RTOS 任务通知
用作轻量级事件组

相关页面：

• RTOS 任务通知
• 使用 RTOS 任务通知作为轻量级二进制信号量
• 使用 RTOS 任务通知作为轻量级计数信号量
• 使用 RTOS 任务通知作为轻量级邮箱

事件组是一组二进制标志（或位）， 应用程序编写者可以对其中的每一个指定含义。 RTOS 任务可以 进入“阻塞”状态以等待组内的一个或多个标志激活。 待验证的 RTOS 任务处于“阻塞”状态时不会占用任何 CPU 时间。

当使用任务通知代替事件组时，接收任务的 通知值被用于代替 事件组，接收任务通知值中的位被用作 事件标志，而 xTaskNotifyWait() API 函数被用于代替事件组的 xEventGroupWaitBits () API 函数。

同样，使用 xTaskNotify() 和 xTaskNotifyFromISR() API 函数（其 eAction 参数设置为 eSetBits）来分别代替 xEventGroupSetBits () 和 xEventGroupSetBitsFromISR () 函数，从而设置位。

与 xEventGroupSetBitsFromISR() 相比，xTaskNotifyFromISR() 具有显著的性能优势， 这是因为 xTaskNotifyFromISR () 完全在 ISR 中执行，而 xEventGroupSetBitsFromISR () 必须将一些处理推迟到 RTOS 守护进程任务中。

与使用事件组时的情况不同的是，接收任务无法指定它 只想在当一个位组合处于活动状态时 结束“阻塞”状态。 相反，任务在任何位变为活动状态时结束阻塞，并且必须 测试位组合本身。

请参阅下面的示例：

/* This example demonstrates a single RTOS task being used to process
events that originate from two separate interrupt service routines -
a transmit interrupt and a receive interrupt.  Many peripherals will
use the same handler for both, in which case the peripheral's
interrupt status register can simply be bitwise ORed with the
receiving task's notification value.

First bits are defined to represent each interrupt source. */
#define TX_BIT    0x01
#define RX_BIT    0x02

/* The handle of the task that will receive notifications from the
interrupts.  The handle was obtained when the task
was created. */
static TaskHandle_t xHandlingTask;

/*-----------------------------------------------------------*/

/* The implementation of the transmit interrupt service routine. */
void vTxISR( void )
{
BaseType_t xHigherPriorityTaskWoken = pdFALSE;

   /* Clear the interrupt source. */
   prvClearInterrupt();

   /* Notify the task that the transmission is complete by setting the TX_BIT
   in the task's notification value. */
   xTaskNotifyFromISR( xHandlingTask,
                       TX_BIT,
                       eSetBits,
                       &xHigherPriorityTaskWoken );

   /* If xHigherPriorityTaskWoken is now set to pdTRUE then a context switch
   should be performed to ensure the interrupt returns directly to the highest
   priority task.  The macro used for this purpose is dependent on the port in
   use and may be called portEND_SWITCHING_ISR(). */
   portYIELD_FROM_ISR( xHigherPriorityTaskWoken );
}
/*-----------------------------------------------------------*/

/* The implementation of the receive interrupt service routine is identical
except for the bit that gets set in the receiving task's notification value. */
void vRxISR( void )
{
BaseType_t xHigherPriorityTaskWoken = pdFALSE;

   /* Clear the interrupt source. */
   prvClearInterrupt();

   /* Notify the task that the reception is complete by setting the RX_BIT
   in the task's notification value. */
   xTaskNotifyFromISR( xHandlingTask,
                       RX_BIT,
                       eSetBits,
                       &xHigherPriorityTaskWoken );

   /* If xHigherPriorityTaskWoken is now set to pdTRUE then a context switch
   should be performed to ensure the interrupt returns directly to the highest
   priority task.  The macro used for this purpose is dependent on the port in
   use and may be called portEND_SWITCHING_ISR(). */
   portYIELD_FROM_ISR( xHigherPriorityTaskWoken );
}
/*-----------------------------------------------------------*/

/* The implementation of the task that is notified by the interrupt service
routines. */
static void prvHandlingTask( void *pvParameter )
{
const TickType_t xMaxBlockTime = pdMS_TO_TICKS( 500 );
BaseType_t xResult;

   for( ;; )
   {
      /* Wait to be notified of an interrupt. */
      xResult = xTaskNotifyWait( pdFALSE,    /* Don't clear bits on entry. */
                           ULONG_MAX,        /* Clear all bits on exit. */
                           &ulNotifiedValue, /* Stores the notified value. */
                           xMaxBlockTime );

      if( xResult == pdPASS )
      {
         /* A notification was received.  See which bits were set. */
         if( ( ulNotifiedValue & TX_BIT ) != 0 )
         {
            /* The TX ISR has set a bit. */
            prvProcessTx();
         }

         if( ( ulNotifiedValue & RX_BIT ) != 0 )
         {
            /* The RX ISR has set a bit. */
            prvProcessRx();
         }
      }
      else
      {
         /* Did not receive a notification within the expected time. */
         prvCheckForErrors();
      }
   }
}