ILI9341 TFT Driver

Introduction

This library is a native implementation of a graphics chip for small TFT screens, the ILI9341. The chip is found in many popular displays, including 320x240 and 480x320 pixel versions, and is capable of driving 16-bit (RGB-565) as well as 18-bit (RGB-666) modes. Although the chip is capable of using 18-bit colors (262,114), that requires three bytes per pixel to be written. Therefore, this driver initializes it in 16-bit colors (65,536) allowing for two bytes per pixel.

Fundamentals

The chip allows access to its framebuffer memory by setting a window by means of (x0,y0)-(x1,y1) coordinates, and then writing pixel data. To implement higher level shapes and fonts, we implement three basic functions on the hardware:

void mgos_ili9341_drawPixel(uint16_t x0, uint16_t y0);

This function sets the window to be exactly 1 pixel and writes the current foreground color to it.

void mgos_ili9341_drawLine(uint16_t x0, uint16_t y0, uint16_t x1, uint16_t y1);

This function can draw a vertical line by setting the window to (x0,y0)-(x0+1,y1), and writing the current foreground color to it. It can draw a horizontal line similarly by setting the window to (x0,y0)-(x1,y0+1). Any other lines can be decomposed into segments of horizontal and vertical lines and single pixels.

void mgos_ili9341_fillRect(uint16_t x0, uint16_t y0, uint16_t w, uint16_t h);

This sets the window to (x0,y0)-(x0+w-1,y0+h-1) and writes the current foreground color to it.

Primitives

Orientations

void mgos_ili9341_set_orientation(uint8_t madctl, uint16_t rows, uint16_t cols);

Depending on how the hardware manufacturer connected the LCD panel to the ILI9341 chip, several registers are provided to determine the true orientation of the screen. From the datasheet, there are 5 bits which determine that orientation, in the MADCTL register, as follows:

• bit2 ILI9341_MADCTL_MH - sets the Horizontal Refresh, 0=Left-Right and 1=Right-Left
• bit4 ILI9341_MADCTL_ML - sets the Vertical Refresh, 0=Top-Bottom and 1=Bottom-Top
• bit5 ILI9341_MADCTL_MV - sets the Row/Column Swap, 0=Normal and 1=Swapped
• bit6 ILI9341_MADCTL_MX - sets the Column Order, 0=Left-Right and 1=Right-Left
• bit7 ILI9341_MADCTL_MY - sets the Row Order, 0=Top-Bottom and 1=Bottom-Top

This bits are defined as ILI9341_MADCTL_* in mgos_ili9341.h header file. By means of example, here's a definition for Adafruit panels:

#define ADAFRUIT_PORTRAIT        (ILI9341_MADCTL_MX)
#define ADAFRUIT_LANDSCAPE       (ILI9341_MADCTL_MX|ILI9341_MADCTL_MY|ILI9341_MADCTL_MV)
#define ADAFRUIT_PORTRAIT_FLIP   (ILI9341_MADCTL_MY)
#define ADAFRUIT_LANDSCAPE_FLIP  (ILI9341_MADCTL_MV)

And here's a definition for the panel used in M5Stack:

#define M5STACK_PORTRAIT         (ILI9341_MADCTL_MV|ILI9341_MADCTL_MY)
#define M5STACK_LANDSCAPE        (0x00)
#define M5STACK_PORTRAIT_FLIP    (ILI9341_MADCTL_MV|ILI9341_MADCTL_MX)
#define M5STACK_LANDSCAPE_FLIP   (ILI9341_MADCTL_MY|ILI9341_MADCTL_ML|ILI9341_MADCTL_MX)

The API call mgos_ili9341_set_orientation() gives full control over these settings, as well as setting the resulting width and height in pixels.

Window and Clipping

The driver works by setting a bounding box around the area to be drawn in:

void mgos_ili9341_set_window(uint16_t x0, uint16_t y0,
                             uint16_t x1, uint16_t y1);

Any operation will be homed within the window, and clipped if it runs outside of the window. For example, setting the window to (20,30)-(99,99) will create a bounding box of 80 pixels wide and 70 pixels high. Subsequently calling mgos_ili9341_drawPixel(0,0) will draw a pixel at the top left of the window, which in this case is physical pixel (20,30). Drawing a pixel at coordinates (80,70) will draw a pixel at the bottom right corner of the window, whereas drawing one at (81,71) will not show anything.

Geometric Shapes

A set of primitives are provided to allow drawing of geometrics shapes such as circles, triangles and boxes:

void mgos_ili9341_drawRect(uint16_t x0, uint16_t y0, uint16_t w, uint16_t h);
void mgos_ili9341_drawRoundRect(uint16_t x0, uint16_t y0, uint16_t w,
                                uint16_t h, uint16_t r);
void mgos_ili9341_fillRoundRect(uint16_t x0, uint16_t y0, uint16_t w,
                                uint16_t h, uint16_t r);

void mgos_ili9341_drawCircle(uint16_t x0, uint16_t y0, uint16_t r);
void mgos_ili9341_fillCircle(uint16_t x0, uint16_t y0, uint16_t r);

void mgos_ili9341_drawTriangle(uint16_t x0, uint16_t y0, uint16_t x1,
                               uint16_t y1, uint16_t x2, uint16_t y2);
void mgos_ili9341_fillTriangle(uint16_t x0, uint16_t y0, uint16_t x1,
                               uint16_t y1, uint16_t x2, uint16_t y2);

Fonts

Fonts can be embedded in the program by including the font files in fonts/*.h. The width and height of a given string can be calculated, and a string can be printed (using the set foreground and background colors). As with geometric shapes, the print function is relative to the window, (0,0) being top-left, and will be clipped at the window borders.

Images

Since the ILI9341 chip expects pixels in 16-bit network byte order, we can transmit a line of pixels from an image. Because the build target of the driver is all Mongoose available platforms (notably ESP8266, ESP32 and CC32xx), memory is an issue, and decompressing PNG, GIF, or JPG files can be CPU as well as memory intensive, a simplistic alternative is provided: DIF

This function can be used for displaying DIF images:

void mgos_ili9341_drawDIF(uint16_t x0, uint16_t y0, char *fn);

DIF file format

DIF stands for dumb image format, and consists of a 16 byte header followed by raw pixel data, 16 bits per pixel, in network byte order. This aleviates CPU and memory pressure on the micro controller, at the expense of file size. The header is as follows:

• The first four bytes (offset 0..3) are the string DIF\001
• The second four bytes (offset 4..7) are the width (an uint32_t in network byte order).
• The third four bytes (offset 8..11) are the height (an uint32_t in network byte order).
• The byte at offset 12 denotes the raw image format that follows:
  • Value of 0 means: 16 bit RGB-565 in network byte order.
• The last three bytes (offset 13..15) are reserved for future use.

The file size will therefor be w * h + 16 bytes.

Converting PNG to DIF

To convert PNG images to DIF images, there is a png2dif utility available in this repo, under contrib/png2dif

Clone this repo, and while inside the contrib/png2dif directory, run make to generate png2dif executable. You can then run png2dif -i input.png -o output.dif to convert a non-transparent png file, to a dif file.

The generated dif file should then be placed inside the fs directory, and can be displayed using mgos_ili9341_drawDIF. As an example, if the filename is output.dif, to display at x cord of 0 and y cord of 0:

mgos_ili9341_drawDIF( 0, 0, "/output.dif" );

Example usage of DIF images, and this library, can be found in the Huzzah Featherwing Example App

Example Application

mos.yml

The driver uses the Mongoose native SPI driver. It is configured by setting up the MOSI, MISO, SCLK pins and assinging one of the three available CS positions, in this example (which was taken from the Huzzah32 ESP32 microcontroller), we are going to use CS0:

config_schema:
  - ["spi.enable", true]
  - ["spi.cs0_gpio", 15]      # The ILI9341 CS pin
  - ["spi.cs1_gpio", -1]
  - ["spi.cs2_gpio", -1]
  - ["spi.mosi_gpio", 18]
  - ["spi.miso_gpio", 19]
  - ["spi.sclk_gpio", 5]
  - ["ili9341.cs_index", 0]  # Use spi.cs0_gpio
  - ["ili9341.dc_pin", 33]

Application

#include "mgos.h"
#include "mgos_ili9341.h"
#include "fonts/FreeMonoBold9pt7b.h"

enum mgos_app_init_result mgos_app_init(void) {
  mgos_ili9341_set_fgcolor(0xff, 0, 0);          // Red
  mgos_ili9341_set_window(20, 30, 119, 59);      // 100x30 pixels
  mgos_ili9341_drawRoundRect(0, 0, 100, 30, 8);  // Draw a rounded rectangle
  mgos_ili9341_set_fgcolor(0, 0xff, 0);          // Green
  mgos_ili9341_set_font(&FreeMonoBold9pt7b);     // Set font
  mgos_ili9341_print(5, 5, "Hello World");

  return MGOS_APP_INIT_SUCCESS;
}

Disclaimer

This project is not an official Google project. It is not supported by Google and Google specifically disclaims all warranties as to its quality, merchantability, or fitness for a particular purpose.