Space Force X: Disaster Strikes is an interactive space defense game implemented on the Basys3 FPGA development board. Players control a defensive tank positioned at the bottom of a VGA display screen to protect against waves of descending alien invaders. This intermediate-level project introduces students to complete embedded system design including Finite State Machines, real-time collision detection, synchronous hardware design patterns, and VGA display interfacing. Every pixel rendered, every collision detected, and every game state transition is a direct result of hardware logic circuits working in concert at the clock cycle level.<\/p>\n
Space Force X simulates a classic top-down space defense scenario that mirrors real-world systems like air defense networks and threat detection systems. The gameplay takes place on a 640\u00d7480 pixel VGA display where time is measured in clock cycles. A player-controlled tank occupies the bottom 30 pixels of the screen. Above it, an enemy formation of aliens descends in organized rows. The objective is straightforward but demanding: destroy all alien invaders before any reach the tank’s position at Y=450 pixels. Success advances the player to progressively harder levels with increased speed and density.<\/p>\n
The beauty of this project lies in its architectural elegance. Rather than using a high-level programming language running on a processor, Space Force X employs dedicated hardware circuits for each function. The tank position is stored in a hardware register. Alien positions are managed by arrays of flip-flops. Collision detection happens combinatorially in the same nanosecond across all entities. The rendering happens in parallel, with every pixel decision made instantly based on current entity coordinates. This parallel processing capability demonstrates why FPGAs excel at real-time systems where responsiveness cannot be delayed by software execution times.<\/p>\n
The game implements a Mealy Finite State Machine with six distinct states: START_SCREEN (awaiting player input), PLAYING_LEVEL_1 (active gameplay on level one), PLAYING_LEVEL_2 (faster difficulty), LEVEL_ADVANCE (brief transition state), PAUSE_STATE (game frozen), and GAME_OVER (end screen). Each state has precisely defined entry conditions and exit conditions. State transitions happen synchronously on clock edges, ensuring deterministic behavior essential for reliable embedded systems. This FSM design teaches professional hardware architecture patterns used in aerospace, automotive, and industrial control applications.<\/p>\n
The game begins with a START_SCREEN displaying “SPACE FORCE X” and waiting for keyboard input. The moment a player presses any key, the FSM transitions to PLAYING_LEVEL_1. The screen now displays a formation of 24 aliens arranged in 3 rows of 8, positioned at the top of the screen. Simultaneously, the tank appears at the bottom center, ready to defend. The player controls movement using arrow keys: A moves the tank left (decrements the tank_x register by 4 pixels per clock cycle), D moves right (increments tank_x by 4), and W fires a bullet upward. The tank cannot move beyond screen boundaries (X must remain between 0 and 600).<\/p>\n
As the game progresses, aliens move downward at a constant rate controlled by a movement counter. Every 8 clock cycles, all alien Y-coordinates decrement by 2 pixels, creating the illusion of smooth descent. When the player presses W, a new bullet object is created at the current tank X position and begins moving upward at 5 pixels per cycle. The collision detection module runs combinatorially every cycle, comparing every bullet’s position against every alien’s position. The moment a bullet’s X-coordinate matches an alien’s X-coordinate (within a 10-pixel tolerance) AND the bullet’s Y-coordinate falls within the alien’s Y range (20 pixels tall), a collision is instantly detected. The alien is removed from the playfield, the bullet is destroyed, and the score increments by 100 points. Visual feedback occurs immediately\u2014some implementations flash the hit location or change colors briefly.<\/p>\n
The game continues until one of two end conditions occurs: (1) All aliens are destroyed, triggering LEVEL_ADVANCE\u2014a brief 2-second pause displaying “LEVEL COMPLETE” before advancing to PLAYING_LEVEL_2 where aliens move 2\u00d7 faster; or (2) Any alien reaches Y=450 (the tank position), triggering an immediate loss of life. The player begins with 3 lives. Each alien reaching the bottom costs one life. When lives reach 0, the game transitions to GAME_OVER, displaying the final score and waiting for the player to press the reset button (U) to return to START_SCREEN. The pause button (P) can freeze the game at any time, maintaining all entity positions and FSM state until pressed again.<\/p>\n
Component<\/strong><\/th>\n| Specification<\/strong><\/th>\n | Where to Buy<\/strong><\/th>\n | Approx Price<\/strong><\/th>\n<\/tr>\n | FPGA Board<\/strong><\/td>\n | Basys3 (Artix-7 XC7A35T)<\/td>\n | Digilent.com, Amazon, RS Components<\/td>\n | $99-120 USD<\/td>\n<\/tr>\n | VGA Monitor<\/strong><\/td>\n | Any 640\u00d7480+ display with VGA input<\/td>\n | Local electronics, eBay, used market<\/td>\n | $30-80 USD (used)<\/td>\n<\/tr>\n | VGA Cable<\/strong><\/td>\n | 15-pin D-sub VGA cable, minimum 2m<\/td>\n | Local computer shops, Amazon<\/td>\n | $5-15 USD<\/td>\n<\/tr>\n | PS\/2 Keyboard<\/strong><\/td>\n | Standard PS\/2 connection (purple connector)<\/td>\n | Computer shops, eBay, local hardware stores<\/td>\n | $10-25 USD<\/td>\n<\/tr>\n | USB Power Cable<\/strong><\/td>\n | Micro-USB to USB-A (included with board)<\/td>\n | Included; any USB power supply<\/td>\n | Included \/ $5-10<\/td>\n<\/tr>\n | Development Tools<\/strong><\/td>\n | Vivado Design Suite (free WebPACK edition)<\/td>\n | xilinx.com (free download)<\/td>\n | Free<\/td>\n<\/tr>\n<\/table>\n | Total Estimated Cost:<\/strong> $150-250 USD. The FPGA board is reusable for all 8 projects, so it’s a one-time investment. Monitors and keyboards may already be available.<\/p>\n Keyboard Input Processing:<\/strong> The PS\/2 keyboard continuously transmits scan codes to the FPGA’s PS\/2 interface module. The module converts these scan codes (which represent physical key positions) into ASCII values. The input handler monitors the ASCII values and sets digital flags: key_left, key_right, key_fire, key_pause, and key_reset. These flags persist as long as keys are held down, allowing smooth continuous movement.<\/p>\n Debouncing:<\/strong> Mechanical switches bounce\u2014they make and break contact multiple times within a few milliseconds. Without debouncing, a single button press would register as multiple rapid presses. The debounce circuit uses a simple counter: after detecting an input transition, it waits 20ms (2,000,000 clock cycles at 100 MHz) before accepting another change. If the input returns to its original state within 20ms, it’s considered noise and ignored. Only changes that persist for 20ms are considered valid inputs.<\/p>\n Multi-key Handling:<\/strong> The FPGA can detect multiple simultaneous key presses. A player can hold A to move left while pressing W to fire, and both actions execute immediately. This simultaneous input handling is one of the advantages of parallel hardware logic versus sequential software execution.<\/p>\n |
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