Difference between revisions of "RTOS Basics : TASK"

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In that case it executes the task with same priority in time slice manner.
 
In that case it executes the task with same priority in time slice manner.
  
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==API Details==
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 +
==API Details==
 
Here we will discuss some of the mostly frequently used APIs related to task.
 
Here we will discuss some of the mostly frequently used APIs related to task.
 
====Task Create function====
 
====Task Create function====
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<syntaxhighlight>
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−
portBASE_TYPE xTaskCreate  
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{|{{Widget:LibCol}}
 +
{{#Widget:LibTable}}
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|-
 +
|Defination ||portBASE_TYPE xTaskCreate  
 
( pdTASK_CODE pvTaskCode,
 
( pdTASK_CODE pvTaskCode,
 
const portCHAR * const pcName,
 
const portCHAR * const pcName,
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unsigned portBASE_TYPE uxPriority,
 
unsigned portBASE_TYPE uxPriority,
 
xTaskHandle *pvCreatedTask);
 
xTaskHandle *pvCreatedTask);
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</syntaxhighlight>
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|-
−
 
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| Input Arguments ||#pdTASK_CODE : Pointer to the task entry function
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Name : xTaskCreate()
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−
 
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−
Arguments :
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−
 
+
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:pvTaskCode : Pointer to the task entry function
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:pcName : A descriptive name for the task. It is a character pointer and is mainly used for debugging.
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#const portCHAR * : A descriptive name for the task. It is a character pointer and is mainly used for debugging.
 
 
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:usStackDepth : The stack size of the task in terms of number of variables not the number of bytes
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#unsigned portSHORT : The stack size of the task in terms of number of variables not the number of bytes
 
 
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:pvParameters : void pointer which will be used as the parameter for the task  
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#void* : void pointer which will be used as the parameter for the task  
 
 
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:uxPriority : The task priority
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#unsigned portBASE_TYPE : The task priority
 
 
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:pvCreatedTask : Used to pass back a handle using which the task can be referenced
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#xTaskHandle* : Used to pass back a handle using which the task can be referenced
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|-
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| Return Value|| Returns TRUE on successful creation of task and adding it to ready list FALSE otherwise.
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|-
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| Description || It creates a task and adds it to ready list.
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|-
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| Usage || xTaskHandle TaskHandle_1;
  
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Return type : Returns TRUE on successful creation of task and adding it to ready list
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static void MyTask1(void* pvParameters);
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+
 
−
:::Returns FALSE otherwise.
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xTaskCreate( MyTask1, ( signed char * )"Task1", configMINIMAL_STACK_SIZE, NULL, 1, &TaskHandle_1 );
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|}
  
−
Function : It creates a task and adds it to ready list.
 
  
 
====Task Delay Function====
 
====Task Delay Function====
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<syntaxhighlight>void vTaskDelay( portTickType xTicksToDelay )</syntaxhighlight>
 
−
 
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Name : vTaskDelay()
 
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−
Arguments : Number of ticks for which task is to be delayed.
 
−
 
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Return type : None
 
  
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Function : It delays the calling task for specified number of ticks.
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{|{{Widget:LibCol}}
 +
{{#Widget:LibTable}}
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|-
 +
|Defination ||void vTaskDelay( portTickType xTicksToDelay )
 +
|-
 +
| Input Arguments ||portTickType : Number of ticks for which task is to be delayed.
 +
|-
 +
| Return Value|| None
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|-
 +
| Description || It delays the calling task for specified number of ticks.
  
 
Set the macro INCLUDE_vTaskDelay to use this API.
 
Set the macro INCLUDE_vTaskDelay to use this API.
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Use the constant portTICK_RATE_MS in the file portmacro.h to calculate real time delay.  
 
Use the constant portTICK_RATE_MS in the file portmacro.h to calculate real time delay.  
 
Its resolution is one tick period.
 
Its resolution is one tick period.
 +
|-
 +
| Usage || vTaskDelay(100);
 +
|}
 +
  
 
====Task Delete Function====
 
====Task Delete Function====
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<syntaxhighlight>void vTaskDelete( xTaskHandle pxTask )</syntaxhighlight>
 
  
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Name : vTaskDelete()
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{|{{Widget:LibCol}}
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{{#Widget:LibTable}}
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Arguments : The handle (of type xTaskHandle) of the task to be deleted. Pass NULL to delete calling task.
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|-
−
 
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|Defination ||void vTaskDelete( xTaskHandle pxTask )
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Return type : None
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|-
−
 
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| Input Arguments ||xTaskHandle : handle to the task to be deleted. Pass NULL to suspend calling task
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Function : It removes the specified  task from the RTOS real time kernels management.
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|-
 +
| Return Value|| None
 +
|-
 +
| Description || It removes the specified  task from the RTOS real time kernels management.
 
But it does not frees the memory allocated by the task code.
 
But it does not frees the memory allocated by the task code.
  
 
Set the macro INCLUDE_vTaskDelete to use this API.
 
Set the macro INCLUDE_vTaskDelete to use this API.
 +
|-
 +
| Usage || vTaskDelete(TaskHandle_1);
 +
|}
  
  
 
====Task Suspend Function====
 
====Task Suspend Function====
−
<syntaxhighlight>
 
−
void vTaskSuspend( xTaskHandle pxTaskToSuspend );
 
−
</syntaxhighlight>
 
  
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Name : vTaskSuspend()
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{|{{Widget:LibCol}}
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+
{{#Widget:LibTable}}
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Arguments :
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|-
−
 
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|Defination ||void vTaskSuspend( xTaskHandle pxTaskToSuspend );
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:pxTaskToSuspend : handle to the task to be suspended. Pass NULL to suspend calling task
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|-
−
 
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| Input Arguments ||xTaskHandle : handle to the task to be suspended. Pass NULL to suspend calling task
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Return Type : None
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|-
−
 
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| Return Value|| None
−
Function : It suspends the specified task
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|-
 +
| Description || It suspends the specified task
  
 
Set INCLUDE_vTaskSuspend to use this API.
 
Set INCLUDE_vTaskSuspend to use this API.
 +
|-
 +
| Usage || vTaskSuspend(TaskHandle_1);
 +
|}
  
 
====Task Resume Function====
 
====Task Resume Function====
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<syntaxhighlight>
 
−
void vTaskResume( xTaskHandle pxTaskToSuspend );
 
−
</syntaxhighlight>
 
  
−
Name : vTaskResume()
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{|{{Widget:LibCol}}
 +
{{#Widget:LibTable}}
 +
|-
 +
|Defination ||void vTaskResume( xTaskHandle pxTaskToSuspend );
 +
|-
 +
| Input Arguments ||xTaskHandle : handle to the task to be resumed. Pass NULL to suspend the calling task.
 +
|-
 +
| Return Value|| None
 +
|-
 +
| Description || It resumes the specified task
  
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Arguments :
+
Set INCLUDE_vTaskSuspend to use this API.
 +
|-
 +
| Usage || vTaskResume(TaskHandle_1);
 +
|}
  
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:pxTaskToSuspend : handle to the task to be resumed. Pass NULL to suspend calling task
 
  
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Return Type : None
 
−
 
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Function : It resumes the specified task
 
−
 
−
Set INCLUDE_vTaskSuspend to use this API.
 
  
 
{{DISQUS}}
 
{{DISQUS}}

Revision as of 14:05, 14 April 2015

Amruta (talk) 13:41, 8 April 2015 (IST)

Intro

In RTOS implementation of a design, the program is divided into different independent functions what we call as a task. These functions are not called anywhere in the program, they are just created. Each tasks runs continuously i.e. in infinite loop.

Now, you may get question, how the application works ?

Here the kernel comes in picture. To know more about kernel check RTOS basics tutorial.

Different states of a task

In the FreeRTOS a task can be in either of four different states viz., Running, Ready, Blocked and Suspended.

Lets see each state one by one.

  • Running

The task which is executing currently is said to be in running state. It owns the CPU.

  • Ready

The task which is neither suspended nor blocked but still not executing will be ready state. Its not in running state because either a high priority or equal priority task is executing.

  • Blocked

A task will go in blocked state whenever its waiting for a event to happen. The event can be completing a delay period or availability of a resource. The blocked tasks are not available for scheduling.

  • Suspended

When vTaskSuspend() is called, the task goes in suspended state. To resume it, xTaskResume() is called. The suspended tasks are also not available for scheduling.

It seems that the tasks in both Blocked and Suspended are in waiting state. But the difference is, a suspended task can't come in Ready state unless xTaskResume() is called. Time out period can be specified only for Blocked tasks but not for suspended tasks.


Deciding Priority of a task

One general rule to decide the task priority is RMS ( Rate Monotonic Scheduling ). According to this rule, the most frequently executed task is given highest priority.

In FreeRTOS, higher is the priority number, higher is the task priority. FreeRTOS also always you to have multiple tasks with same priority. In that case it executes the task with same priority in time slice manner. 


==API Details==

Here we will discuss some of the mostly frequently used APIs related to task.

Task Create function

{{#Widget:LibTable}}
Defination portBASE_TYPE xTaskCreate

( pdTASK_CODE pvTaskCode, const portCHAR * const pcName, unsigned portSHORT usStackDepth, void *pvParameters, unsigned portBASE_TYPE uxPriority, xTaskHandle *pvCreatedTask);

Input Arguments #pdTASK_CODE : Pointer to the task entry function
  1. const portCHAR * : A descriptive name for the task. It is a character pointer and is mainly used for debugging.
  1. unsigned portSHORT : The stack size of the task in terms of number of variables not the number of bytes
  1. void* : void pointer which will be used as the parameter for the task
  1. unsigned portBASE_TYPE : The task priority
  1. xTaskHandle* : Used to pass back a handle using which the task can be referenced
Return Value Returns TRUE on successful creation of task and adding it to ready list FALSE otherwise.
Description It creates a task and adds it to ready list.
Usage xTaskHandle TaskHandle_1;

static void MyTask1(void* pvParameters);

xTaskCreate( MyTask1, ( signed char * )"Task1", configMINIMAL_STACK_SIZE, NULL, 1, &TaskHandle_1 );


Task Delay Function

{{#Widget:LibTable}}
Defination void vTaskDelay( portTickType xTicksToDelay )
Input Arguments portTickType : Number of ticks for which task is to be delayed.
Return Value None
Description It delays the calling task for specified number of ticks.

Set the macro INCLUDE_vTaskDelay to use this API.

Use the constant portTICK_RATE_MS in the file portmacro.h to calculate real time delay. Its resolution is one tick period.

Usage vTaskDelay(100);


Task Delete Function

{{#Widget:LibTable}}
Defination void vTaskDelete( xTaskHandle pxTask )
Input Arguments xTaskHandle : handle to the task to be deleted. Pass NULL to suspend calling task
Return Value None
Description It removes the specified task from the RTOS real time kernels management.

But it does not frees the memory allocated by the task code.

Set the macro INCLUDE_vTaskDelete to use this API.

Usage vTaskDelete(TaskHandle_1);


Task Suspend Function

{{#Widget:LibTable}}
Defination void vTaskSuspend( xTaskHandle pxTaskToSuspend );
Input Arguments xTaskHandle : handle to the task to be suspended. Pass NULL to suspend calling task
Return Value None
Description It suspends the specified task

Set INCLUDE_vTaskSuspend to use this API.

Usage vTaskSuspend(TaskHandle_1);

Task Resume Function

{{#Widget:LibTable}}
Defination void vTaskResume( xTaskHandle pxTaskToSuspend );
Input Arguments xTaskHandle : handle to the task to be resumed. Pass NULL to suspend the calling task.
Return Value None
Description It resumes the specified task

Set INCLUDE_vTaskSuspend to use this API.

Usage vTaskResume(TaskHandle_1);


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