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 byte 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 Specs:

• 800 x 600 @ 60 Hz output signal with a 40 MHz pixel clock
• 8-colour RGB output (Red, Green, Blue, Cyan, Magenta, Yellow, Black and White)
• Double-width horizontal pixels, giving 400 x 600 output
• 8x16 font glyphs
• MS-DOS CodePage 850 character set
• 48 column x 36 row text output
• One foreground/background pair stored per character cell
• Single-buffered
  • Uses unsafe mutation of a static, 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 was unable to get 40 MHz SPI output functioning correctly on my particular micro. 20 MHz works just fine.

Tested on Monotron using a Texas Instruments Tiva-C TM4C123 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.
• 8x8 pixel font, more suited to the graphics modes

Contributing

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