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Lights-Out Game (3x3)

This project implements a simple 3x3 Lights-Out game in Verilog.

The goal of the game is to turn all LEDs off. Pressing a button toggles that LED and its neighboring LEDs.

Reset → board is empty.
Press any button → random board loads.
Press buttons to toggle LEDs.
When all LEDs are off → DONE goes high.
Press any button again → a new random game starts.

Wokwi Simulation: https://wokwi.com/projects/446364136220529665
Breakout Board KiCAD: https://github.com/BMellor/tiny-tapeout-breakout

How it works

1. High-Level Overview

The design includes:

A 3x3 LED matrix
3 row button inputs (matrix-scanned to detect 9 buttons)
LED multiplexing logic
Button debouncing
A 16-bit LFSR for random game generation
Lights-Out toggle logic
A DONE signal when all LEDs are off

2. LED Matrix Representation

The game board is stored in:

reg [8:0] leds;

This 9-bit register represents the 3x3 grid in row-major order:

Index layout:

0 1 2  
3 4 5  
6 7 8

If a bit is:
1 → LED is ON
0 → LED is OFF

3. LED Multiplexing (Column Scanning)

The design uses column multiplexing:

reg [1:0] active_col;

active_col cycles 0→1→2 repeatedly.
Only one column is active at a time.
Row outputs are driven according to:
- The active column
- The stored LED state

This allows a 3×3 grid to be driven using:

3 row lines
3 column lines

Because scanning happens at 1kHz, the display appears continuously lit.

4. DONE Signal (Win Detection)

DONE is assigned as:

DONE = !(|leds);

If any LED is on, DONE = 0.
If all LEDs are off, DONE = 1.

The player wins when the board is completely cleared.

5. Button Matrix and Debouncing

There are 3 physical button inputs:

BTN_ROW0  
BTN_ROW1  
BTN_ROW2

Because columns are scanned, each row line corresponds to a different button depending on the active column:

Column 0 → buttons 0, 3, 6
Column 1 → buttons 1, 4, 7
Column 2 → buttons 2, 5, 8

Each of the 9 buttons has a 16-bit shift register:

reg [15:0] btn_shift[0:8];

These are used for debouncing.

How it works:

On each clock cycle, the current button value shifts into the register.
If the register becomes 16'h7FFF (15 stable 1s after a 0), a clean rising-edge press is detected.
When this happens, btn_debounced[i] is set to 1 for one clock cycle.

This produces a single clean pulse per valid press.

6. Random Game Generation (LFSR)

A 16-bit Linear Feedback Shift Register (LFSR) runs continuously. Each clock cycle, the LFSR shifts and computes a new feedback bit.

Polynomial:
x^16 + x^14 + x^13 + x^11

When the board is empty (leds == 0) and any button is pressed, the lower 9 bits of the LFSR are loaded as a new random board.

leds <= lfsr[8:0];

This allows a new game to start after a win.

7. Lights-Out Toggle Logic

Each button has a predefined toggle mask such as:

localparam [8:0] TOGGLE_MASK0 = 9'b000001011;

Each mask determines:

The pressed LED
Its neighboring LEDs (up, down, left, right)

When a button is pressed the XOR operation flips the selected bits.

leds <= leds ^ TOGGLE_MASKx;

The board update is the XOR of:

The current board
The mask for any pressed button

Generally only one button press is processed at a time due to debouncing.

External Hardware

There is a pre-designed breakout board at the following repo: https://github.com/BMellor/tiny-tapeout-breakout

I/O Pin Description

Pin Name	Signal Name	Function	Connection / Usage
IN_0	BTN_ROW0	Row 0 button input for scanned button matrix.	Connect to row 0 of 3x3 button matrix (pull-down resistor required).
IN_1	BTN_ROW1	Row 1 button input for scanned button matrix.	Connect to row 1 of 3x3 button matrix (pull-down resistor required).
IN_2	BTN_ROW2	Row 2 button input for scanned button matrix.	Connect to row 2 of 3x3 button matrix (pull-down resistor required).
OUT_0	LED_ROW0	Drives LED row 0 of the matrix.	Connect to row 0 of LED matrix (Cathode of LED's through current-limiting resistor).
OUT_1	LED_ROW1	Drives LED row 1 of the matrix.	Connect to row 1 of LED matrix (Cathode of LED's through current-limiting resistor).
OUT_2	LED_ROW2	Drives LED row 2 of the matrix.	Connect to row 2 of LED matrix (Cathode of LED's through current-limiting resistor).
OUT_3	COL0	Column 0 select for LED drive and button scan.	Connect to column 0 of LED matrix (Anode of LED's) and button matrix.
OUT_4	COL1	Column 1 select for LED drive and button scan.	Connect to column 1 of LED matrix (Anode of LED's) and button matrix.
OUT_5	COL2	Column 2 select for LED drive and button scan.	Connect to column 2 of LED matrix (Anode of LED's) and button matrix.
OUT_6	DONE	Indicates win condition (all LEDs off).	Connect to anode of indicator LED

Notes

The LED and button matrix share the same 3 column lines (COL0–COL2).
Add pull-down resistors on button row inputs to prevent them from floating when the buttons aren't pressed.
Add current limiting resistors on LED row outputs, not the columns.

428 Lights-Out

428 : Lights-Out