65816+VGA Progress Update

Continued from 65816: Driving VGA Through Dual-port SRAM...


I made some progress today on my 65816 with VGA 2.0.

  • Learned a bit about 65816 assembly for 24-bit I/O addressing. Thanks to u/tmrob4 for the help!

  • Added pulldown resistors on the address and data buses on the video circuit.

  • Added two additional 32K SRAMs to the stack. I potentially will add a fourth to the "final" design.

  • Tested faster clock speeds.

  • Fixed an issue where content was being drawn at the wrong location. I had an address wire connected to the wrong location.

  • I still have a mess of wires.

Observations

  • The pulldown resistors on the VGA address and data buses are absolutely required. Without them, I am unable to maintain a video signal. This makes sense, as the address decode logic with enable signals will result in momentary points where the buses are floating. Right now, I'm using 10K pulldowns. I'm wondering if I should try 3.3K or 1K pulldowns to see if they make things better or worse. If you have suggestions, please let me know!

  • I can run two SRAMS at 0.5 MHz. Adding a third causes instability at this speed. I can run three SRAMs at slower speeds. Considering the mess of wiring, this isn't surprising. I have not tried speeds higher than 0.5 MHz.


Here's a quick video showing where I'm at.


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