCursor (int row, int column) |
| Set Cursor Position by Pixel | int oledSetCursorByPixel (int row, int pixel) |
| Clear the Screen | int oledClear () |
| Write a Single Byte | int oledWriteByte (int byte) |
| Write a Single Character | int oledWriteChar (char c) |
| Write a String | int oledWrite (char *msg) |
| Read Image Data | int oledReadLcdFile (char* file, uint8_t *buffer) |
| Write Image Data to the Display | int oledDraw (uint8_t *buffer, int bytes) |
| Horizontal Scrolling | int oledScroll (int direction, int scrollSpeed, int startPage, int stopPage) |
| Diagonal Scrolling | int oledScrollDiagonal (int direction, int scrollSpeed, int fixedRows, int scrollRows, int verticalOffset, int startPage, int stopPage) |
| Stop Scrolling | int oledScrollStop () |
Initialization Function - int oledDriverInit ()
This function programs the initialization sequence on the OLED Expansion, after this step is completed, the other various OLED functions can be used with success:
Functions to Adjust Settings
There is a series of functions that adjust various settings on the OLED Display. The adjustable settings are: * Screen on/off * Colour inversion * Setting screen brightness * Dim the display * Setting the memory mode * Defining the width of each page * Manually * For rendering text * For rendering images
Turn the Screen On and Off - int oledSetDisplayPower (int)
The screen can be turned on and off while still preserving the displayed contents:
``` c int oledSetDisplayPower (int bPowerOn);
Note: turning on a screen that is already on, or turning off a screen that is already off will have no effect.
Arguments
The bPowerOn argument is detemines whether to turn the display on or off
| Value | Meaning |
|---|---|
| 0 | Turn the screen off |
| 1 | Turn the screen on |
Examples
Turn the screen off:
Turn the screen on:
Invert Display Colours - int oledSetDisplayMode (int)
The screen driver has the ability to invert the display colours, meaning that black becomes white and vice versa:
Arguments
The bInvert argument is detemines whether to invert the colours or not
| Value | Meaning |
|---|---|
| 0 | Normal colour settings |
| 1 | Inverted colour settings |
Examples
Invert the colours:
Return the colours to normal:
Set the Display Brightness - int oledSetBrightness (int)
The brightness of the display can be adjusted in a granularity of 256 steps:
The default brightness after initialization is 207.
Arguments
The brightness argument is detemines the display brightness with a range of 0 to 255, with 255 being the brightest setting and 0 being the dimmest.
Examples
Set to maximum brightness:
Set to lowest possible brightness:
Set to middle brightness:
Dim the Display - int oledSetDim (int)
This function implements a ‘dim’ and a ‘normal’ setting for the display:
While the brightness adjustment function described just above implements the same feature, this function simplifies the procedure with two very disctinct settings.
Arguments
The dim argument is detemines whether to enable the dim setting
| Value | Meaning |
|---|---|
| 0 | Normal brightness: 207 |
| 1 | Dimmed screen: 0 |
Examples
Dim the display:
Set the display to normal brightness:
Set Memory Mode - int oledSetMemoryMode (int)
Implements the ability to select the display’s memory mode:
The memory mode affects how the cursor is automatically advanced when the display memory is written to with text or images.
Horizontal Addressing Mode
After each write, the column cursor advances horizontally to the right along the page, once the end of the page is reached, the page cursor is advanced onto the next page.
Vertical Addressing Mode
After each write, the page cursor advances vertically downward through the pages, once the last page is reached, the column cursor is advanced to the next pixel column
Page Addressing Mode
The column cursor advances horizontally to the right along each page, once the end of the page is reached, the column cursor wraps around to column 0 and the page cursor remains the same.
By default, Horizontal Addressing is used.
Arguments
The mode argument detemines which memory mode is active, it is recommended to use the eOledExpMemoryMode enumerated type:
| Memory Mode | Input |
|---|---|
| Horizontal Addressing Mode | OLED_EXP_MEM_HORIZONTAL_ADDR_MODE |
| Vertical Addressing Mode | OLED_EXP_MEM_VERTICAL_ADDR_MODE |
| Page Addressing Mode | OLED_EXP_MEM_PAGE_ADDR_MODE |
Examples
Set to page addressing mode:
Set Column Addressing - int oledSetColumnAddressing (int, int)
This function is used to define where each page starts and ends horizontally:
Changing the start and end pixels changes the width of each page. By default, the column addressing is set from 0 to pixel 127. This means that each page has 128 usable columns, once the cursor reaches the last column, the display knows this page is filled.
For example, when writing text, the oledWrite function will set the columns to run from 0 to 125 in order for the characters to display properly. Since each character is 6 columns, 21 full characters can be displayed on a single line, the end pixel is set to 125 so that the 22nd character gets placed on the next line instead.
Note: the column cursor is not preserved after a column addressing change.
Arguments
The startPixel argument sets the starting column for each page.
The endPixel argument sets the end column for each page.
Both arguments must be between 0 and 127 and startPixel must be less than endPixel.
Examples
Set the column addressing to display text:
Set the column addressing to the full display width
Set each column to start halfway across and to end three quarters of the way across the display:
Set Columns for Text - int oledSetTextColumns ()
A function exists to define the column addressing specifically for text:
It sets the start pixel to 0 and the end pixel to 125. This allows for 21 text characters per line. This function should be run before setting the cursor for writing text.
Set Columns for Images - int oledSetImageColumns ()
Also, a function exists to define the column addressing to cover the entire screen:
It sets the start pixel to 0 and the end pixel to 127. This enables the use of the entire screen real estate.
Set Cursor Position
Any data written to the screen gets written to the current position of the cursor. This position can be adjusted.
Two methods exist: * Specifying the page row and character column * Specifying the page row and pixel column
Set Cursor Position by Character Column - int oledSetCursor (int, int)
This function is used to position the cursor on a specific page and character column. After this call, the next bytes written to the screen will be displayed at the new position of the cursor:
Note: since the column cursor is not preserved after a column addressing change, the column addressing needs be set to 0-125 using the oledSetTextColumns() function to properly display text before the cursor position is programmed.
Arguments
The row argument sets the page for the cursor, so the range is 0 to 7
The column argument sets the character column position of the cursor, the range is 0 to 20
Examples
Set the cursor to the start of the last page:
Set the cursor to the middle of the 4th row:
Set the cursor to the starting position at the top-left
Write ‘hello’ at the top left of the screen, and then write ‘hi there’ on the middle of the 7th row:
Set Cursor Position by Pixel - int oledSetCursorByPixel (int, int)
This function is used to position the cursor on a specific page and pixel row. This gives more fine grain control than setting by character column.
After this call, the next bytes written to the screen will be displayed at the new position of the cursor:
It allows for cursor positions that are not aligned with the 21 possible character columns.
Arguments
The row argument sets the page for the cursor, so the range is 0 to 7
The pixel argument sets the horizontal pixel position of the cursor, the range is 0 to 127
Examples
Set the cursor to the start of the last page:
Set the cursor to the middle of the 4th row:
Set the cursor to the bottom-left
Write ‘hello’ at the top left of the screen, and then write ‘hi there’ on the middle of the 7th row:
Clear the Screen - int oledClear ()
To clear the screen and move the cursor to the starting position at the top-left of the screen:
Writing Text to the Display
Listed below are the functions that write bytes, characters, strings, or images to the display.
Write a Single Byte - int oledWriteByte (int)
Write a single byte, eight vertical pixels, to the current position of the cursor:
Arguments
The byte argument holds the eight bits that will be written to the screen. The Least Significant Bit (LSB) in the byte corresponds to the top-most pixel in the column, the Most Significant Bit (MSB) corresponds to the bottom-most pixel in the column.
After the byte is set, the cursor will automatically move to the next page column.
Examples
Draw the following pattern:
Write a Single Character - int oledWriteChar (char)
Write a single character to the current position of the cursor:
Arguments
The c argument is the character that is to be written to the screen.
Make sure to check the asciiTable dynamic array found in oled-exp.h, it defines the bitmap pattern of each allowed character. Any characters not found in the table will be ignored.
Examples
Write an ‘O’
Write an open bracket, an x, then a close bracket:
Write a String - int oledWrite (char*)
Write an entire string of characters, starting at the current position of the cursor:
Underneath, this uses the oledWriteChar function to display characters and also implements newline functionality.
The newline functionality is implemented by using a global variable to keep track where the cursor is in each row (I know… shameful…). Once the characters ‘' and ’n’ are detected, the oledWrite function will print spaces to the display until it reaches the end of the current page and the cursor increments its position to the next page.
Note: the column addressing will be set to 0-125 to properly display text during this call, at the end of the function it will be reset back to 0-127.
Arguments
The msg argument is the string to be written to the display. Any characters not found in the asciiTable array will be ignored.
Examples
Write ‘Onion Omega’:
Write ‘Onion Omega’, then ‘Inventing the Future’ on the next line, and then ‘Today’ two lines below:
Drawing Images on the Display
The OLED Screen can also be used to display images. The Console can be used to convert existing images into a format that is compatible with the OLED Expansion and save the output to an Omega. Functions in the C library can read the image data and display it on the OLED Expansion. Alternatively, a buffer can be created programatically and displayed on the OLED.
The functions that interact with images are:
Creating Image Files
The Console OLED App can be used to create OLED Expansion compatible image files. Navigate to the Image tab of the OLED App:
Once an image has been selected, a button and form will appear that allow you to save the OLED image file to your Omega:
After the image name and location are selected, click the Save to Omega button.
OLED Image File Details
The OLED image files store the image data as 1024 bytes represented in hexadecimal. Each byte represents eight vertical pixels, with the first 128 bytes representing the columns in Page 0, the following 128 bytes representing the columns in Page 1, and so on.
If this is unclear, see the Understanding the Display Section for details on how the display is addressed.
Read Image Data - int oledReadLcdFile (char*, uint8_t*)
First, the data from the file needs to be loaded into a buffer:
Arguments
The file argument is the path to the OLED image file.
The buffer argument is a pointer to the memory that will hold the image data. It needs to be able to hold 1024 bytes (1 byte for each column in a page).
Write Image Data to the Display - int oledDraw (uint8_t *buffer, int bytes)
Then, write the data from the buffer to the display:
Arguments
The buffer argument is the pointer to memory that holds the image data.
The bytes is the number of bytes to be written. This will usually be 1024 bytes, this can be represented with macros: OLED_EXP_WIDTH*OLED_EXP_HEIGHT/8.
Example
Read an image file located at /root/image.lcd and draw it on the OLED display:
int status;
uint8_t *buffer = malloc(OLED_EXP_WIDTH*OLED_EXP_HEIGHT/8 * sizeof *buffer); // allocate memory for the buffer
// read data from file
status = oledReadLcdFile(param, buffer);
// draw on display
if (status == EXIT_SUCCESS) {
status = oledDraw(buffer, OLED_EXP_WIDTH*OLED_EXP_HEIGHT/8);
}It is also possible to programmatically fill a buffer and then draw it on the OLED display:
int i, status;
uint8_t data;
uint8_t *buffer = malloc(OLED_EXP_WIDTH*OLED_EXP_HEIGHT/8 * sizeof *buffer); // allocate memory for the buffer
// programatically fill the buffer
data = 0;
for (i = 0; i < OLED_EXP_WIDTH*OLED_EXP_HEIGHT/8; i++) {
buffer[i] = data;
// increment the data for the next column
if (data == 0xff) {
data = 0; // reset to 0
}
else {
data++; // increment by 1
}
}
// draw on display
status = oledDraw(buffer, OLED_EXP_WIDTH*OLED_EXP_HEIGHT/8);Scrolling the Display Contents
The OLED can scroll the contents of the screen horizontally or upwards at a 45 degree angle. The displayed content will wrap around when it reaches the edge of the display.
Horizontal Scrolling - int oledScroll (int, int, int, int)
Scroll all or part of the screen horizontally:
Arguments
The direction argument indicates whether to scroll to the left or right:
direction Value |
Scrolling Direction |
|---|---|
| 0 | Left |
| 1 | Right |
The scrollSpeed argument determines the number of “frames” between each scroll step, it is recommended to use the eOledExpScrollSpeed enumerated type:
* OLED_EXP_SCROLL_SPEED_5_FRAMES
* OLED_EXP_SCROLL_SPEED_64_FRAMES
* OLED_EXP_SCROLL_SPEED_128_FRAMES
* OLED_EXP_SCROLL_SPEED_256_FRAMES
* OLED_EXP_SCROLL_SPEED_3_FRAMES
* OLED_EXP_SCROLL_SPEED_4_FRAMES
* OLED_EXP_SCROLL_SPEED_25_FRAMES
* OLED_EXP_SCROLL_SPEED_2_FRAMES
The startPage argument defines at which page to start scrolling, and stopPage defines at which page to start the scroll. The range for both is 0 to 7, and startPage must be less than stopPage.
Examples
Scroll the entire screen to the left:
Quickly scroll the bottom half of the screen to the right:
Slowly scroll pages 1 to 5:
Diagonal Scrolling - int oledScrollDiagonal (int, int, int, int, int, int, int)
Scroll all or part of the screen diagonally upwards:
Arguments
The direction argument indicates whether to scroll upwards to the left or right:
direction Value |
Scrolling Direction |
|---|---|
| 0 | Upwards and Left |
| 1 | Upwards and Right |
The scrollSpeed argument determines the number of “frames” between each scroll step, it is recommended to use the eOledExpScrollSpeed enumerated type:
* OLED_EXP_SCROLL_SPEED_5_FRAMES
* OLED_EXP_SCROLL_SPEED_64_FRAMES
* OLED_EXP_SCROLL_SPEED_128_FRAMES
* OLED_EXP_SCROLL_SPEED_256_FRAMES
* OLED_EXP_SCROLL_SPEED_3_FRAMES
* OLED_EXP_SCROLL_SPEED_4_FRAMES
* OLED_EXP_SCROLL_SPEED_25_FRAMES
* OLED_EXP_SCROLL_SPEED_2_FRAMES
The fixedRows argument defines the amount of pixel rows, starting from the top, will not have vertical scrolling. The range is 0 to 63.
The scrollRows argument defines the amount of pixel rows, starting from the top, that will have vertical scrolling.
The verticalOffset argument defines the number of vertical rows to scroll by each frame.
The startPage argument defines at which page to start scrolling, and stopPage defines at which page to start the scroll. The range for both is 0 to 7, and startPage must be less than stopPage.
Examples
Scroll the entire screen upwards to the left:
int status;
status = oledScrollDiagonal ( 0,
OLED_EXP_SCROLL_SPEED_4_FRAMES,
0,
OLED_EXP_HEIGHT-1,
1,
0,
OLED_EXP_CHAR_ROWS-1
);Slowly Scroll the entire screen upwards to the right:
int status;
status = oledScrollDiagonal ( 1,
OLED_EXP_SCROLL_SPEED_64_FRAMES,
0,
OLED_EXP_HEIGHT-1,
1,
0,
OLED_EXP_CHAR_ROWS-1
);
Scroll the entire screen to the left, but only the bottom half upwards:
int status;
status = oledScrollDiagonal ( 0,
OLED_EXP_SCROLL_SPEED_3_FRAMES,
OLED_EXP_HEIGHT/2-1,
OLED_EXP_HEIGHT-1,
1,
0,
OLED_EXP_CHAR_ROWS-1
);Scroll pages 3 to 7 to the right, but only pages 4 to 7 upwards:
Stop Scrolling - int oledScrollStop ()
Disables all active scrolling:
Examples
Scroll the entire screen to the left, then stop it: