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Solo Squash

This is a simple hardware video game that I'm developing as a possible student submission for the Zero to ASIC course.

It is a Verilog HDL project that implements a primitive digital-logic-based video game that resembles Pong, but with one player just bouncing a ball within a 3-walled space, resembling a game of squash but with just 1 paddle.

License

This repo is licensed with Apache 2.0.

Details

The main (generic) design is src/solo_squash.v.

This is intended to drive a VGA display at 640x480 resolution, ~60Hz, and it gets adapted to different targets, including FPGA, Verilator-based VGA simulation, and Caravel ASIC.

It has been tested on a DE0-Nano FPGA board (Altera Cyclone IV), and I hope to submit this as part of the Google Skywater project to be made into an ASIC.

An older, more-compact version (without colour or patterns) has also been tested on an XC9572XL CPLD, but it's a tight fit!

Hardware

SCHEMATIC COMING SOON

Key parts:

  • 25.175MHz clock source (though 25.000MHz is good enough).
  • Buttons (all with pull-ups): Up, Down, Pause, New Game, optional Reset
  • VGA connector: HSYNC and VSYNC via 100Ω resistors and R, G, B each via 270Ω resistors (assuming 3.3V outputs rather than 5V).
  • speaker signal optionally connected in series with a piezo speaker and 670Ω resistor to GND.

There is a Quartus project and top wrapper interface in de0nano for running this specifically on a Terasic DE0-Nano board (using an Altera Cyclone IV FPGA), but with a little extra external hardware as mentioned above. The file solo_squash_de0nano.v should be inspected to see the mapping of DE0-Nano GPIOs for working out where to connect the external hardware.

Visual simulation with Verilator

Make sure SDL2 dev packagers are installed:

sudo apt-get update
sudo apt install libsdl2-dev libsdl2-ttf-dev

Then hopefully you can run the following and it will build and run the simulator, popping up a window that shows the game in action.

make clean sim

Use the up and down arrows to control the paddle.

You can also simulate with various init states, too:

# Each of these builds and simulates, with all unassigned bits starting at...
make clean sim          # ...0
make clean sim_ones     # ...1
make clean sim_seed     # ...predictable random values based on SEED (set in Makefile or overridden via command-line)
make clean sim_random   # ...unpredictable random values each time.

Reset is not asserted automatically at the start of simulation. This is evident if you run make clean_sim run_sim_random, in which case the ball and paddle will start in random (and sometimes invalid) positions.

Virtual VGA display

Verilator simulating solo_squash

NOTES about what you see in the screenshot above:

  • The background is grey and not black because the region that got rendered between SDL refreshes gets its lower bits set to visually show what's being updated each time. To toggle this, use the H key.
  • Purple bars are a visualisation of when the speaker signal is on. This is hard to simulate especially when the video speed isn't realtime, and visualisation of exactly when it turns on (and off) in relation to the video rendering is probably more useful anyway. I think I could probably get away with an audio sim just buffering until the end of the frame, and then playing it in realtime when VSYNC arrives. This should at least "feel" right and sound at the right tone.
  • Regions outside the main playfield area are typically called "overscan" and allow us to visualise the "front porch", "sync", and "back porch" signals for each of HSYNC (red) and VSYNC (blue).
  • Regions even further outside this (seen as black to the right and at the bottom) should not get any video signal crossing into them, except maybe during the simulator's initial attempt to lock on to the video signal. Anything that DOES make it into this region should decay back to black after a short while.
  • Faint horizontal and vertical lines are showing what a VGA monitor would probably sense as the actual exact visible area of the display.

Simulator Hotkeys

Key Function
Space Pause simulator
H Toggle refresh highlight
N New game signal
P Assert Pause signal to game
Q Quit
R Reset
V Toggle VSYNC logging
X Turn on eXamine mode: Pause simulator if last frame had any tone generation
S Step-examine: Unpause, but with examine mode on again
F NOT IMPLEMENTED: Step by 1 full frame
+ (Keypad) Increase refresh period by 1000 cycles
- (Keypad) Decrease refresh period by 1000 cycles
1 Refresh after every pixel (VERY slow)
2 Refresh after every line
3 Refresh after every 10 lines
4 Refresh after every 80 lines
5 Refresh exactly on every frame
6 Refresh exactly every 3 frames
9 Refresh after every 100 pixels (better for observing repaint within frames)

Examine mode is currently programmed to help observe what happens with tone generation:

  1. Hit X to turn on examine mode.
  2. As soon as a frame completes that included the speaker being turned on, go into PAUSE.
  3. You can either just resume with P, or step through each subsequent examine trigger with S.

NOTE: With the current implementation, speaker tones will typically "spill over" into the very top of the "next frame" (as detected by the end of VSYNC) and so this will trip examine mode again. This is normal; just expect examine mode to trip at least 2 times per ball hit.

Running tests

Run:

make clean test

This will run tests in test/ based on cocotb, using Icarus Verilog. It will move test results into test_results/, including:

  • results.xml
  • solo_squash.vcd: Dump file of waveforms generated by the main design.

To see the results in GTKWave, run:

make show_results

Caravel ASIC target

More to come.

When tested within Caravel (using cocotb tests), I think we have the following hierarchically:

  • User code: solo_squash_caravel_tb
  • ...then Caravel code: → uut (caravel) → mprj (user_project_wrapper)
  • ...finally more user code: → adapter (solo_squash_caravel) → game (solo_squash)

Note that adapter was originally a 2nd level called mprj from the caravel_user_project example. With this name change, the existing FP_PDN_MACRO_HOOKS parameter in a caravel_user_project/openlane/user_project_wrapper/config.json file would also need to be updated.

Note that solo_squash_caravel_tb includes a few extra wire definitions that assign names to the Caravel GPIOs (etc) that we've chosen to use, hence meaning that most of the code from the original tests could still use those names for convenience. In fact, my aim will be to make better universal tests that work with both the Caravel-wrapped design and the bare generic solo_squash design.

Contents

  • src/: Verilog source for the project.
    • solo_squash.v is the main design.
    • solo_squash_caravel.v is for when we want to implement this as an ASIC, using Caravel. This file is a bridge between the main design and the Caravel user_project_wrapper (UPW), and we have it so we can have additional logic (that otherwise must be excluded from the UPW) that makes our design specifically compatible with Caravel.
  • sim/: C++ code for Verilator-driven SDL-based VGA simulation.
  • test/: Where we'll start to put files that are needed for formal verification.
    • __init__.py: (empty file) needs to be in here so that Python/cocotb finds our tests.
  • caravel_stuff/: Things we'll probably need later for making a proper Caravel UPW. See its own README for more info.

Requirements

TBC!

Not all of these are necessarily required together. Some are just for different types of tests:

  • Icarus Verilog (iverilog)
  • Python 3.8+, cocotb 1.7.2+, pytest
  • Verilator + SDL2

Installing pytest 7.1.x (which cocotb uses to improve its assertions output):

pip install --upgrade pytest