StickHero

Project Overview

StickHero is an FPGA hardware implementation of the popular mobile game where players control a stick figure traversing platforms by extending and retracting a bridge. The objective is to accurately time the bridge's extension to cross the gap between platforms. Overextending or undershooting the gap causes the player to fall into the abyss, triggering a game-over state.

The system is built entirely on a Digilent Nexys A4 FPGA using Verilog, implementing graphics generation, game state control, and user input directly in hardware logic without a CPU.

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Technical Architecture

The project is structured modularly with a clear separation between display signals, coordinate control, and the main game logic:

• vga_top.v: The top-level design module that wires together the clock, reset, physical buttons, and display outputs.
• display_controller.v: Manages the VGA display signals hSync and vSync to control horizontal and vertical pixel scan positions.
• block_controller.v: Directs physical user input (buttons) and pixel-generation logic for the game graphics.
• StickHero.v: The core of our design, which contains the main game loops, coordinates, and rules.

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Game State Machine

The game rules and transitions are managed through a 5-state Finite State Machine (FSM) implemented inside the game logic:

1. INIT (Initialization)

• Initializes all active game variables.
• Sets the initial starting coordinates for the starting platform.

2. WAIT

• Sets up the target platform dynamically.
• Platforms are generated with widths ranging from $20\text$ to $200\text$ based on the player’s current score (decreasing in width as score increases)[cite: 1].
• Calculates the horizontal position ($X$) of the next platform relative to the character's coordinate and the next platform's width[cite: 1].

3. PLAY

• Up Button Held: The stick's length increments along the vertical ($Y$) axis, setting a register flag[cite: 1].
• Up Button Released: The vertical stick is instantly rotated (copied) onto the horizontal ($X$) axis to simulate the falling bridge motion[cite: 1].

4. MOVE

• Validates the length of the horizontal stick[cite: 1].
• If the bridge is too short or long, a gameOverFlag is set[cite: 1].
• If successful, coordinates shift to slide the platforms to the left, creating a smooth camera-scrolling effect, and increments the score[cite: 1].

5. GAMEOVER

• Controls the downward physics animation of the character[cite: 1].
• Both blocks composing the player character decrement their height coordinates to simulate a plummet[cite: 1].
• Triggers a fallingFlag which turns the screen red to signal defeat[cite: 1].

Key Engineering Challenges & Bugs

The Platform Generation Bug

A persistent challenge during development was creating robust procedurally-sized platforms[cite: 1]. Specifically, a mathematical bug was identified where every time the score becomes a multiple of 5, platform generation fails, leaving the stick character "floating" in mid-air[cite: 1].

The bug is traced to coordinate offsets and modulo operations when recalculating boundaries[cite: 1]:

// placement for curr platform inside StickHero.v
if (score == 0) begin
    currxpos   <= 200;
    currypos   <= 515;
    currWidth  <= 200;
    currHeight <= 150;
end
else begin // Define currPlatform dimensions as previous nextPlatform dimensions
    currWidth  <= 200 - (((score - 1) % 5) * 35);
    currHeight <= 150;
    currxpos   <= (guyxpos + 20) - nextWidth - 35; // <-- Platform bug occurs here
    currypos   <= 515;
end

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How to Run It

1. Open Xilinx Vivado.
2. Create a new project targeting the Nexys A4 (XC7A100T).
3. Import all .v files from the stickHero Repository.
4. Add the nexys4.xdc constraint file to map the VGA pins, clock, and buttons.
5. Generate Bitstream and program your FPGA board!

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Resources:

• StickHero Repository
• Full Report