Archive - April 22, 2017

Now that I have the 4MB SRAM board installed, I now have a frame buffer that can keep up with the video bandwidth rate.

The resolution and color depth is currently 800 x 480 x 12-bit. The computing shield I’m using only supports 12-bits, but my LCD can do 24-bit.

Essentially how my frame buffer works is that you write the 12-bit color value into an address in memory that corresponds to a location on the screen.

See Framebuffer at Wikipedia.

The 4MB SRAM gets mapped to $820 000. The color values are stored as 16-bit words like this 0000_RRRR_GGGG_BBBB. Top 4 bits are not used, for now.

One 68K instruction, for example,  “move.w #$0F00,$820000” will put a single red pixel, at location 0,0.

I also have text generation working:

This works a similar way with a character buffer. Another part of ram is being used to store a 100 x 30 character buffer. If you write a 7-bit ASCII value to the lower portion of a 16-bit word at an address that corresponds to the location, then that character will be displayed.

The text layer is independent of the frame buffer, and is overlaid on top of that graphics layer.

This adds pretty powerful capabilities to the badge computer, and now I have to start writing some utility routines to do higher level functions like drawing a line or a circle.