vga-experiments

GOAL: Create a dual port video RAM memory using a static RAM chip and a few 74HTC245 octal bus transceivers. (WTF is a dual port RAM? -> https://en.wikipedia.org/wiki/Dual-ported_RAM)

Current status: Working PoC with Arduino able to write (but not read) to the video memory and contents of video RAM displayed on a screen. Current resolution is 800x600 but using 4x1 pixels (due to using 10MHz pixel clock, instead of 40MHz required for 800x600 resolution). This gives us graphic with 200x150 4x4 pixels. Currently only single color is supported.

Prerequisites

Programming ATF16V8B Chips (PLD or GAL)

Follow this tutorial:

1. https://mike42.me/blog/2021-08-a-first-look-at-programmable-logic
2. If you don't like WinCUPL you may try the open source alternative that the author recommends in a
   follow up https://mike42.me/blog/2021-10-programming-plds-with-open-source-software.

I was using WinCUPL under Linux via Wine and the program was starting and compiling just fine. The UI is really outdated and cumbersome to use. But you don't have to stick with it, you may call the compiler yourself:

cupl.exe -m1lxfjnabe -u C:\WinCUPL\Shared\Atmel.DL C:\MY_PROJECT\MY_FILENAME.pld

This command line comes from docs for this VSCode extension (https://marketplace.visualstudio.com/items?itemName=tlgkccampbell.code-cupl). I did not test the extension so I cannot recommend it.

Remember that the output JED file will have the same name as the one used in Name declaration in PLD file (avoid spaces BTW):

Name            PALSamp1;
Partno          EE459;
Revision        01;
Date            2/3/04;
Designer        A. Weber;
Company         USC EE-Systems;
Location        None;
Assembly        None;
Device          g22v10;

(from: https://ece-classes.usc.edu/ee459/library/samples/PLD/PALSamp1.pld).

The PLD language reference: https://ece-classes.usc.edu/ee459/library/documents/CUPL_Reference.pdf

There are also a few examples if you scroll down to PLDs section: https://ece-classes.usc.edu/ee459/library/

Web Archive copy: https://web.archive.org/web/20240707073231/https://ece-classes.usc.edu/ee459/library/

So that was for the software, but we also need a programmer. I was using the popular MiniPRO TL866 II Plus together with minipro software (see: https://gitlab.com/DavidGriffith/minipro).

If your programmer is quite old you may need to upgrade the firmware first. Instructions to upgrade firmware: https://github.com/blurpy/minipro?tab=readme-ov-file#update-firmware. Don't bother updating above xgproV1181_setup.rar, the unar (yes, unAr) command won't be able to unpack it. The 1181 version allowed me to program ATF16V8B without any problems.

TIP: A USB cable that you use with MiniPRO TL866 II Plus must be able to withstand high current spikes. Use a short, thick cable. If you encounter errors when programming ATF's try a few different cables before looking for another solution.

You can find my PLD/JED files in PLD directory.

I really wanted to use .R/.S extension (that is RS switch on the output of the device) but looks like ATF16V8B does not support it in "Simple" mode. I was forced to use "Simple" mode as I needed both CLK & OE pins configured as regular inputs. I plan to switch to a more powerful ATF22V10 chip in the final build.

BTW You can select mode by specifying a suffix to the device in PLD file, e.g. G16V8AS means a simple mode while G16V8MS means a registered mode. You can find more about those in ATF16V8B datasheet (the truth is out there).

VGA Signal

This video https://youtu.be/5exFKr-JJtg?feature=shared is all that you need.

If you need a slower introduction you may want to check Ben Eater's videos https://eater.net/vga.

VGA Timings http://tinyvga.com/vga-timing, if it's gone there is a web-archive copy.

Alternative designs: https://www.youtube.com/watch?v=RcMilPYFlr0. The guy is building a PC compatible computer using breadboards. But uses a dedicated chips for it's VGA card. It is a more complicated design but supports multiple colors and is PC compatible.

Schematics

schematics directory contains both PNG & PDFs.

VGA Timing signals generator and shifting register:

A "half" dual-port memory:

Arduino connection:

Additionally each chip has a small decoupling capacitor (0.1uF). Power distribution over breadboards must be very solid for this to work properly. I used LM317T voltage regulator and powered it from 9V 2.5A wall adapter and LM317T was getting quite hot.

Results

The completed build:

Video: https://www.youtube.com/shorts/JnYwsofQ0bc

Conclusions:

• I tested 3 different memory chips: AS7C256B and AS6C4008 from Alliance Memory and a generic SRAM chip UM6264B.
  UM6264B was too slow for this application. Both AS chips where fine.
• There is a snowing problem when updating the screen this can be easily fixed by doing the video memory writes only during blanking
  period. This of course will reduce the speed of writing.