picodvi VS vga-rpi

The RP2040 is a great MCU for playing with graphics as it can bit bang VGA and DVI/HDMI. There's some info on the DVI here: https://github.com/Wren6991/PicoDVI

I wrote a couple of articles on how to do bit banged VGA on the RP2040 from scratch: https://gregchadwick.co.uk/blog/playing-with-the-pico-pt5/ and https://gregchadwick.co.uk/blog/playing-with-the-pico-pt6/ plus an intro to PIO https://gregchadwick.co.uk/blog/playing-with-the-pico-pt4/

A few years back someone was bit banging DVI and 720p 30fps video straight off the RP2040 GPIO pins with just inline resistors between not and the HDMI cable.

https://github.com/Wren6991/PicoDVI

I have the Adafruit DVI board and have been hacking on it some. It's more fun than should be allowed.

There's a project to get this running using Rust[1]. Currently it outputs the blue channel with sync, but has trouble with the other two channels. I don't have a hardware lab with scope, logic analyzer, etc., so it's not easy for me to debug.

In the meantime, I've been doing some experiments in the C codebase as well[2], mostly in the direction of proportionally spaced bitmap text, using the fonts in the X11 distribution.

[1]: https://github.com/DusterTheFirst/pico-dvi-rs

[2]: https://github.com/Wren6991/PicoDVI/pull/48

There is even bit-banged DVI library for pico.

Apparently, there is a digital video output project for the RP2040 already out there [0]! As for why digital video is more rare, it is because the common digital video formats are higher bandwidth and require higher signal integrity.

[0] https://github.com/Wren6991/PicoDVI

I've seen people implement DVI with PIO, and I'm sure basic missile commands wouldn't be as bandwidth-intensive as that.

For example this guy (https://github.com/Wren6991/PicoDVI) essentially 'bit bangs' CML. Sure its not a completely compliant implementation, but it's pretty close... In his case using a push pull with 3.3V/0V. I don't know how he's getting a 0.8V swing.

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.