Installing the world’s worst video card
Ben Eater・29 minutes read
A video card outputs low-resolution images with limited colors, while a 6502 computer aims to connect for enhanced graphics, requiring changes in ROM to RAM for control. Glue logic and specific pins facilitate the connection between the video card and computer, creating intriguing screen displays with the potential for further development.
Insights
- The video card outputs a VGA signal with a resolution of 100x75 pixels and 64 colors, displaying a single hard-coded image, with the possibility of expanding graphics capabilities by connecting it to a 6502 microprocessor computer.
- Implementing dual-ported RAM, buffers like the 74HCT245, and transceivers are crucial for enabling communication between the video card and CPU, allowing for seamless control and access to RAM, ROM, and I/O, ultimately enhancing the computer's graphics processing capabilities.
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Recent questions
How does the video card generate images?
By using binary counters for pixel positioning.
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Summary
00:00
"Building VGA Video Card for 6502 Computer"
- The video card outputs a VGA signal with a resolution of 100x75 pixels and 64 colors, displaying a single hard-coded image.
- A 6502 microprocessor computer is built, currently displaying text, with the idea to connect it to the video card for more graphics.
- Kits for the video card and computer are available on eater.net/vga for project replication.
- The video card generates horizontal and vertical timing signals, controlled by binary counters for pixel positioning.
- The ROM in the video card determines the displayed image based on a 14-bit binary address.
- To enable the computer to change the displayed image, the ROM needs to be replaced with RAM.
- Dual-ported RAM allows simultaneous access by the video card and CPU, but modern RAM types like DDR SDRAM are more common.
- The CPU can be halted using the ready pin to allow the video card to access RAM during blanking intervals.
- Connecting the ready pin and bus enable pin halts the CPU and prevents it from using the bus, allowing the video card to control it.
- Buffers like the 74HCT245 are used to control bus access for the video card, as it lacks built-in control mechanisms.
12:07
"Video Card Interface for Computer Bus"
- A transceiver enables the direction of gates in one or the other way, with an enable line that can disable it in either direction, isolating one side from the other.
- The 6502 uses a bus enable line to disable its internal bus transceiver, similar to what is needed for the video card to enable or disable it when connected to the computer bus.
- Two 74 HCT 245 chips are added for the 16-bit address bus, with each chip buffering eight signals, connecting to the computer's address bus.
- The video card always sets the address of the RAM, necessitating a hardcoded direction for writing the address out to the bus.
- The 74 HCT series is used to interface between the video card and computer, allowing for bus mastering where one device controls the bus at a time.
- The video card can access RAM, ROM, or I/O, requiring mapping of the 14 address lines to the computer's 16 address lines.
- The frame buffer in RAM is set to start at address 2000 to 3FFF, with the top three bits hardcoded to 001 when the video card is active.
- The video card's xy counters are connected to the address bus, passing an address in RAM between 2000 to 3FFF for displaying pixel values.
- A 74 HCT 245 buffer is used for the data bus to ensure the video card only reads from it, preventing random data output during the blanking interval.
- The data bits from the buffer are connected to resistors to generate the correct voltages for red, green, and blue, allowing the video card to display pixel values on the VGA monitor.
24:03
"CPU bus control and VGA monitor logic"
- To enable the CPU to control the bus during the blanking interval, specific pins like the ready pin and bus enable pin need to be utilized along with buffer enable pins, requiring the addition of glue logic to facilitate the toggling at precise moments. In the upcoming video, the control logic for the VGA monitor to showcase the computer's RAM content will be established, alongside the creation of programs for intriguing screen displays. For those interested in this project, eater.net offers kits and further details, while subscribing to the channel ensures updates on the project's progress.
