DispHSTX - DVI (HDMI) and VGA display driver for Pico2 RP2350, using HSTX peripheral
====================================================================================
Version 1.01, March 2025

Copyright (c) 2023-2025 Miroslav Nemecek

[email protected]
[email protected]
https://github.com/Panda381/DispHSTX
https://www.breatharian.eu/hw/disphstx/index_en.html
https://github.com/pajenicko/picopad
https://picopad.eu/en/


Basic description
-----------------
DispHSTX is a driver for Pico2 RP2350 microcontroller, both in ARMv8 and RISC-V
Hazard3 mode, allowing to generate DVI (HDMI) and VGA video signal, using HSTX
peripheral. It supports 40 different image formats - 9 modes of pixel graphics,
2 text modes, attribute compression, RLE and HSTX compression. Functions for
drawing geometric shapes, images and texts are provided for 8 basic graphic
modes. The drawing library supports a back buffer - it updates the boundaries
indicating the modified "dirty area". If memory is insufficient, the back
buffer can be used in strip mode. Several different image formats can be
combined on one screen simultaneously by dividing the screen into strips and
slots. In both modes, DVI (HDMI) and VGA, color resolution up to 16 bits per
pixel is possible. Although in VGA mode the output is only via a 6-bit RGB
converter, higher colour resolution is achieved by the Pulse Pattern Modulation
PPM, the display is visually close to the 15-bit display. The timing of the
generated video signal is limited only by the overclocking capabilities of the
microcontroller used. It is usually possible to achieve a resolution of up to
1440x600 pixels.

DispHSTX is built on the PicoLibSDK library, but interface to original
Raspberry PicoSDK library is ready too.


License
-------
DispHSTX driver source code is completely free to use for any purpose,
including commercial use. It is possible to use and modify all or parts of the
driver source code without restriction.


Schematic wiring diagram
------------------------
The DVI (HDMI) and VGA connectors are connected to the same processor pins,
GPIO 12 to GPIO 19. The selection of pins 12 to 19 is determined by the
processor and cannot be changed. In the driver configuration there are 3
options for the pin order for the DVI (HDMI) output, but any other pin order
can be selected after editing the code. It is recommended to use the default
configuration 0 (see schematic diagram), which is based on the wiring of the
popular Pico DVI socket. Similarly, it is possible to select any pin order for
the VGA connector, but it is recommended to keep the order used in the
schematic above to allow VGA signal output from the PIO controller, which
requires keeping the pin order.

The 400 and 800 ohm resistors used for the VGA output should be used within a
1% tolerance. If necessary, close values such as 390 and 820 ohms can be used,
but the smoothness of the gradient transition may be degraded for multi-color
modes (12 to 16 bits per pixel).

On the RGB outputs of the VGA connector 220pF capacitors are used to filter
Pulse Pattern Modulation PPM. For video modes with a low system frequency, it
may be beneficial to increase the value of the capacitors to reduce potential
image grain. For video modes with high system frequency, it may be beneficial
to increase the value of the capacitors to increase the sharpness of the image.

Any GPIO pin can be used for the switch to select VGA/DVI mode, or the switch
does not need to be used. If the switch is not used or it is in the middle
position, an automatic detection of the VGA monitor connection can be used to
select the mode.


Files and building
------------------
The DispHSTX driver is primarily prepared for use with the PicoLibSDK library.
The main driver files can be found in the _display/disphstx folder. The
disphstx_dvi.* files provide support for the DVI (HDMI) output. The
disphstx_vga.* files are used to handle the VGA output. The
disphstx_vmode_time.* files contain video mode timing definitions - you can use
them to define your own video mode. The disphstx_vmode_format.* files contain
definitions of graphic formats. Rendering of graphic formats is done in the
*_render.* files in the C codes - these functions are used only as a reference
to the correct code functions. The actual rendering is done using functions
optimized by the assembler, in the *.S files.

The _config.h file contains the default definitions of the driver parameter
settings. If you need to change a configuration switch, specify it in the
config.h file in the project folder. Default switches are used if they are not
found in the project's config.h file. The main switch you will need to include
in config.h is "#define USE_DISPHSTX 1". This will ensure that the DispHSTX
driver functions are compiled and made available. The second important switch
you may need to include is "#define USE_DRAWCAN 1". This will provide access to
the DrawCan drawing library functions. For more info about projekt
configuration switches, see chapter "Configuration".

The HSTX driver is by default run in the second core of the processor. This
is required by the fact that the driver operation must not be disturbed by
anything to avoid dropping the generated signal. For this reason, no interrupts
other than those from the HSTX driver may be running in the second core of the
processor. All time-consuming operations must be performed from RAM, not from
Flash - the linker must provide a "time_critical" section in RAM for this
purpose.

Driver functionality is provided for both ARMv8 processor mode and RISC-V
Hazard3 mode. However, it is necessary to take into account that the code for
RISC-V mode is slightly slower (about 15%) and so the HSTX driver in RISC-V
mode may load the processor a bit more than in ARM mode.

The driver in the basic full setup can take quite a lot of RAM for functions
and auxiliary buffers (about 50 KB). You can reduce the memory requirements by
limiting functionality - disabling VGA mode, reducing the maximum resolution,
or disabling unused image formats. See the "Configuration" chapter for details.


History of versions
-------------------
02/24/2025 version 1.00
03/02/2025 version 1.01, Raspberry PicoSDK library build