pico-loser

This is an old game, Loser Corps, ported to the Raspberry Pi Pico. It features:

• VGA output with 64 colors at 320x240, 60Hz
• Sound output using 22050Hz/mono/8 bits (a simple MOD player for music plus up to 4 simultaneous sound effects)
• Wii nunchuk and Wii classic controller support via I2C

Here's the Pico connected to a VGA plug, a speaker and a Wii classic controller:

Compilation

The code contains a CMake project that uses the official pico-sdk. To build it using the command line, install the Pico SDK and then:

# set the pico-sdk path (not needed if you used the pico-sdk installation script)
export PICO_SDK_PATH=/path/to/your/pico-sdk

# download this repository
git clone https://github.com/moefh/pico-loser.git
cd pico-loser

# build the game executable
mkdir build
cd build
cmake ..
make

This will create the file game/pico-loser.uf2 inside the build directory you created. To run the game, start the Pico in BOOTSEL mode (plug the USB cable while holding the BOOTSEL button) and copy the file pico-loser.uf2 to the Pico folder that appears.

VGA Output

The VGA output has 2 bits per color channel (which can generate up to 64 colors). As shown in the photo above, I use a homemade perfboard with a few resistors.

The VGA output pins must be sequential and are configured by default according to the table below (the lowest pin number can be changed in game/main.c):

Pico GPIO	Output signal
2	Red low
3	Red high
4	Green low
5	Green high
6	Blue low
7	Blue high
8	Horizontal sync
9	Vertical sync

The vertical sync and horizontal sync pins should each be connected to the monitor cable through an appropriate resistor (I use 300 Ω, but 330 Ω would probably work as well).

For each color component (red, green, blue), a resistor ladder or some other form of DAC is needed to convert the 2 digital pin outputs to the corresponding analog voltage expected by the VGA monitor. I use this simple setup with 2 resistors for each color:

This setup doesn't match the VGA monitor impedance (the output impedance is 1/(1/470+1/1000)=320Ω, but the monitor input impedance is 75Ω) but the image doesn't look bad, even with my old CRT monitor. My old attempt at matching the impedance with 3 resistors (270Ω for high, 480Ω for low, and a 130Ω to ground) exceeds the maximum output current for the Pico by a bit (when adding all pins), so it's not recommended.

Here is full schematic of all required connections to a VGA female port:

Video Modes

The default output mode is 320x240 at 60 Hz, but the video signal generation code is able to output different video modes. For that, the code needs to know the pixel and line timings of the desired mode. A few standard video modes are defined in the code, see "VGA mode definitions" in game/lib/vga_6bit.c for a list of the included modes and an explanation of how the timings are configured.

The Internet is full of resources describing the standard VGA mode timings, but beware that not all monitors support all modes, and some old CRT monitors get really angry when supplied with an unsupported or invalid signal (the default 320x240 is pretty safe, as it will be seen by the monitor as the ultra-standard 640x480@60Hz). Some great resources are:

• http://tinyvga.com/vga-timing
• https://projectf.io/posts/video-timings-vga-720p-1080p

If you want to use a different video mode, note that the vertical and horizontal polarities of the selected video mode must be baked in the image bits. The images in the code have both vertical and horizontal bits set to 1, since the default 320x240 mode uses negative polarities for both vertical and horizontal sync pulses -- this means that the sync pulses are low, so outside the pulses (like when the visible pixels are being sent) the vertical and horizontal signals must be high. If you select a video mode with a different polarity (see the values for (h,v)pol in the video mode definitions), then you must change the sync bits in the image conversion script and re-run it to rebuild the images with the new sync bits.

Sound Output

The sound is produced as a single stream of 8-bit samples at 22050 Hz using a very simple MOD player and a mixer that takes the output of the MOD player and up to 4 samples at the same time (this limit can easily be increased at the cost of increased CPU use).

Since the Pico doesn't have a DAC, the resulting 8-bit samples are converted and sent to GPIO 16 as PWM. The code that does this conversion is based on Greg Chadwick's PWM sound generator described in this blog post.

The PWM output from GPIO 16 is filtered with a 100nF capacitor and a few resistors. The result is fed to an LM386 amplifier, which is connected to an 8 Ω speaker. The whole thing is described in this schematic:

Controller Support

The game supports the Wii Nunchuk and the Wii Classic Controller using I2C. The controller pins should be connected as follows:

Pico Pin	Controller
3V3 OUT (pin 36)	3V3
Any GND	GND
GPIO 12	SDA (data)
GPIO 13	SCL (clock)

To make the connections, you can use a breakout board like this one from Adafruit, or (if you're OK with destroying you controller's plug) follow this tutorial.

License

The source code and images are released under GPL 2.0

Includes the song "The Soft-liner" by Zilly Mike, licensed under CC BY 3.0. No changes were made other than the conversion to a header file for inclusion in the code.