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:

• HAGL
• Pico extras, scanvideo part
• Pico playground, scanvideo part
• Pimoroni VGA Demo Base
• 640x480x16 framebuffer
• HAGL GD
• MEMU - Memotech EMUlator
• PicoBB - BBC BASIC for Raspberry Pi Pico
• Rumbledethumps' Picocomputer 6502

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

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.

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

• 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

CMakeLists.txt is inspired by:

• Memotech Bill's Pico VGA framebuffer
• Tuupola's HAGL Pico MIPI

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

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

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

• ^[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 ;-)

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

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