STM32F446 Button LED State Machine

Embedded C firmware project for the STM32 Nucleo-F446RE developed using STM32CubeIDE.

This project demonstrates interrupt-driven button input, short-press vs long-press detection, and a finite state machine controlling multiple LED modes. The firmware also includes UART debug output to monitor system state transitions.

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Features

• Short press cycles LED modes:

  • Solid ON
  • Quick flash
  • SOS pattern

• Long press turns LED OFF from any mode

• Uses EXTI interrupts instead of polling

• Implements software debouncing

• Detects button press and release using rising and falling edge interrupts

• Uses interruptible timing so flashing and SOS patterns stop immediately when the button is pressed

• UART debug output via USART2 reports LED mode transitions

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Hardware

• STM32 Nucleo-F446RE
• Breadboard
• Pushbutton
• LED
• Current-limiting resistor
• Jumper wires

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Hardware Overview

Button ───> PA8 (EXTI interrupt input)

LED ──────> PB5 (GPIO output)

UART ─────> USART2 → ST-LINK Virtual COM Port → PC Terminal

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Pin Configuration

Pin	Function
PA8	Button input
PB5	LED output

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GPIO Setup

Button input

PA8 → GPIO_MODE_IT_RISING_FALLING
PA8 → GPIO_PULLDOWN

LED output

PB5 → GPIO_MODE_OUTPUT_PP

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Behavior

Short press cycles through LED modes:

OFF → ON → FLASH → SOS → ON

Long press from any mode turns the LED off:

(any mode) → OFF

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LED Modes

Solid ON

LED remains continuously on.

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Quick Flash

LED remains mostly off with a brief flash.

OFF 1000 ms
ON  100 ms

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SOS Pattern

Standard Morse code SOS pattern:

S = short short short
O = long long long
S = short short short

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Debug Output

The firmware provides UART debug messages via USART2 to report LED mode transitions.

Example terminal output:

MODE: ON
MODE: FLASH
MODE: SOS
MODE: OFF

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Concepts Demonstrated

This project demonstrates several important embedded firmware concepts:

• Embedded C programming
• STM32 HAL usage
• GPIO configuration
• External interrupt handling (EXTI)
• Button debouncing
• Short-press vs long-press detection
• Finite state machines
• Timing using HAL_GetTick()
• Interrupt-safe shared variables (volatile)
• Interruptible delay patterns
• UART debug logging

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Development Environment

• STM32CubeIDE
• STM32 HAL Drivers
• ARM GCC Toolchain

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Project Structure

Core/
Drivers/
STM32F446RETX_FLASH.ld
STM32F446RETX_RAM.ld
Interrupt1.ioc
README.md
.gitignore

The .ioc file allows the project to be reopened and modified using STM32CubeMX inside STM32CubeIDE.

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Purpose

This project was created as a learning exercise to explore real embedded firmware design patterns beyond simple LED blinking, including:

• interrupt-driven input handling
• state machine architecture
• responsive firmware behavior
• serial debugging

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Author

Michael Berry