STM32 Bare-Metal RTOS (Cooperative Task Scheduler)

This project implements a minimal, no-libraries cooperative RTOS from scratch on the STM32F446RE microcontroller. It manages multiple tasks using a simple round-robin scheduler and displays system output on a 16x2 LCD. All hardware interactions are done through direct register access.

Features

• Written entirely in C with no HAL or CMSIS libraries
• Basic cooperative multitasking (3 tasks: LCD print, heartbeat, button press)
• Uses SysTick for millisecond timing and task delay control
• LCD1602 display integration via 4-bit GPIO interface
• Button handling on PA9 with software debounce
• Task scheduling and internal state debugged using GDB

Demo

Tasks Implemented

Task Name	Behavior	Interval
task_blink	Displays "Blinking..."	1000 ms
task_heartbeat	Displays "Task 2 is Alive"	2000 ms
task_button_check	Detects button press on PA9 and prints "Button Pressed!"	50 ms debounce check

Tools

• STM32F446RE Nucleo Board
• ARM-GCC toolchain
• GDB for live debugging
• Makefile build system (no IDE bloat)

How It Works

• Each task is a function with an associated delay and ready flag.
• SysTick_Handler increments a global time counter every millisecond.
• update_task_delays() is called each loop to check if tasks should be reactivated.
• Tasks self-schedule their next delay inside their own logic.
• run_scheduler() loops through task slots and runs ready tasks.

Getting Started

1. Clone repo
2. Flash to STM32F4 with your preferred tool (ST-Link, OpenOCD, etc.)
3. Connect LCD (PA5–PA8 for data, PA0, PA1, PA10 for control)
4. Connect pushbutton to PA9 and GND
5. Power on and watch tasks rotate live on LCD

Roadmap

• Add task state monitor to LCD
• Implement task counters
• Add UART logging
• Support dynamic task creation
• Preemptive scheduling via timer ISR

Author

Steven Reynoso
M.S. Computer Engineering, University of Michigan - Dearborn
https://github.com/StevenReynoso