The Gist

I had some ATF22V10C’s lying around and tossed together a quick VGA card.

NOTE: I happen to have a few 25.175MHz “Standard Clock” TCXO’s and a pile of PLD’s lying around. Admittedly, this is a somewhat luxurious state of affairs. On the one hand, it makes the implementation significantly more straightforward. On the other, those aren’t likely to be super common “just lying around” parts for home hackers…

• Pixel Clock: 25.175MHz
• Resolution: 640x480
• Refresh: 59.9Hz
• 3 x ATF22V10C’s
• 1 x SN74F21 (Dual, 4-input, positive, AND — mind your Logic Family!)
• Inputs/Outputs:
  • HSync + VSync,
  • “Visible” bool
  • VBlank flag
  • 20-bit pixel address output (10-bit H, 10-bit V)
  • analog RGB input

The POC is some pretty shoddy wire-pasta, but it works brilliantly…and is fun:

Development

• Written for GALasm (I use Galette)
• Programmed with a T48 using Minipro
• For help: make help
• To build and upload a unit: UNIT=<my-file.pld> make clean compile upload

PLD Sources

Schematics available in the repo, vga-22v10.

• VGASync.pld (ATF22V10C): HSync, VSync, Visible, and VBlank
• mod800.pld (ATF22V10C): % 800 counter
• mod525.pld (ATF22V10C): % 525 counter
• Four-input AND on (V32, V64, V128, V256)

Notes

• I’m sure it’s possible to reduce chip-count or make this more elegant — e.g. moving some of the sync counter logic from one IC into VGASync.pld / using some outputs in tri-state mode. This isn’t the optimal solution; it’s just an easy, off-the-cuff, solution with proper timing.
• Pinouts: these are malleable! The current disposition is informed, more than anything else, by where the IC’s happened to land on my breadboard. Just mind your product terms! (The number of product terms available to each macrocell varies by pin).
• If you don’t have “spare” 22V10’s, you can replace the mod800/mod525 counters with synchronous counters or ripple-counters + latches. If you spec out your parts, mind your gate delays, and maybe add some additional product terms to your equations, you might be able to get away with a couple of 10+ bit ripple counters too!

Misc / Fun

Analog Switch Pixel Gate

Use the Visible output of VGASync to toggle 3 x SPST or SPDT switches for red, green, and blue.

NOTE: Watch your propagation delays!

If you use an analog switch that has a large ON/OFF (or Inhibit) propagation delay, relative to the pixel clock, you’ll want to turn it on and off early to avoid offsetting your pixels horizontally.

Boolean Grids

Have some fast CMOS or TTL logic gates lying around? Taking the output from one or more bits from VAddr and HAddr, passing them through some gates, and AND‘ing them with the Visible flag is a great way to check your pixel alignment (this is how the grid pattern in the screenshot below was generated).

Screenshots!