VGA Framebuffer

This crate implements basic VGA text output on an embedded microcontroller, using nothing more than some timer peripherals and a few GPIO pins.

This crate is written to be hardware-agnostic.

• You supply an object implementing the Hardware trait.
• The function isr_sol() must be called at the start of every scan-line, e.g. with a Timer interrupt.
• This will then call write_pixels() on the given Hardware object with the RGB values for every octet (or block of 8 pixels) in that scan-line.
• These pixel values should be buffered (e.g. in an SPI FIFO) and then sent to the screen at the appropriate moment (e.g. when the line_start timer fires).
• The configure() callback gives you the appropriate timings for all of this.

The example code renders to a console using ANSI escape sequences to set the colour and printing a '█' character of the appropriate colour for each pixel. Obviously on a system with real VGA output you need to send the pixels to the display as analog values between 0V and 0.7V, along with appropriate Horizontal and Vertical sync signals. That is left as a platform-specific exercise for the reader, but the Monotron is one example for a specific embedded platform.

The Specs:

• 800 x 600 @ 60 Hz output signal with a 40 MHz pixel clock
• 400 x 600 effective resolution in text mode (runs pixel clock at half-speed)
• 8-colour RGB output (Red, Green, Blue, Cyan, Magenta, Yellow, Black and White)
• 8 pixel by 16 pixel bitmap font, taken from FreeBSD
• MS-DOS CodePage 850 character set
• 48 column x 36 row text output
• Double-height text support
• One foreground/background pair stored per character cell
• Single-buffered
  • Mutates shared memory through a pointer, but the worst case scenario is a minor screen glitch which is fixed on the next frame
• Optional Mono 384x288 framebuffer, coloured with text-mode attributes.
  • Doubles each horizontal line to keep memory usage down
  • Uses an extra 13,824 bytes over text mode
  • Will suffer terrible attribute-clash, just like a ZX Spectrum :)
  • Can set a start and end scan-line for the graphics mode, to allow a mixed text/graphics split-screen with reduced memory consumption

The pixels are double-width as I didn't have the CPU power to render colour pixels at 40 MHz. I do have experimental 40 MHz support in a branch, but only in black-and-white.

Tested on Monotron using a Texas Instruments Tiva-C TM4C123, which has a Cortex-M4F core clocked at 80 MHz.

Coming soon...

• Optional 3-bits-per-pixel RGB 192x288 framebuffer mode.
  • Uses an extra 20,736 bytes over text mode.
  • No attribute clash, but half the resolution.
• 8 pixel by 8 pixel font, more suited to the graphics modes
• Borderless 80 column by 25 row monochrome text mode (using a 10x24 bitmap font)

Contributing

I'll happy accept a patches to enable other resolutions and/or other text resolutions.