The FreeRTOS Tutorial Books
provide additional detailed information on tasks and their behaviour.
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
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.
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:
See the Embedded software application design section for more information on
using an idle hook.
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.
- Create an idle priority task to implement the functionality.
This is a more flexible solution but has a higher RAM usage overhead.
To create an idle hook:
It is common to use the idle hook function to place the microcontroller CPU into
a power saving mode.
- Set configUSE_IDLE_HOOK to 1 in FreeRTOSConfig.h.
- Define a function that has the following name and prototype:
void vApplicationIdleHook( void );
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