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• Demonstration video - Bouncing Balls

  George Foot • 10/12/2020 at 14:58 • 0 comments

  Just a quick note - I've uploaded a demonstration video showing my current software sprite rendering system, bouncing balls around the screen.  This is mostly trying out some systems that I'll need to rely on in a more "proper" game.
  
  Check it out here and let me know what you think!

• Next Steps

  George Foot • 10/10/2020 at 18:17 • 0 comments

  I've been thinking about next steps for this project.  It's been quite successful for its intended purpose, allowing me to experiment with different timings and resolutions.  The 640x256 upgrade last week has been working well since then, and I've started making a little game on it.

  There are a few definite directions I still want to take the video circuitry in though.  The main ones I've always had in mind are adding colour, proper 640x512 support, alternate output types (VGA, component), and higher resolutions (e.g. 1024x720, 1920x1080) as I'm interested in what's possible and what's beyond reach on a breadboard.

  In the past week a new interesting extension has come onto my list as well - decoupling the CPU, clock, etc from the video circuit more, so that the CPU can do its own thing when it's not accessing video memory.  This could become a module that's simple to plug into any 6502-based system and get a fully-fledged graphical display, much like how PC video cards used to work, or like what Debug Innovations' TTL VGA Terminal already does a great job of doing for text-based output.
  

  The trouble is, the better what I already have continues to behave, the less inclined I am to risk breaking it by extending it.  Some of these things are significant compromises - increasing the resolution would require a lot more RAM; adding colour would also require more RAM or would require lowering the resolution; and any increase in bandwidth will make the CPU struggle to fill the memory fast enough.  I'm still interested in climbing the mountain "because it's there", but I also have other projects on the go that require the existing system to keep working as it is.

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• Upgrading to 640x256

  George Foot • 10/04/2020 at 21:48 • 0 comments

  I've generally been running this circuit at 384x256 resolution, and that's how it's configured in the schematics.  The main clock is 16MHz, but this is halved to 8MHz to from the pixel clock.  It's divided down a couple more times to 1MHz to form the CPU clock, and the interleaved memory accesses also occur at that frequency.  This is based on how the BBC Micro's video circuitry worked, and these timings match its "Mode 4", which was 320x256 (same timings, but a shorter line length for a non-widescreen display).
  

  The BBC Micro also had a higher resolution 640x256 mode, "Mode 0".  The way this worked was by doubling the pixel clock to 16MHz, and fetching from RAM at 2MHz.  This also requires the CPU to run at 2MHz or more, as a 1MHz CPU needs access to RAM for too long at a time.

  Unfortunately my 6502 CPU was a cheap clone and wasn't capable of running higher than 1MHz, so I wasn't able to get this doubling of horizontal resolution.  However, since building the circuit I have got some WDC 65C02s that can run at much higher clock speeds, and this weekend I decided to go ahead and do the upgrade.

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