Introduction to RTOS Part 2 - Getting Started with FreeRTOS | Digi-Key Electronics

Introduction to FreeRTOS on ESP32

Overview of the Series

• The series will cover general RTOS concepts and demonstrate their implementation using FreeRTOS on an ESP32.

• The Arduino IDE will be used for simplicity, starting with downloading the FreeRTOS source code.

Getting Started with FreeRTOS

• Visit freeRTOS.org to download the source code; documentation is available under Kernel > Getting Started for building demo applications.

• Including FreeRTOS in a project requires specific C and header files, along with selecting a heap management file. A microcontroller timer must be assigned as the RTOS tick timer.

Resources for Learning FreeRTOS

Recommended Reading

• For assistance, download "Mastering the FreeRTOS Real-Time Kernel" and the reference manual from the books link on freeRTOS.org.

Understanding Library Structure

• The FreeRTOS library functions as a scheduler; additional drivers are included in the FreeRTOS Plus library primarily for networking (TCP/UDP).

• Example projects can be found in the demo folder, which includes config files defining important variables necessary for your project setup.

ESP32 Specific Considerations

Modified Version of FreeRTOS

• The ESP32 uses a modified version of FreeRTOS within its ESP-IDF framework to support its dual-core processor architecture (SMP).

Limiting Core Usage

• This series will limit tasks to one core for simplicity; understanding multi-core support can be explored later through ESP32 documentation.

Setting Up Arduino IDE for ESP32

Installation Steps

• Install Arduino IDE and add Espressif's board manager URL: https://dl.espressif.com/dl/package_esp32_index.json.

Configuring Board Settings

• Search and install the latest version of Espressif Systems' ESP32 package via Tools > Board > Board Manager.

FreeRTOS Configuration on ESP32

Accessing Configuration Files

• Locate FreeRTOS config.h file in your Arduino board packages directory to understand how FreeRTOS is configured specifically for ESP32.

Key Configuration Insights

• The configuration allows up to 25 priority levels and sets a minimum task stack size of 768 bytes. Use this file as a reference when adjusting settings.

Creating Tasks in FreeRTOS

Defining Tasks

• Define constants that restrict task creation to one core; typically not recommended but useful for learning purposes.

Implementing Blinky Program

Understanding RTOS and Task Management in FreeRTOS

Tick Timer and Task Scheduling

• Most Real-Time Operating Systems (RTOS) utilize a tick timer, which is a hardware timer that interrupts the processor at regular intervals.

• The interval of these interrupts is referred to as a "tick," allowing the scheduler to determine which task should run during each tick.

• In FreeRTOS, the default tick period is set to one millisecond, with portTICK_PERIOD_MS defined as one.

Creating Tasks in FreeRTOS

• The function vTaskDelay requires ticks for delay rather than milliseconds; thus, desired milliseconds must be divided by the tick period.

• To create tasks, xTaskCreatePinnedToCore can be used to specify which core will execute the task; this function is not available in Vanilla FreeRTOS.

• While xTaskCreate works in ESP-IDF, it allows the scheduler to choose any core for task execution.

Task Parameters and Configuration

• When creating a task, parameters include specifying the function to call, naming the task, setting stack size (minimum 768 bytes), and defining priority levels (0 being lowest).

• A pointer or handle can be assigned for managing tasks from other tasks or loops; this helps monitor status and memory usage.

Execution Flow in ESP32 Environment

• In Arduino framework for ESP32, setup and loop functions operate within their own tasks separate from main program entry points.

• New tasks begin executing automatically without needing to call vTaskStartScheduler, as it has already been invoked prior to setup.

Practical Application: LED Blinking Challenge

• After configuring everything correctly, an LED should blink once per second as a basic outcome of the setup process.