Video Chip

Update as of 2013-01-29: Initial release.

I created a drop-in replacement for a TMS9918 or TMS9929 video chip (and the VRAM it is connected to) and used it to confirm the rest of my Memotech MTX computer was working fine. Turns out it was the 4116 VRAMs which were at fault (quelle surprise).

This is really a cheap and nasty approximation to the F18A project. Part of the reason I did it was because I happened to have a spare GODIL and I had implemented the VHDL already for my REMEMOTECH project.

I use a GODIL that has :-

• an XC3S500E Spartan 3E FPGA, with
  • 500K "gates", equivalent to 10476 "logic cells"
  • 360Kb block RAM, ie: at least 40KB of on-chip RAM
• SPI flash configuration for FPGA
• bus switches bridging between the 3.3V on the board, and the 5V on the DIL pins
• 49.152MHz clock - 49.152/2MHz is close enough to use as the VGA dot-clock
• 40-pin DIL

I connect this to a VGA breakout board.

Here you can see them plugged into a Memotech MTX motherboard :-

I can generate a 640x480 @ 60Hz VGA signal, with 2 bits R, 2 bits green and 2 bits blue. This is a reasonable approximation to the TMS9918, which actually has 16 colours using 3 bits of each.

The VGA related output of the GODIL actually needs to go through 74HCT04 hex inverters so as to clean up the voltages and hide the charge and discarge effects from the FET based bus-switches on the GODIL. If I don't do this, I'll get effects like this (example from an 80 column video card implemented in a GODIL) :-

This project still needs work to make a daughter board to connect to the GODIL to contain the inverters and the VGA connector. This is problematic, as it would make the assembly too large to physically fit where the TMS9929 is in my MTX computers. Maybe for some other folks it would be fine.

To get the Xilinx ISE tooling working on Linux, I need to follow guidance I've previously documented on my REMEMOrizer Design page.

For now at least, this project is on hold. But I'll share it as it stands, in case anyone is interested. As far as it goes, it does work fine.

Full package downloadable from here.