FreeRTOS软件定时器xTimerCreate

头文件：timers.h

FreeRTOS软件定时器xTimerCreate函数通过计时、计时清零、函数调用的方式，建立一个需要不断运行的任务。该任务不被调度器打断，保证运行稳定。常用在LED灯闪烁等功能的开发上面。

函数原型： 

TimerHandle_t xTimerCreate
                 ( const char * const pcTimerName,
                   const TickType_t xTimerPeriod,
                   const UBaseType_t uxAutoReload,
                   void * const pvTimerID,
                   TimerCallbackFunction_t pxCallbackFunction );

Creates a new software timer instance and returns a handle by which the timer can be referenced.

For this RTOS API function to be available:

1. configUSE_TIMERS and configSUPPORT_DYNAMIC_ALLOCATION must both be set to 1 in FreeRTOSConfig.h (configSUPPORT_DYNAMIC_ALLOCATION can also be left undefined, in which case it will default to 1).
2. The FreeRTOS/Source/timers.c C source file must be included in the build.

Each software timer requires a small amount of RAM that is used to hold the timer's state. If a timer is created using xTimerCreate() then this RAM is automatically allocated from the FreeRTOS heap. If a software timer is created using xTimerCreateStatic() then the RAM is provided by the application writer, which requires an additional parameter, but allows the RAM to be statically allocated at compile time. See the Static Vs Dynamic allocation page for more information.

Timers are created in the dormant state. The xTimerStart(), xTimerReset(), xTimerStartFromISR(),xTimerResetFromISR(), xTimerChangePeriod() and xTimerChangePeriodFromISR() API functions can all be used to transition a timer into the active state.

Parameters:

pcTimerName	A human readable text name that is assigned to the timer. This is done purely to assist debugging. The RTOS kernel itself only ever references a timer by its handle, and never by its name.
xTimerPeriod	The period of the timer. The period is specified in ticks, and the macro pdMS_TO_TICKS() can be used to convert a time specified in milliseconds to a time specified in ticks. For example, if the timer must expire after 100 ticks, then simply set xTimerPeriod to 100. Alternatively, if the timer must expire after 500ms, then set xTimerPeriod to pdMS_TO_TICKS( 500 ). pdMS_TO_TICKS() can only be used if configTICK_RATE_HZ is less than or equal to 1000.
uxAutoReload	If uxAutoReload is set to pdTRUE, then the timer will expire repeatedly with a frequency set by the xTimerPeriod parameter. If uxAutoReload is set to pdFALSE, then the timer will be a one-shot and enter the dormant state after it expires.
pvTimerID	An identifier that is assigned to the timer being created. Typically this would be used in the timer callback function to identify which timer expired when the same callback function is assigned to more than one timer, or together with the vTimerSetTimerID() and pvTimerGetTimerID() API functions to save a value between calls to the timer's callback function.
pxCallbackFunction	The function to call when the timer expires. Callback functions must have the prototype defined by TimerCallbackFunction_t, which is: void vCallbackFunction( TimerHandle_t xTimer ); .

Returns:

If the timer is created successfully then a handle to the newly created timer is returned. If the timer cannot be created because either there is insufficient FreeRTOS heap remaining to allocate the timer structures, or the timer period was set to 0, then NULL is returned.

Example usage:

 #define NUM_TIMERS 5

 /* An array to hold handles to the created timers. */
 TimerHandle_t xTimers[ NUM_TIMERS ];

 /* Define a callback function that will be used by multiple timer
 instances.  The callback function does nothing but count the number
 of times the associated timer expires, and stop the timer once the
 timer has expired 10 times.  The count is saved as the ID of the
 timer. */
 void vTimerCallback( TimerHandle_t xTimer )
 {
 const uint32_t ulMaxExpiryCountBeforeStopping = 10;
 uint32_t ulCount;

    /* Optionally do something if the pxTimer parameter is NULL. */
    configASSERT( pxTimer );

    /* The number of times this timer has expired is saved as the
    timer's ID.  Obtain the count. */
    ulCount = ( uint32_t ) pvTimerGetTimerID( xTimer );

    /* Increment the count, then test to see if the timer has expired
    ulMaxExpiryCountBeforeStopping yet. */
    ulCount++;

    /* If the timer has expired 10 times then stop it from running. */
    if( ulCount >= ulMaxExpiryCountBeforeStopping )
    {
        /* Do not use a block time if calling a timer API function
        from a timer callback function, as doing so could cause a
        deadlock! */
        xTimerStop( pxTimer, 0 );
    }
    else
    {
       /* Store the incremented count back into the timer's ID field
       so it can be read back again the next time this software timer
       expires. */
       vTimerSetTimerID( xTimer, ( void * ) ulCount );
    }
 }

 void main( void )
 {
 long x;

     /* Create then start some timers.  Starting the timers before
     the RTOS scheduler has been started means the timers will start
     running immediately that the RTOS scheduler starts. */
     for( x = 0; x < NUM_TIMERS; x++ )
     {
         xTimers[ x ] = xTimerCreate
                   ( /* Just a text name, not used by the RTOS
                     kernel. */
                     "Timer",
                     /* The timer period in ticks, must be
                     greater than 0. */
                     ( 100 * x ) + 100,
                     /* The timers will auto-reload themselves
                     when they expire. */
                     pdTRUE,
                     /* The ID is used to store a count of the
                     number of times the timer has expired, which
                     is initialised to 0. */
                     ( void * ) 0,
                     /* Each timer calls the same callback when
                     it expires. */
                     vTimerCallback
                   );

         if( xTimers[ x ] == NULL )
         {
             /* The timer was not created. */
         }
         else
         {
             /* Start the timer.  No block time is specified, and
             even if one was it would be ignored because the RTOS
             scheduler has not yet been started. */
             if( xTimerStart( xTimers[ x ], 0 ) != pdPASS )
             {
                 /* The timer could not be set into the Active
                 state. */
             }
         }
     }

     /* ...
     Create tasks here.
     ... */

     /* Starting the RTOS scheduler will start the timers running
     as they have already been set into the active state. */
     vTaskStartScheduler();

     /* Should not reach here. */
     for( ;; );
 }