Previously I was using the LCD as a display on my 6502 computer. The LCD worked fine to display text but it was not a good choice for displaying images. The only had 8 custom characters which could be used for custom images. This meant a maximum resolution of 20x16 with only two colors per pixel. The LCD was also very slow. The LCD was not designed for fast updating meaning making any complex game was very difficult. I was able to get it up to a few FPS. A much better display is a TV. I decided to communicate with the TV using the VGA standard. I also decided to have a resolution of 640x480 at 60 FPS. I would also be able to display up to 256 colors instead of two. This is a massive improvement from the little LCD display. I started off by making a timing circuit. This coordinates the TV and the graphics card. Then once I got a signal working I was able to add in color using an EEPROM. The EEPROM stores the color data of the pixels and outputs a certain color based on the pixel x y coordinates. You may notice, when looking at the image, the resolution does not look like 640x480. You would be right. Each pixel is actually 8x8 pixels because in the future the 8x8 pixels will not be colors but they will be tiles. Right now I just am using them like a big pixel.
One feature currently working in the graphics card is scrolling. In the gif to the left the image of the pyramids is scrolling up and to the left. The X and Y components of the scrolling can be controlled individually but right now they are controlled by a counter which is constantly counting up.
Problems with color
On the left is the image the graphics card is supposed to create and on the right is the image it is outputting. Anyone can see that the colors are not as vibrant. I did not notice this issue for a while because I thought it came from the monitor being dim. I had two problems relating to the color that I did not know how to fix. Eventually I figured out the color was wrong because I made my r2r ladders incorrectly. After remaking them the color output was correct. The dimness was still an issue. I talked to a professor at Cornell and he told me how the monitors had built in 75 ohm resistors. I had not factored this in when making my resistor dividers to drop the voltage. After correcting this mistake I was able to fix both issues!
Bellow is a picture and a gif of the colors before (left) and the colors after (right). For some reason the gif makes the tv seem darker when in fact it is very bright. On the left you can see that everything looks tinted green including the sky. On the right the sky is the correct blue color without the green tint.
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