Attributes and Display RAM

A project log for FPGA Serial Terminal

Instant-on VT100-style serial terminal implemented in minimal FPGA board, with VGA display and PS/2 keyboard.

Howard JonesHoward Jones 09/13/2015 at 21:010 Comments

Data is fetched from display RAM now, and the attribute byte is decoded correctly. I used an ANSI-art editor in DOSBox to produce a test screen and saved it as a .bin file, which is a VGA memory dump. The FPGA tools don't really distinguish internal between ram and rom, so it's perfectly OK to have RAM but preloaded with data at startup. The display RAM is full-dual port, dual-clock, so the Z80 can live in its slow 4MHz 8-bit world while the VGA side of things grabs 16-bit words at a time. For updates from a serial port, I'm not too concerned about tearing etc. The screen is showing fg and bg colour changes, and a full-ish ASCII character set. Attribute decoding worked first time, pretty much.

There's no bounds-checking on the display though, so outside of the 80x25 main area, there is random overflow/wrapped data. I should be suppressing before the first real character, on the left, and after column 80 (81). A real VGA textmode runs in 720x400, not 800x600, so I have a bunch of extra space on the screen. Unfortunately, without switching to using external RAM, I can't really use it to display anything, so it'll most likely be blank, or I'll figure out how the Dynamic Clock Manager works to generate the right 35.5MHz dot clock for 720x400@85.

You can also see where I haven't yet implemented the column-9 extension stuff for box graphics.

Still haven't figured out the whistling noise. Tying the relevant I/O pin to a constant-0 output has not helped.

Next steps:

  1. column-9 extension
  2. test flashing
  3. bounds-checking and/or resolution change
  4. CPU!