Tutorial: FreeRTOS Multi-Task Basics and GPIO Control

0. Experiment Overview

This experiment is a mandatory introduction to Embedded Real-Time Operating Systems (RTOS). By creating two tasks with different priorities to control two LEDs blinking at different frequencies, it visually demonstrates FreeRTOS’s multi-task scheduling, priority preemption, and time management mechanisms.

Learning Objectives

1. Understand the concept of a FreeRTOS “Task” and how it differs from bare-metal while(1).
2. Master the method of configuring FreeRTOS tasks using STM32CubeMX.
3. Deeply understand the meaning of each parameter in osThreadNew when creating a task.
4. Learn to analyze the scheduling behavior of tasks with different priorities.

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1. Prerequisites

1.1 What is a FreeRTOS Task?

In a bare-metal program, we typically execute code sequentially within a dead loop while(1). In an RTOS, the program is split into multiple independent “tasks”. Each task appears to 独占 CPU, but the OS kernel (Scheduler) rapidly switches between tasks based on priority and time slice, achieving macroscopic “parallelism”.

1.2 Hardware Basics

• MCU: STM32F407 (Cortex-M4)
• LED Connections:
  • LED1 -> PB2 (Push-Pull Output)
  • LED2 -> PB8 (Push-Pull Output)
• Clock: System main frequency 168MHz, SysTick or TIM14 provides the heartbeat tick.

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2. Principle Analysis

2.1 Task Priority and Scheduling

This experiment creates two tasks:

• Task PB2: Priority Normal (higher)
• Task PB8: Priority Low (lower)

Scheduling Rules: FreeRTOS uses priority-based preemptive scheduling.

• When a higher-priority task is ready, the CPU immediately executes it.
• Only when the higher-priority task enters blocking state (e.g., calling osDelay) does the lower-priority task get a chance to run.
• In this experiment, both tasks spend most of their time in osDelay blocking, so they alternate execution without interfering with each other.

2.2 Time Delay (osDelay)

• osDelay(1000): Means the task is suspended for 1000 system ticks.
• Assuming the system tick frequency is 1000Hz (1ms), the delay time is 1 second.
• Key Point: During the delay, this task does not consume CPU resources; the CPU can execute other ready lower-priority tasks. This is the core of efficient multi-tasking.

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3. In-Depth Code Analysis

The following teaching breakdown is based on Core/Src/main.c.

3.1 Task Attribute Definitions

In the global variable section of main.c, the task’s “ID card” is defined.