HAGL Pico VGA Board

BIG FAT WARNING

THIS PROJECT IS NOT PRODUCTION READY!

SOME VGA MODES IMPLY OVERCLOCKING AND OVERVOLTING RP2040!
USE AT YOUR OWN RISK!

Introduction

This is an HAGL HAL for the Raspberry Pi Pico demo VGA board, based on scanvideo from the pico-extras repository that comes alongside SDK.

This follows discussions on Raspberry Pi Pico's SDK forum Understanding Pico VGA Code and CMakelists? dating back from 2021.

The following projects are either bases for this one or sources of inspiration:

To manage this project, I use Github's project board to be able to organize myself between ideas, TODOs, WIPs & done items.

Features

As of February 2024, this project has the following features:

Framebuffer, statically allocated at compile time (see below for corruption using malloc()),
1/2/4/8/16 bits per pixel modes, leading to 2/4/16/256/32768 colors at once,
8 bits or 16 bits mode choosed at compile time, too,
Double buffering for smooth animation at lower resolutions, as it uses twice the memory
VGA modes from 640x400 to 1280x1024, with 1x/2x/4x/8x scale in X and Y directions (not using scanvideo's yscale_denominator)
Letterbox mode to handle for example MSX / ZX Spectrum 256x192 inside 320x240 standard VGA
Handling of A/B/C buttons (with clever? use in example allows going to next/reset demo, cycling through palettes & changing borders ramdomly)
Native execution with Pico Host SDL using PICO_NO_HARDWARE, allowing to debug display algorithms before using real hardware
Serial console on GPIO20/GPIO21 with cutted tracks

There is an example project with some demos.

BUGS

Find why allocating framebuffer dynamically with malloc() leads to corrupted display (memory alignment problems?)

TODO

VGA mode change at runtime, either:
- only color depth and resolution (for example to implement modes 0=160x200/16, 1=320x200/4, 2=640x200/2 of Amstrad CPC)
- other VGA modes, only being limited by the amount of statically allocated memory
Using multiplane capabilities of scanvideo:
- Framebuffer with its own palette
- Text mode layer, using another palette if desired
- Tilemap and tileset (with pixel scrolling, window or full screen,...)
- Sprites, as many as possible, eventually with scaling / rotation
- only 3 will be available at the same time
Optimizations:
- Blitting operations either in C or assembly to speed up rendering and capabilities
- Packing of HAGL's bitmaps when in 1/2/4 bits per pixel modes, or implement them as VGA board only
Replacement of buttons with Pico Host SDL target

Build Instructions

CMakeLists.txt is inspired by:

pico-sdk and pico-extras from Raspberry Pi Foundation are obviously required.

VGA modes

Most VGA timings come from:

VGA Signal Timing
Pico Extras itself

N.B.:

Most modes are intended to be stretched by 2 (or more) in horizontal and/or vertical directions to reduce memory usage
Wider modes have higher pixel clocks that limit system overclocking coefficient to 3, 2 or even 1, that's why I'm trying to provide "halved" versions which are tricky to achieve
For h_sync_polarity and v_sync_polarity, 1 means NEGATIVE, and 0 means POSITIVE

Working modes

These modes should work on any reasonable LCD display, I don't own any CRT monitor anymore so I can't make any tests.

A/R	VGA Mode	PC^1	PC^2	Notes
16:10	640x400	25.175	25.200	Only mode using 70Hz refresh rate, all other use something very near of 60 Hz
4:3	640x480	25.175	25.200	Most standard, if only one can work, that must be this one
4:3	768x576	34.960	35.000	Divisible by 3, fast system clock (280 MHz at 1.20V^3 on my Pico B0 & B1)
4:3	800x600	40.000	40.000	I think 768x756 is better ;-)
4:3	1024x768	65.000	65.000	Even my 15" LCD from 2001 or 2002 has this as native resolution!
16:9	1280x720	74.250	74.000	Only 16:9 mode for now
5:4	1280x1024	108.000	108.000	I own an 19" LCD with this native resolution, too...
16:10	1680x1050	147.140	147.000	Divisible by 3 or 5^4, fastest system clock (294 MHz at 1.30V^3 on my Pico B1)

^[1] Specifications Pixel Clock in MHz
^[2] Rounded Pixel Clock in MHz as achievable by RPi Pico (as given by vcocalc tool)
^[3] Be aware that these values may either not work on specific Picos, shorten their lifespan or even destroy them!, see ALLOW_VREG_VOLTAGE_OVERRIDE
^[4] That Makes a very good fit at 336x210 with 320x200 letterbox, very tight margins at native resolution on my LG L204WT monitor ;-)

Experimental modes

These modes are flaky on my own monitors or even do not work at all.

A/R	VGA Mode	PC^1	PC^2	Notes
5:4	640x512	56.000	56.000	Half of 1280x1024, rounding v_pulse to 1 or 2 does not seem good...
16:10	840x525	73.125	73.500	Half of 1680x1050, should satisfy my LG L204WT monitor but is "out of range"
16:9	1024x576	46.500	46.500
16:9	1600x900	108.000	108.000	Native resolution of my 17" laptop: good for fullscreen tests, KO on my LG monitor
16:10	1280x800	83.460	83.200	Double of 640x400, can be used to make 640x400@60

License

The MIT License (MIT).

See LICENSE for more information.

There's a mix with BSD 3 Clause license between HAGL and Pico SDK / extras that should be fine, but IANAL.

CHiPs44.

EOF

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