Now that we have a VGA synchronization circuit we can move on to designing a pixel generation circuit that specifies unique RGB data for certain pixels (i.e. an image). Before we actually go there, I thought I would separately talk a little bit about how to store image data on an FPGA. This discussion will focus mainly with using a Xilinx FPGA, more specifically the Basys 3, which uses 12-bit color.
Raw images are arrays of pixel data. Each pixel has a number of bits that specifies the intensity of the red, green, and blue color components. Assuming that an image is stored in 24-bit “True Color”, there are 8 bits specifying each respective color component. Since we are using the Basys 3 FPGA, we will need only the upper four bits of each color component (3 colors * 4 bits/color = 12 bits). So we need 12 bits per pixel to represent color, and y*x pixels, where y is the image height, and y is the image width.
The image of Yoshi shown above is scaled up such that we can easily see each pixel as a block of color on our screen. This image has a height of 32 pixels, and a width of25 pixels. That means that in total we have 800 pixels, each needing 12 bits to represent color, so 9600 bits altogether. There are a few ways to store all of these bits.
Learning how to directly drive a VGA monitor with an FPGA opens up a window for many potential projects: video games, image processing, a terminal window for a custom processor, and many more. To get started, we will need to learn how to drive the necessary signals to display things on a VGA monitor. We will then use a test circuit to display some colors on the screen. In future posts I will detail how to design pixel generating circuits for displaying custom graphics and animations. More on that soon!
Computer monitors used to be bulky cathode ray tube (CRT) devices. VGA technology was developed with driving the physical CRT in mind, so knowing how that device works can be instructive in understanding why VGA signals are driven the way they are. That being said, computer monitors nowadays are LCD monitors without a CRT, yet the VGA interfaces for these monitors still use the same signals to display images on their screens. Instead of going into the details of CRTs in this post, we will instead inspect the necessary signals for a VGA monitor and their timing diagrams, and implement a synchronization circuit in Verilog HDL.
Keep in mind that different FPGA development boards have different color depth capabilities. I will focus here on using the Basys 2 which has 8-bit color and the Basys 3 which has 12-bit color. While I am covering the Basys 2 here, in the future I will focus on and use the more capable Basys 3 for my VGA projects.