PICO TTL2VGA

RPi Pico TTL to VGA Converter

Simple TTL (MDA/CGA/EGA) to VGA converter based on RPi Pico.
VGA output is 720x400 (-31.4kHz/+70Hz).
Input video modes:

mode	resolution	frequency / polarity	pixel clock	VSYNC	total lines
MDA	720x350	H:+18.4kHz V:-50Hz	16.257MHz	16 lines	369
CGA	640x200	H:+15.7kHz V:+60Hz	14.318MHz	3 lines	262
EGA Mode 1	640x200	H:+15.7kHz V:+60Hz	14.318MHz	3 lines	262
EGA Mode 2	640x350	H:+21.8kHz V:-60Hz	16.257MHz	13 lines	364

Using the buttons, the pixel clock (horizontal width) can be adjusted. In MDA and CGA/EGA the pallete can be toggled (GP21).

Tested only with OAK067 and ET3000 EGA/VGA graphics cards.

Parts

RPi Pico (RP2040)
11 $\times$ 499 Ω resistors
11 $\times$ 1k Ω resistors (group 8 in resistor network)

• optional
  2 $\times$ 100 Ω resistors
  DB9 male connector
  DB15 female connector
  4 buttons
  1nF capacitor (optional, if VSYNC is noisy)

Schematic

Compile

cf. RPi Pico c/C++ SDK

cd src
# (copy pico_sdk_import.cmake from the SDK)
mkdir build && cd build
cmake .. -DPICO_SDK_PATH=<path_to_pico_sdk>
make

Pico VGA output

cf. https://vanhunteradams.com/Pico/VGA/VGA.html

Remarks

1. Use at your own risk! Don't expect too much.

2. The 114 MHz system clock reduces jitter in the VGA output.
   The input pixel clock will probably introduce jitter. The clock divider for the pixel clock can be adjusted (GP20/19). The parameters can be displayed (GP18).

3. If scanning starts too early, the last line would be missing. For normal use, a frame buffer of 720x351 (or 724x351) is recommended (see below). Once the best settings have been found, they can be set in ttl_in.h, e.g.

   #define DIV16M_FRAC  207  // EGA2
   #define DIVMDA_FRAC  232  // MDA
   #define DIV14M_FRAC   77  // CGA/EGA

   In EGA Mode 2 there should be 350 visible lines, 13 lines vertical sync, and 364 total lines, i.e. not much room for front/back porch.
   640x350 EGA Mode 2:
   
   640x200 CGA/EGA:
   
   720x350 MDA:
   
   

4. Use test images (e.g. CheckIt) to find the right pixel clocks.
   640x200 EGA:
   
   640x350 EGA Mode 2:
   
   (The distortion is due to the camera angle.)

5. If USB output is enabled, the clock divider fractional part for the current video mode can be seen in a serial monitor. However, if pico_enable_stdio_usb(ttl2vga 1) is enabled, the frame buffer is limited to 720x350 (or 724x349). If pico_enable_stdio_usb(ttl2vga 0) is disabled, vga_out.h:YACTIVE and ttl_in.h:YLNS_MDA should be set to 351 lines. This would take into account variations between different graphics cards.

6. Hercules Monochrome Graphics: