vga-rpi VS pico-playground

I've been sort of pondering the thought of using a microcontroller as the base of an ISA "soft video" card for vintage PCs.

I was looking at things like the Graphics Gremlin and the kit to build a Trident 9000-based ISA VGA card, and realizing that 1) I don't think my skills or gear are up to SMD soldering, 2) at least the Trident part is 20-year-old stock with limited supply and questionable reliability, and 3) you still end up with an analogue output that needs a chain of adapters to drive a DisplayPort or HDMI monitor. I may as well just use my old Realtek RTG3106 card.

I'm picturing a board that did some minimal decode of the ISA bus to filter accesses to upper memory or desired registers, then plumb the rest of the bus onto the RasPi GPIOs, and just run some emulation code to interpret the requests and maintain a framebuffer-- drawn to a HDMI output.

I figure by doing the emulation in software, you could cover a wide variety of cards and trivially switch (i. e. toggle between Tandy 1000/PCjr style CGA+, Hercules, and VGA, with some software command rather than having to physically change cards or detach the monitor).

It feels like something like this would exist already, but the closest I've seen (https://github.com/eyalabraham/vga-rpi) is not a drop-in replacement for a standard PC card-- it seems to expect custom hardware and BIOS shimming.

The RP2040 (as used by the Raspberry Pi Pico) can generate some impressive video thanks to its PIO. E.g. Luke Wren's work giving DVI output https://github.com/Wren6991/picodvi and the scanvideo library: https://github.com/raspberrypi/pico-playground

I did a write up going over how to generate VGA video from scratch to produce some SNES like graphics:

https://gregchadwick.co.uk/blog/playing-with-the-pico-pt4/

The example sketch you can use without any modifications nessasory

https://github.com/raspberrypi/pico-playground https://github.com/raspberrypi/pico-examples

I used the USB sound card example: https://github.com/raspberrypi/pico-playground/blob/master/apps/usb_sound_card

You'll need to edit this file after you download it first. Add the lines PICO_AUDIO_I2S_DATA_PIN=28 and PICO_AUDIO_I2S_CLOCK_PIN_BASE=26 in the compile definitions. After that, you can compile it as normal.