TIM2 Interrupt LED Modes with UART Logging — STM32 NUCLEO-F446RE

A bare-metal STM32 project that uses TIM2 update interrupts to toggle the onboard LED and print timer tick information over USART2.

The firmware starts in normal blink mode, automatically switches to fast mode, then switches to slow mode.

---

Project Overview

This project demonstrates timer-based event generation using STM32 hardware timers.

Instead of using a software delay loop, the LED is toggled from the TIM2_IRQHandler() interrupt service routine.

The main loop prints timer tick updates over UART.

---

Behavior

Mode	Timer ARR	Approx. Interrupt Period	LED Behavior
Normal	999	1 second	LED toggles once per second
Fast	199	200 ms	LED toggles quickly
Slow	1999	2 seconds	LED toggles slowly

Automatic Mode Changes

The firmware changes modes automatically:

Start
  |
  | Tick 0 to 49
  v
Normal Mode
  |
  | At tick 50
  v
Fast Mode
  |
  | At tick 250
  v
Slow Mode

When a mode change happens, the STM32 prints a message over UART.

---

Expected UART Output

Example output:

Timer Started
==================
Tick: 0
Tick: 1
Tick: 2
...
Tick: 50

>>> Switching to FAST mode!

Tick: 51
Tick: 52
...
Tick: 250

>>> Switching to SLOW mode!

---

Hardware Required

• STM32 NUCLEO-F446RE
• USB cable

No external components are required.

---

Pin Usage

Function	Pin
Onboard LED	PA5
USART2_TX	PA2
USART2_RX	PA3

USART2 is connected to the PC through the onboard ST-Link Virtual COM Port.

---

UART Settings

Setting	Value
USART	USART2
Baud rate	9600
Data bits	8
Parity	None
Stop bits	1
Flow control	None

---

Concepts Demonstrated

• Bare-metal STM32 programming
• Direct register manipulation
• GPIO output configuration
• USART2 transmit
• TIM2 hardware timer configuration
• Timer prescaler, TIM2_PSC
• Auto-reload register, TIM2_ARR
• Timer update interrupt
• NVIC interrupt enabling
• Interrupt Service Routine, ISR
• Runtime timer period changes
• Volatile variables shared between ISR and main loop
• UART number printing
• Event flag pattern using mode_change_flag

---

Software Architecture

TIM2 Hardware Timer
        |
        v
Update Event
        |
        v
NVIC triggers TIM2_IRQHandler()
        |
        v
ISR toggles LED
        |
        v
ISR increments timer_ticks
        |
        v
ISR changes mode when tick threshold is reached
        |
        v
main() prints tick and mode change messages over UART

---

Timer Configuration

The project assumes a 16 MHz timer clock.

TIM2_PSC = 15999;
TIM2_ARR = 999;

This produces an initial 1-second timer interrupt:

16 MHz / (15999 + 1) = 1000 Hz
1000 counts per second using ARR = 999

Later, the firmware changes TIM2_ARR:

TIM2_ARR = 199;   // Fast mode
TIM2_ARR = 1999;  // Slow mode

---

Important Note

timer_ticks counts timer update events, not always real seconds.

In normal mode, one tick is approximately 1 second.

In fast mode, one tick is approximately 200 ms.

In slow mode, one tick is approximately 2 seconds.

---

Build and Flash

1. Open STM32CubeIDE.
2. Create a project for NUCLEO-F446RE.
3. Replace Core/Src/main.c with the code from src/main.c.
4. Build the project.
5. Flash it to the board.
6. Open a UART terminal at 9600 baud.
7. Watch timer tick messages and LED mode changes.

---

Linux Serial Terminal

Find the serial port:

ls /dev/ttyACM* /dev/ttyUSB* 2>/dev/null

Open with minicom:

minicom -D /dev/ttyACM0 -b 9600

If no message appears, press the reset button on the NUCLEO board after opening the terminal.

More details:

docs/terminal_setup.md

---

Project Structure

stm32-tim2-interrupt-led-uart/
│
├── README.md
├── .gitignore
├── docs/
│   ├── wiring.md
│   └── terminal_setup.md
└── src/
    └── main.c

---

Future Improvements

• Add button-controlled mode switching
• Use separate real-time seconds counter
• Print current mode with every tick
• Add UART command interface to change timer speed
• Use timer interrupts for accurate non-blocking delays
• Add PWM output using timer channels
• Add low-power sleep while waiting for timer interrupt