Crate vga_framebuffer

Expand description

VGA Frame Buffer for Embedded Microcontrollers

Generates an 800 x 600 @ 60 Hz SVGA signal from a 48 column x 36 row monochrome text buffer. The image has a border.

TODO: Implement smooth scrolling in the vertical direction with an extra text row.

Width = 400 double width pixels => 400 = 8 + (48 x 8) + 8

Height = 600 pixels => 600 = 12 + (36 x 16) + 12

<-------------- 400 px, pixel doubled to 800 px ------------->
+------------------------------------------------------------+
|<--> 8 pixel border     ^                8 pixel border <-->|
|                        | 12 px border                      |
|                        v                                   |
|    +--------------------------------------------------+    |
|    | <--^------ 48 chars x 8 px = 384  px ----------->|    |
|    |    |                                             |    |
|    |    |                                             |    |
|    |    | 36 rows x 16 px = 576 px                    |    |
|    |    |                                             |    |
|    |    |                                             |    |
|    |    v                                             |    |
|    +--------------------------------------------------+    |
|                          ^                                 |
|                          | 12 px border                    |
|                          v                                 |
+------------------------------------------------------------+

Requires pixels to be emitted with a 20 MHz pixel clock (against a nominal 40 MHz pixel clock, in order to acheive the horizontal doubling).

In order to maintain performance, only one font size is supported: 8x16 pixels. But you can substitute your own font if required (e.g. for Teletext support).

There is optional cursor support. Rather than try and check each text cell at render time to see if it is in the cursor position, we swap chars in and out of the text buffer as the cursor moves. It’s a little more expensive, but the cost is at text write time, not at render time (and so it won’t break sync).

See https://github.com/thejpster/monotron for an example.

Modules§

freebsd_cp850
freebsd_teletext

Structs§

Attr: This structure describes the attributes for a Char. They’re all packed into 8 bits to save RAM.
Col: Describes a vertical column on the screen. Zero is on the left.
FrameBuffer: This structure represents the framebuffer - a 2D array of monochome pixels.
Mode0TextRow
Mode2: Represents Mode2 1-bpp graphics
ModeInfo: Describes a video mode.
Point: A point on the screen. The arguments are X (column), Y (row)
Position: Describes a place on the screen. (0, 0) is the top left.
Row: Identifies a horizontal row on the screen. Zero is at the top.
XRGBColour: Represents 8 pixels, each of which can be any 3-bit RGB colour

Enums§

Char: This MS-DOS CodePage 850. It offers a compromise between the box characters of CodePage 437 and the accents of ISO 8859-1 / Latin-1.
Colour
ControlCharMode: How to handle Control Characters
DoubleHeightMode: You can set this on a row to make the text double-height. This was common on the BBC Micro in Mode 7/Teletext mode.
EscapeCharMode
SpecialChar: Special types of character we need to interpret

Constants§

MODE0_HORIZONTAL_OCTETS: How many words in a line (including the border)
MODE0_TEXT_MAX_COL: Highest X co-ord for text
MODE0_TEXT_MAX_ROW: Highest Y co-ord for text
MODE0_TEXT_NUM_COLS: How many characters in a row
MODE0_TEXT_NUM_ROWS: How many rows of characters on the screen
MODE0_USABLE_COLS: Number of columns in frame buffer
MODE0_USABLE_HORIZONTAL_OCTETS: How many words in a line (excluding the border)
MODE0_USABLE_LINES: Number of lines in frame buffer
MODE2_USABLE_LINES: Number of scan-lines in an image in Mode 2. Note, we print each one twice.
MODE2_WIDTH_PIXELS: Number of pixels in a scan-line in Mode 2

Traits§