This article explains the method of using an FPGA to control a VGA display and show images.
1: Black and White Picture RGB Extraction
If the image is black and white, you can use Image2LCD software to convert it into a BMP format. Then, use BMPtoMIF to convert the BMP file into a MIF file that can be imported into ROM. However, this approach only provides 3-bit RGB, which results in limited color options.
RGB values are as follows:
- 000 = Black
- 100 = Red
- 001 = Blue
- 101 = Purple
- 010 = Green
- 110 = Yellow
- 011 = Cyan
- 111 = White
2: Convert to 8-bit Display (r[2:0]g[2:0]b[1:0])
The following code maps the 3-bit ROM data to an 8-bit VGA output:
if(rom_data == 3'd7) vga_rgb <= 8'b111_111_11;
else if(rom_data == 3'd6) vga_rgb <= 8'b111_111_00;
else if(rom_data == 3'd5) vga_rgb <= 8'b111_000_11;
else if(rom_data == 3'd4) vga_rgb <= 8'b111_000_00;
else if(rom_data == 3'd3) vga_rgb <= 8'b000_111_11;
else if(rom_data == 3'd2) vga_rgb <= 8'b000_111_00;
else if(rom_data == 3'd1) vga_rgb <= 8'b000_000_11;
else if(rom_data == 3'd0) vga_rgb <= 8'b000_000_00;
This conversion allows the display to use 8 bits, where r[2:0], g[2:0], and b[1:0] represent the red, green, and blue components respectively.
3: Color Image Acquisition
To extract color information from a picture, use WinHex software. The first 54 bytes contain metadata, while the next 1024 bytes form the color palette. The remaining data represents pixel positions within the color table.
During processing, discard the first 54 bytes, copy the rest into a C array, and save it as a text file. Open the file in Excel and use the formula `=OFFSET($A$1,(ROW(A1)-1)/16, MOD(ROW(A1)-1,16))` to arrange the data into a column. Finally, convert it into a MIF file and import it into the ROM.
4: Overall Project Setup
The project involves storing both the color table and image data in ROM. The color table is then converted into 256 different colors and stored in RAM. When displaying the image, the VGA signal uses the RGB values from the color table.
For example:
- 2'd0: rr_din[7:6] <= rom_data[7:6]; // 2 bits for blue
- 2'd1: rr_din[5:3] <= rom_data[7:5]; // 3 bits for green
- 2'd2: rr_din[2:0] <= rom_data[7:5]; // 3 bits for red
5: Important Notes
When reading data from RAM or ROM, there is a delay of two clock cycles after addressing. This must be taken into account when designing the timing for data retrieval and display.
By carefully handling the data conversion and timing, you can successfully display images on a VGA monitor using an FPGA.
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