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freertos: Update FreeRTOS overview document
This commit updates "freertos.rst" to act as an overview document of FreeRTOS in ESP-IDF, outlining the different FreeRTOS implementations (IDF vs Amazon) and various supplemental features. The details of each implementation will be separated to their own documents.
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Darian Leung
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FreeRTOS
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========
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FreeRTOS (Overview)
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===================
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Overview
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--------
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This section contains documentation of FreeRTOS types, functions, and macros. It is automatically generated from FreeRTOS header files.
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FreeRTOS is an open source real-time operating system kernel that acts as the operating system for ESP-IDF applications and is integrated into ESP-IDF as a component. The FreeRTOS component in ESP-IDF contains ports of the FreeRTOS kernel for all the CPU architectures used by ESP targets (i.e., Xtensa and RISC-V). Furthermore, ESP-IDF provides different implementations of FreeRTOS in order to support SMP (Symmetric Multiprocessing) on multi-core ESP targets. This document provides an overview of the FreeRTOS component, the FreeRTOS implementations offered by ESP-IDF, and the common aspects across all implementations.
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Implementations
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---------------
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The `official FreeRTOS <https://www.freertos.org/index.html>`_ (henceforth referred to as Vanilla FreeRTOS) is a single-core RTOS. In order to support the various multi-core ESP targets, ESP-IDF supports different FreeRTOS implementations, namely **ESP-IDF FreeRTOS** and **Amazon SMP FreeRTOS**.
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ESP-IDF FreeRTOS
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^^^^^^^^^^^^^^^^
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ESP-IDF FreeRTOS is a FreeRTOS implementation based on Vanilla FreeRTOS v10.4.3, but contains significant modifications to support SMP. ESP-IDF FreeRTOS only supports two cores at most (i.e., dual core SMP), but is more optimized for this scenario by design. For more details regarding ESP-IDF FreeRTOS and its modifications, please refer to the :doc:`freertos_idf` document.
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.. note::
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ESP-IDF FreeRTOS is based on Vanilla FreeRTOS v10.4.3
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ESP-IDF FreeRTOS is currently the default FreeRTOS implementation for ESP-IDF.
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- For more information about the SMP changes of ESP-IDF FreeRTOS, see :doc:`/api-guides/freertos-smp`
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- For more information about the features added to ESP-IDF FreeRTOS, see :doc:`/api-reference/system/freertos_additions`.
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Amazon SMP FreeRTOS
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^^^^^^^^^^^^^^^^^^^
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Amazon SMP FreeRTOS is an SMP implementation of FreeRTOS that is officially supported by Amazon. Amazon SMP FreeRTOS is able to support N-cores (i.e., more than two cores). Amazon SMP FreeRTOS can be enabled via the :ref:`CONFIG_FREERTOS_SMP` option. For more details regarding Amazon SMP FreeRTOS, please refer to the `official Amazon SMP FreeRTOS documentation <https://freertos.org/symmetric-multiprocessing-introduction.html>`_.
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.. warning::
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The Amazon SMP FreeRTOS implementation (and its port in ESP-IDF) are currently in experimental/beta state. Therefore, significant behavioral changes and breaking API changes can occur.
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Configuration
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-------------
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Vanilla FreeRTOS allows ports and applications to configure the kernel by adding various ``#define config...`` macros to ``FreeRTOSConfig.h``. Through these macros, the kernel's scheduling behavior and various kernel features can be enabled or disabled. **However, in ESP-IDF FreeRTOS, the ``FreeRTOSConfig.h`` file is considered a private and must not be modified by users**. Any FreeRTOS configuration that is exposed to the user will be done so via menuconfig.
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Kernel Configuration
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^^^^^^^^^^^^^^^^^^^^
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ESP-IDF FreeRTOS can be configured in the project configuration menu (``idf.py menuconfig``) under ``Component Config/FreeRTOS``. The following section highlights some of the ESP-IDF FreeRTOS configuration options. For a full list of ESP-IDF FreeRTOS configurations, see :doc:`/api-reference/kconfig`
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Vanilla FreeRTOS requires that ports and applications configure the kernel by adding various ``#define config...`` macros to ``FreeRTOSConfig.h``. Vanilla FreeRTOS supports a list of kernel configuration options which allow various kernel behaviors and features to be enabled or disabled.
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- :ref:`CONFIG_FREERTOS_UNICORE` will run ESP-IDF FreeRTOS only on CPU0. Note that this is **not equivalent to running Vanilla FreeRTOS**. Futhermore, this option may affect behavior of components other than :component:`freertos`. For more details regarding the effects of running ESP-IDF FreeRTOS on a single core, refer to :ref:`freertos-smp-single-core`. Alternatively, users can also search for occurrences of ``CONFIG_FREERTOS_UNICORE`` in the ESP-IDF components.
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**However, for all FreeRTOS ports in ESP-IDF, the ``FreeRTOSConfig.h`` file is considered private and must not be modified by users**. A large number of kernel configuration options in ``FreeRTOSConfig.h`` are hard coded as they are either required or not supported in ESP-IDF. All kernel configuration options that are configurable by the user will be exposed via menuconfig under ``Component Config/FreeRTOS/Kernel``.
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- :ref:`CONFIG_FREERTOS_ASSERT_ON_UNTESTED_FUNCTION` will trigger a halt in functions in ESP-IDF FreeRTOS that have not been fully tested in an SMP context.
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For the full list of user configurable kernel options, see :doc:`/api-reference/kconfig`. The list below highlights some commonly used kernel configuration options:
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- :ref:`CONFIG_FREERTOS_TASK_FUNCTION_WRAPPER` will enclose all task functions within a wrapper function. In the case that a task function mistakenly returns (i.e. does not call :cpp:func:`vTaskDelete`), the call flow will return to the wrapper function. The wrapper function will then log an error and abort the application, as illustrated below::
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E (25) FreeRTOS: FreeRTOS task should not return. Aborting now!
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abort() was called at PC 0x40085c53 on core 0
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- :ref:`CONFIG_FREERTOS_UNICORE` will run FreeRTOS only on CPU0. Note that this is **not equivalent to running Vanilla FreeRTOS**. Furthermore, this option may affect behavior of components other than :component:`freertos`. For more details regarding the effects of running FreeRTOS on a single core, refer to :ref:`freertos-smp-single-core` (if using ESP-IDF FreeRTOS) or the official Amazon SMP FreeRTOS documentation. Alternatively, users can also search for occurrences of ``CONFIG_FREERTOS_UNICORE`` in the ESP-IDF components.
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.. only:: CONFIG_FREERTOS_UNICORE
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.. note::
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As {IDF_TARGET_NAME} is a single core SoC, the :ref:`CONFIG_FREERTOS_UNICORE` configuration is always set.
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- :ref:`CONFIG_FREERTOS_ENABLE_BACKWARD_COMPATIBILITY` enables backward compatibility with some FreeRTOS macros/types/functions that were deprecated from v8.0 onwards.
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.. _freertos-applications:
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Port Configuration
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^^^^^^^^^^^^^^^^^^
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ESP-IDF FreeRTOS Applications
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-----------------------------
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All other FreeRTOS related configuration options that are not part of the kernel configuration are exposed via menuconfig under ``Component Config/FreeRTOS/Port``. These options configure aspects such as:
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Unlike Vanilla FreeRTOS, users must not call :cpp:func:`vTaskStartScheduler`. Instead, ESP-IDF FreeRTOS is started automatically. The entry point is a user defined ``void app_main(void)`` function.
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- The FreeRTOS ports themselves (e.g., tick timer selection, ISR stack size)
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- Additional features added to the FreeRTOS implementation or ports
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- Typically, users would spawn the rest of their applications task from ``app_main``.
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Using FreeRTOS
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--------------
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Application Entry Point
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^^^^^^^^^^^^^^^^^^^^^^^
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Unlike Vanilla FreeRTOS, users of FreeRTOS in ESP-IDF **must never call** :cpp:func:`vTaskStartScheduler` and :cpp:func:`vTaskEndScheduler`. Instead, ESP-IDF will start FreeRTOS automatically. Users must define a ``void app_main(void)`` function which acts as the entry point for user's application and is automatically called on ESP-IDF startup.
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- Typically, users would spawn the rest of their application's task from ``app_main``.
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- The ``app_main`` function is allowed to return at any point (i.e., before the application terminates).
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- The ``app_main`` function is called from the ``main`` task.
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- The ``app_main`` function is called from the ``main`` task.
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The ``main`` task is one of multiple tasks that are automatically spawned by ESP-IDF during startup. These tasks are:
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Background Tasks
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^^^^^^^^^^^^^^^^
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.. only:: not CONFIG_FREERTOS_UNICORE
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During startup, ESP-IDF and FreeRTOS will automatically create multiple tasks that run in the background (listed in the the table below).
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.. list-table:: List of Tasks Created During Startup
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:widths: 25 25 5 50
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:header-rows: 1
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.. list-table:: List of Tasks Created During Startup
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:widths: 10 75 5 5 5
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:header-rows: 1
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* - Task Name
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- Affinity
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- Priority
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- Description
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* - Main Task (``main``)
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- CPU0
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- 1
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- Task that simply calls ``app_main``. This task will self delete when ``app_main`` returns
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* - Idle Tasks (``IDLEx``)
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- CPU0 and CPU1
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- 0
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- Idle tasks created for (and pinned to) each CPU
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* - IPC Tasks (``ipcx``)
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- CPU0 and CPU1
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- 24
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- IPC tasks created for (and pinned to ) each CPU. IPC tasks are used to implement the IPC feature. See :doc:`/api-reference/system/ipc` for more details.
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* - Task Name
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- Description
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- Stack Size
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- Affinity
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- Priority
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* - Idle Tasks (``IDLEx``)
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- An idle task (``IDLEx``) is created for (and pinned to) each CPU, where ``x`` is the CPU's number.
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- :ref:`CONFIG_FREERTOS_IDLE_TASK_STACKSIZE`
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- CPUx
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- ``0``
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* - FreeRTOS Timer Task (``Tmr Svc``)
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- FreeRTOS will create the Timer Service/Daemon Task if any FreeRTOS Timer APIs are called by the application.
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- :ref:`CONFIG_FREERTOS_TIMER_TASK_STACK_DEPTH`
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- CPU0
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- :ref:`CONFIG_FREERTOS_TIMER_TASK_PRIORITY`
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* - Main Task (``main``)
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- Task that simply calls ``app_main``. This task will self delete when ``app_main`` returns
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- :ref:`CONFIG_ESP_MAIN_TASK_STACK_SIZE`
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- :ref:`CONFIG_ESP_MAIN_TASK_AFFINITY`
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- ``1``
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* - IPC Tasks (``ipcx``)
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- When :ref:`CONFIG_FREERTOS_UNICORE` is false, an IPC task (``ipcx``) is created for (and pinned to) each CPU. IPC tasks are used to implement the Inter-processor Call (IPC) feature.
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- :ref:`CONFIG_ESP_IPC_TASK_STACK_SIZE`
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- CPUx
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- ``24``
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* - ESP Timer Task (``esp_timer``)
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- ESP-IDF will create the ESP Timer Task used to process ESP Timer callbacks.
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- :ref:`CONFIG_ESP_TIMER_TASK_STACK_SIZE`
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- CPU0
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- ``22``
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.. only:: CONFIG_FREERTOS_UNICORE
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.. note::
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Note that if an application uses other ESP-IDF features (e.g., WiFi or Bluetooth), those features may create their own background tasks in addition to the tasks listed in the table above.
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.. list-table:: List of Tasks Created During Startup
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:widths: 25 25 5 50
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:header-rows: 1
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* - Task Name
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- Affinity
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- Priority
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- Description
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* - Main Task (``main``)
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- CPU0
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- 1
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- Task that simply calls ``app_main``. This task will self delete when ``app_main`` returns
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* - Idle Tasks (``IDLEx``)
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- CPU0 and CPU1
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- 0
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- Idle task created for (and pinned to) each CPU
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.. note::
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Low priority numbers denote low priority tasks.
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Task API
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--------
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.. include-build-file:: inc/task.inc
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Queue API
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---------
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.. include-build-file:: inc/queue.inc
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Semaphore API
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-------------
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.. include-build-file:: inc/semphr.inc
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Timer API
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---------
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.. include-build-file:: inc/timers.inc
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Event Group API
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---------------
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.. include-build-file:: inc/event_groups.inc
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Stream Buffer API
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-----------------
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.. include-build-file:: inc/stream_buffer.inc
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Message Buffer API
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FreeRTOS Additions
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------------------
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.. include-build-file:: inc/message_buffer.inc
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ESP-IDF provides some supplemental features to FreeRTOS such as Ring Buffers, ESP-IDF style Tick and Idle Hooks, and TLSP deletion callbacks. See :doc:`freertos_additions` for more details.
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