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[More about tasks...]

The FreeRTOS Tutorial Books provide additional detailed information on tasks and their behaviour.

The Idle Task

The idle task is created automatically when the RTOS scheduler is started to ensure there is always at least one task that is able to run. It is created at the lowest possible priority to ensure it does not use any CPU time if there are higher priority application tasks in the ready state.

The idle task is responsible for freeing memory allocated by the RTOS to tasks that have since been deleted. It is therefore important in applications that make use of the vTaskDelete() function to ensure the idle task is not starved of processing time. The idle task has no other active functions so can legitimately be starved of microcontroller time under all other conditions.

It is possible for application tasks to share the idle task priority (tskIDLE_PRIORITY). See the configIDLE_SHOULD_YIELD configuration parameter for information on how this behaviour can be configured.

The Idle Task Hook

An idle task hook is a function that is called during each cycle of the idle task. If you want application functionality to run at the idle priority then there are two options:
  1. Implement the functionality in an idle task hook.

    There must always be at least one task that is ready to run. It is therefore imperative that the hook function does not call any API functions that might cause the idle task to block (vTaskDelay(), or a queue or semaphore function with a block time, for example). It is ok for co-routines to block within the hook function.

  2. Create an idle priority task to implement the functionality.

    This is a more flexible solution but has a higher RAM usage overhead.

See the Embedded software application design section for more information on using an idle hook.

To create an idle hook:

  1. Set configUSE_IDLE_HOOK to 1 in FreeRTOSConfig.h.

  2. Define a function that has the following name and prototype:

    void vApplicationIdleHook( void );

It is common to use the idle hook function to place the microcontroller CPU into a power saving mode.

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