xTimerStartFromISR
[定时器 API ]

 BaseType_t xTimerStartFromISR
               (
                  TimerHandle_t xTimer,
                  BaseType_t *pxHigherPriorityTaskWoken
               );

可从中断服务例程调用的 xTimerStart() 的版本。

参数：

xTimer	正在启动/重新启动的定时器的句柄。
pxHigherPriorityTaskWoken	定时器服务/守护进程任务大部分时间 都处于阻塞状态，等待消息到达定时器 命令队列。 调用 xTimerStartFromISR() 会将消息写入定时器 命令队列，从而让定时器服务/守护进程 任务有可能脱离阻塞状态。 如果调用 xTimerStartFromISR() 导致 定时器服务/守护进程任务脱离阻塞状态，且定时器服务/ 守护进程任务的优先级等于或大于当前执行的 任务（被中断的任务），那 *pxHigherPriorityTaskWoken 将在 xTimerStartFromISR() 函数内部被设置为 pdTRUE。 如果 xTimerStartFromISR() 将此值设置为 pdTRUE， 那应在中断退出前执行上下文切换。

返回：

如果启动命令无法发送至定时器命令队列， 则返回 pdFAIL。 如果命令成功发送至定时器命令队列， 则返回 pdPASS。 实际处理命令的时间 取决于定时器服务/守护进程任务相对于系统中其他任务的优先级， 尽管定时器到期时间 是相对于实际调用 xTimerStartFromISR() 的时间而言。 定时器服务/守护进程 任务优先级由 configTIMER_TASK_PRIORITY 配置常量设置。

/* This scenario assumes xBacklightTimer has already been created.  When a
key is pressed, an LCD back-light is switched on.  If 5 seconds pass
without a key being pressed, then the LCD back-light is switched off.  In
this case, the timer is a one-shot timer, and unlike the example given for
the xTimerReset() function, the key press event handler is an interrupt
service routine. */

/* The callback function assigned to the one-shot timer.  In this case the
parameter is not used. */
void vBacklightTimerCallback( TimerHandle_t pxTimer )
{
    /* The timer expired, therefore 5 seconds must have passed since a key
    was pressed.  Switch off the LCD back-light. */
    vSetBacklightState( BACKLIGHT_OFF );
}

/* The key press interrupt service routine. */
void vKeyPressEventInterruptHandler( void )
{
BaseType_t xHigherPriorityTaskWoken = pdFALSE;

    /* Ensure the LCD back-light is on, then restart the timer that is
    responsible for turning the back-light off after 5 seconds of
    key inactivity.  This is an interrupt service routine so can only
    call FreeRTOS API functions that end in "FromISR". */
    vSetBacklightState( BACKLIGHT_ON );

    /* xTimerStartFromISR() or xTimerResetFromISR() could be called here
    as both cause the timer to re-calculate its expiry time.
    xHigherPriorityTaskWoken was initialised to pdFALSE when it was
    declared (in this function). */
    if( xTimerStartFromISR( xBacklightTimer,
        &xHigherPriorityTaskWoken ) != pdPASS )
    {
        /* The start command was not executed successfully.  Take appropriate
        action here. */
    }

    /* Perform the rest of the key processing here. */

    /* If xHigherPriorityTaskWoken equals pdTRUE, then a context switch
    should be performed.  The syntax required to perform a context switch
    from inside an ISR varies from port to port, and from compiler to
    compiler.  Inspect the demos for the port you are using to find the
    actual syntax required. */
    if( xHigherPriorityTaskWoken != pdFALSE )
    {
        /* Call the interrupt safe yield function here (actual function
        depends on the FreeRTOS port being used). */
    }
}