This article explains how to control a VGA display using an FPGA. It covers the process of converting images into data that can be displayed on a VGA monitor through an FPGA-based system.
**1: Black and White Picture RGB Extraction**
For black and white images, you can use image2LCD to convert the image into a BMP format. Then, use BMPtoMIF software to convert it into a MIF file that can be imported into a ROM. However, this method only provides 3-bit RGB representation, which is limited in color depth. The basic colors are defined as follows:
- RGB = 000 → Black
- RGB = 100 → Red
- RGB = 001 → Blue
- RGB = 101 → Purple
- RGB = 010 → Green
- RGB = 110 → Yellow
- RGB = 011 → Cyan
- RGB = 111 → White
**2: Conversion to 8-Bit Display (r[2:0]g[2:0]b[1:0])**
To display these colors on a VGA monitor, we need to map the 3-bit RGB values into an 8-bit format. For example:
- If rom_data == 3'd7 → vga_rgb <= 8'b111_111_11
- If rom_data == 3'd6 → vga_rgb <= 8'b111_111_00
- If rom_data == 3'd5 → vga_rgb <= 8'b111_000_11
- If rom_data == 3'd4 → vga_rgb <= 8'b111_000_00
- If rom_data == 3'd3 → vga_rgb <= 8'b000_111_11
- If rom_data == 3'd2 → vga_rgb <= 8'b000_111_00
- If rom_data == 3'd1 → vga_rgb <= 8'b000_000_11
- If rom_data == 3'd0 → vga_rgb <= 8'b000_000_00
This allows for an 8-bit output where r[2:0], g[2:0], and b[1:0] represent the red, green, and blue components respectively.
**3: Color Image Acquisition**
To work with color images, you can use WinHex to extract image information. The first 54 bytes contain header information, followed by a 1024-byte color table, and then the actual pixel data that references positions in the color table. When processing the image, the first 54 bytes are ignored. The remaining data is copied into a C array, saved as a .txt file, and then opened in Excel. Using the formula `=OFFSET($A$1,(ROW(A1)-1)/16, MOD(ROW(A1)-1,16))`, the data is arranged into columns. Finally, it is converted into a MIF file according to the required format and imported into the ROM.
**4: Overall System Design**
The overall design involves storing both the color table and image data in the ROM. The color table is converted into 256 different color entries and stored in RAM. When displaying the image, the system reads from the RAM to generate the appropriate RGB signals for the VGA output. The following code shows how the data is mapped:
- 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 considered during timing calculations to ensure proper synchronization with the VGA signal.
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