The system is structured around the following hardware and software modules:

Hardware Modules:

• ATmega328P X-Mini — the central processing unit
• Passive Buzzer — driven by PWM (Timer1) to produce variable-frequency alarm tones
• Push Buttons (4x) — used to input the password digits (e.g. a 4-button combination lock)
• Reset Button — triggers a software or hardware reset of the system state
• Green LED — indicates correct password entry
• Red LED — indicates wrong password entry
• 16×2 LCD Display — displays system status messages

Bill of Materials:

Electrical Notes:

• All push buttons are wired with the internal pull-up resistors enabled (INPUT_PULLUP equivalent in bare metal: set DDRx bit to 0 and PORTx bit to 1). Buttons are active-low.
• LEDs are connected through 220Ω current-limiting resistors to GND.

Algorithms and Structures:

• Password Validation FSM:

The system uses a simple finite state machine with 5 states:

1. `IDLE` — system waiting, buzzer silent
2. `ARMED` — alarm active, buzzer playing alarm melody via PWM
3. `INPUT` — user is entering the password sequence
4. `CORRECT` — correct password: green LED on, buzzer off, LCD shows “ACCESS GRANTED”, auto-return to IDLE after timeout
5. `WRONG` — wrong password: red LED on, buzzer plays intensified tone, LCD shows “ACCESS DENIED”, auto-return to ARMED

Software design

• Editor: VS Code with the C and “AVR Utils” extensions for syntax highlighting and register lookup
• Debugging: LED-based state signaling and logic analyzer on PWM/GPIO lines
• Target: ATmega328P X-Mini

Libraries and 3rd-party sources:

• `avr/io.h` — direct register-level access to all ATmega328P peripherals (DDRx, PORTx, PINx, TCCRx, OCRx, etc.)
• `avr/interrupt.h` — enabling global interrupts (sei/cli) and defining ISR handlers
• `util/delay.h` — used exclusively in the LCD initialization sequence and LED hold timers; all alarm timing is handled via hardware timers

Block Design

Laboratories Used:

*The implementation of this project is based on the concepts studied in the following laboratories:

1. Laboratory 0 GPIO - Used for basic interfacing. I configured the pins for the LEDs as outputs and the pins for the push buttons as inputs with internal pull-up resistors enabled.
2. Laboratory 1 UART - Integrated for debugging purposes. The system transmits the current state and the entered sequence to a Serial Monitor, allowing for real-time monitoring of the logic.
3. Laboratory 2 Interrupts - Used for the reset button and potentially for handling button presses to ensure immediate response from the microcontroller without constant polling in the main loop.
4. Laboratory 3 Timers - Essential for the acoustic feedback.