自製數位相框 DIY photo frame with BL-TFT240320PLUS

Build photo frame with AVR and BL-TFT240320PLUS

Build it self photo frame. Display on QVGA 240x320 pixel using AVR small 8 bit microcontroller.

Objective

• DIY photo frame
• Using AVR microcontroller.
• Display on QVGA TFT 3.2” 240x320 pixel 16 bit color
• Using BMP file format with out convert.
• File store on SD card.
• Keep small program size.
• Fast display as fast as possible.

 Hardware

• Atmega8,16,32 8 bit microcontroller from www.atmel.com. 
• SD card
• QVGA BL-TFT240320PLUS from circuitidea.com. It come with include SD card slot. 8/16 bit interface.

    

Understanding BMP file format

BMP file contain 4 group of data structure.

1. BMP File Header Stores general information about the BMP file.
2. Bitmap Information Stores detailed information about the bitmap image.
3. Color Palette Stores the colors use for indexed color bitmaps. This is for 1,4,8 bits per pixel.
4. Bitmap Data Stores the actual image, pixel by pixel. 

In this project we are interesting in part 1, 2 and 4. The color palette take more ram to keep color index table.

The yellow mark is interesting point for project.

BMP File Header

00h	2 bytes	the “BM” characters identify the BMP file.
02h	4 bytes	size of the BMP file in bytes
06h	2 bytes	reserved; actual value depends on the application that creates the image
08h	2 bytes	reserved; actual value depends on the application that creates the image
0Ah	4 bytes	the bmp data offset starting address

 Bitmap Information

0Eh	4 bytes	size of this header ( 40 bytes ).
12h	4 bytes	bitmap width in pixels (signed integer).
16h	4 bytes	bitmap height in pixels (signed integer).
1Ah	2 bytes	number of color planes being used. This must be set to 1.
1Ch	2 bytes	number of bits per pixel. values are 1, 4, 8, 16, 24 and 32.
1Eh	4 bytes	compression method being used.
22h	4 bytes	image size. This is the size of the raw bitmap data.
26h	4 bytes	horizontal resolution of the image. (Pixel per meter, signed integer)
2Ah	4 bytes	vertical resolution of the image. (Pixel per meter, signed integer)
2Eh	4 bytes	number of colors in the color palette, or 0 to default to 2n.
32h	4 bytes	number of important colors used.

To understand raw data format. . You can found example, The Example of a 2x2 Pixel Bitmap with 24 bits/pixel encoding ) athttp://en.wikipedia.org/wiki/Windows_bitmap#Example_of_a_2x2_Pixel.2C_24-Bit_Bitmap.

Schematic

AVR Atmega16,32

AVR atmega8 (ARDUINO hardware)

Software

            Concept is very easy. Read BMP files from SD card and display to LCD.

LCD driver

            BL-TFT240320PLUS TFT LCD board (4) using SSD1289. The code from www.circuitidea.com

SD card driver

            AVR microcontroller has a small ram. Our scope is reading and display BMP file with out convert. It’s take more ram and give more speed to display. I found many SD card libraries for AVR on internet. Petit FatFs(3) is good for this solution. It’s take a small code. 

Petit Fatfs Features:

• Read only file system.
• Very small RAM consumption (46 bytes work area + certain stack).
• Very small code size (2.5K-3.5K bytes).
• FAT12, FAT16 and FAT32.
• Single volume and Single file. 

Display BMP

1. Start scan SD card to find bmp file.

2. Open file and check condition to fit in solution. 

Check image to fit screen	Address 0x12h contains the width of the bitmap in pixels. Address 0x16h contains the height of the bitmap in pixels.
Display 24 bit per pixel	Address 0x1Ch contains number of bit per pixel.
No image processing function.	Address 0x1Eh contains the compression. MUST be 0.

       3. Scan data and put to LCD 

Set area to display. Adjust center image if need.	Image size must be equal or less than screen area.
Start of picture data	Address 0x0Ah keeps the point to bitmap data.
Scan and display from bottom to top	Image data store from the bottom line of the image
Scan each line of image.	Image data store in each lines. Address 0x12h contains the width of the bitmap in pixels.
Convert color to RGB	Color is present in BGR
Skip padding data form some image	There are padding data to keep the line of data in multiples of four image width.

Compiler

            SD buffer set to 512 byte. Using AVR atmega32 program size is 8166 (24.9%). Data size 23 byte ( internal buffer use 512 byte ).

Atmega16 program size is 8166 (49.8%).  Atmega8 (using 8 bit mode) program size is 7220 (88.1%).

Source code

Release 01

            Atmega16,32 schematic and hex include in Photoframe.zip

            Atmega8 ( using only arduino hardware ) schematic and hex include in Photoframe Arduino

Release 02

           Atmega8 ( using only arduino hardware ) schematic and hex include in Photoframe Arduino R02

Arduino

          Arduino atmega168 ( Source code in Arduino library, LCD and demo. Must use with SD card library) ( board Version 1)  www.circuitidea.com/dev-board/BL-TFT240320PLUS.html

          Code and library for Arduino ( board Version 1 and 2 ) www.circuitidea.com/dev-board/BL-TFT240320PLUS-V2.html

Sample picture

 

  

Reference

(1) hackaday : How-to: Digital picture frame, 100% DIY

(2) BMP file format : en.wikipedia.org/wiki/Windows_bitmap

(3) Petit FAT File System Module : elm-chan.org/fsw/ff/00index_p.html

(4) LCD TFT BL-TFT240320PLUS: www.circuitidea.com/dev-board/BL-TFT240320PLUS.html