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.
The kernel is responsible for the management of all the tasks. Check this tutorial to know working of kernel and task scheduling.
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
| Defination | portBASE_TYPE xTaskCreate (
|
| Input Arguments |
|
| 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
| 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
| 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
| 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
| 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); |
All Tasks Suspend Function
| Defination | void vTaskSuspendAll(void); |
| Input Arguments | None |
| Return Value | None |
| Description | It suspends all real time kernel activity while keeping interrupts (including the kernel tick) enabled.
After calling vTaskSuspendAll () the calling task will continue to execute until xTaskResumeAll () is called. |
| Usage | vTaskSuspendAll(); |
All Tasks Resume Function
| Defination | portBASE_TYPE xTaskResumeAll( void ); |
| Input Arguments | None |
| Return Value | Returns TRUE if resuming the scheduler caused a context switch, FALSE otherwise |
| Description | Resumes real time kernel activity following a call to vTaskSuspendAll () |
| Usage | vTaskSuspendAll();
. . . xTaskResumeAll(); |
Get Task Priority
| Defination | unsigned portBASE_TYPE uxTaskPriorityGet ( xTaskHandle pxTaskToSuspend ); |
| Input Arguments | xTaskHandle : handle to the task to be queried. Pass NULL to query the calling task. |
| Return Value | unsigned portBASE_TYPE : the priority of the specified task. |
| Description | It is used to get the priority of a task.
Set the constant INCLUDE_vTaskPriorityGet to 1. |
| Usage |
unsigned int task_prio; task_prio = xTaskPriorityGet (TaskHandle_1); |
Set Task Priority
| Defination | void vTaskPrioritySet( xTaskHandle pxTask, unsigned portBASE_TYPE
uxNewPriority ) |
| Input Arguments |
|
| Return Value | None |
| Description | It is used to set the priority of a task.
Set the constant INCLUDE_vTaskPrioritySet to 1. |
| Usage |
xTaskPrioritySet (TaskHandle_1, 4); |