pico-composite8

Based on pico-mposite by breakingtoprogram:

A hacked together demo for the Raspberry Pi Pico to output a composite video signal from the GPIO Using a resistor ladder to translate a 5-bit binary number on the Pico GPIO to a voltage between 0v and 1v.

8-Bit R2R Resistor DAC

The input impedance of the Composite Monitor is 75Ω; the output impedance of the R2R ladder is R. In order to get 1V across a 75Ω load with a 3.3V source, R needs to be 2.3 * 75 = 172.5Ω. Closest 'standard' values are 180Ω/360Ω.
Unfortunately using those values doesn't work for 8-bits. The values are low enough that the internal resistance on the Pico GPIO pins becomes important, and causes visible errors on the 7- and 8-bit. Measuring the voltage drop on the pins gives a calculated series resistance of about 40Ω, so you will need to reduce the 2R value by that much to compensate. I used two 160Ω resistors in series:

The R2R DAC will draw about 19mA total from the Pico when all the bits are '1', with a maximum of 6.5mA on any single pin. Here's the simulation:

You can access the simulation here: R2R DAC Simulation

Bitmaps

A portion of the NTSC signal is reserved for SYNC and BLANKing:

On a 256 scale, BLACK works out to about 86 (255/140 * 47.5), so the values from 86 - 255 are the range from BLACK to WHITE, or 170 values. The scripts take the 256-gray image, compress it to 170-gray, and shift it up by 86 to leave room for the SYNC.

The 8-bit gray images are created with Gimp:

• convert to grayscale, 8-bit
• flatten image
• scale image and canvas size to get 512x384 image
• export as raw image data (*.data) File size should be exactly 196608 bytes

Use scripts/bit2h.pl to create an '.h' file which can be included and compiled with the program. There's only enough room for one 512x384 image in memory though. Remove the comment in front of #define TESTPATTERN in cvideo.c to enable that behavior.

The images can also be stored in flash, which has room for about 8 images; for that you'll need to install picotool. Put the *.data files in a directory and run scripts/mkbin.pl. The resulting 'flash.bin' file can be copied to the Pico with scripts/flashbin.sh. The program will step through the images in a slideshow.

Timing

The 640x480 resolution is accomplished by interlacing 640x240 fields with this timing:

The program is doing that and can render the Indian test pattern:

Note the concentric circles around the 30's and the vertical/horizontal resolution lines.

Compiling/Uploading

Compile using VSCode with the GCC 9.2.1 arm-none-eabi kit (which unfortunately isn't available on the Raspberry Pi (yet)).

To deploy from the development platform (e.g. X86 Ubuntu) to the Pico using SWD, you need a Raspberry Pi running openOCD. The Pico connects to the Raspberry Pi like this:

On the Raspberry Pi, execute openOCD:

openocd -c "bindto 0.0.0.0" -f interface/raspberrypi-swd.cfg -f target/rp2040.cfg

On the Development Machine, click the Run and Debug icon in the left-side menu - it will execute the .vscode/launch.json included in the project. Click the triangle next to Pico Deploy in the top-side menu, and enter the address of the Raspberry Pi SWD Host in the box to the right:

The program will compile, upload to the Pico, and break at main(). You can either remove the programmed Pico, or continue with debug commands, top-side middle menu.

the BUG

The interlace fails and only gives half the vertical resolution unless there's a 30 usec delay in cvideo.c at the start of scan line #0 - before the interlace starts:

15 usec isn't enough, only 30 usec will work. Other than that, it follows the timing above... Sort of stumbled into it by accident: in an early version there was a memcpy there that took 30 usec to complete. When I took that out, interlace failed until I added an equivalent 30 usec delay. Don't know why...